Sunday, September 8, 2019

Adhesion and Cohesion

What are adhesion and cohesion? To start with, they are properties of matter!

Cohesion is the tendency of molecules to stick to other molecules of the same substance. This is why drops of water bead up on a slick surface. In fact, this is why water forms drops in the first place!

The stronger the force of cohesion, the more molecules that will be able to stick to each other. Drops will be bigger!

Cohesion is also responsible for surface tension. If you've ever skipped a rock or ridden in a fast boat or jet ski, you have surface tension to thank!

Adhesion is the tendency of molecules to stick to other molecules of different substances. You probably already know what an adhesive does! An adhesive adheres to two things and makes them stick together. So, a good adhesive has a high degree of adhesion between itself and other things.

Consider this very fancy diagram (right)!

The GLUE adheres to the mug and it also adheres to the handle. Thus, the handle sticks to the glue which is stuck to the mug. The final result is that the handle sticks to the mug!

Adhesion exists beyond tapes and glues, too. The adhesive properties of water allow you to use it to stick the shower curtain to the tiles. If you have ever licked a plastic decal and stuck it to glass (or wiped it with water and stuck it to glass), it was the adhesive properties of water that allowed that to work.

Flash back to those beads of water… Car waxes brag that they cause water to bead up and run off.
What's going on there? There is at play a combination of cohesion between the water molecules and adhesion between the water and the surface.

When water falls onto a surface, there is created a balance between the forces of attraction between the molecules to themselves and the surface. Wax makes the surface "slicker." That means that the adhesive force decreases, so the cohesive forces have a bigger effect. The water sticks to itself better than it does to the wax, so the beads of water become larger.

Friday, September 6, 2019

More Physical Properties of Matter

It has been established that…

A physical property of matter is any attribute, quality, or characteristic of a material that can be observed or measured without changing the composition of the substances in the material. 

Among the many properties of matter there are:

Melting and Boiling Points

These properties were introduced HERE.

There are many more physical properties of matter, and some of them will be introduced in this article.

Some physical properties are independent of the amount of the substance preset. Some physical properties change as the amount present changes.

If the property changes based on the how much is present, it is said to be an extensive property. Some factor outside the makeup of the material—some external factor is connected to the property. For instance, mass is an extensive property; the more of something you have, the more mass you have.

If the property does not change based on how much is present, it is said to be an intensive property. The property is independent of the amount present. For instance, color is an intensive property; no matter how much you have, the color is the same.

Physical Properties of Matter

Appearance (intensive)
How does it look? What is visually identifiable about the substance?

Some aspects of a physical appearance include its color, texture, or sheen. Uniformity of these things could also be a factor, or variances might indicate that the sample has some impurities in it.

Odor (intensive)

IMPORTANT NOTE: Don't sniff chemicals! You could die!

While it is dangerous to inhale chemicals, some of them do stimulate the olfactory nerves which results in the perception of odor.

Solubility (intensive)

Solubility is the degree to which a substance (solute) dissolves in a given solvent (other substance). Not everything dissolves in everything! There is a vast degree of variance in what will dissolve in what and to what degree! You need only watch a few TV commercials to know that Brand A dish soap will dissolve grease better than all the other! Let's just skip the laundry ads!

Solubility for a given substance might be noted in relationship to various solvents. How well does it dissolve in water? How about alcohol? How about mineral spirits? How about acetone?

Nail polish is a very intuitive example. Nail polish does not dissolve very well if at all in water. (If it did, it would come off with every hand wash!) It does, however dissolve readily in acetone (nail polish removers often are acetone).

Magnetivity / Magnetism (intensive*)

The degree to which a substance is attracted to or repelled by magnets and magnetic fields. Basically, do magnets stick to it or does it stick to iron?

*Larger samples of a magnetic substance will create a larger (i.e. stronger) magnetic pull, but the property is uniform regardless of sample size)

Ductility (intensive)

The degree to which a substance can elongate when pulled.

Examples: Chewing gum is very ductile. You can pull it and stretch it really far out of your mouth (although that is gross and germy). Carrots are not, compared to gum, very ductile. They snap off.

Specific Heat (intensive)

The capacity of a substance to hold energy in the form of heat.

If you have a dishwasher and have ever tried to unload it right after it was done, you know that the glass bowls will burn your fingers more than the plastic ones, but when the door opens, they are all the same temperature. Glass can hold heat better than plastic. Thus, the specific heat of glass is higher than the specific heat of plastic.

Specific heat is measured in a unit of energy per a unit of mass such as cal/gr or J/gr.

Opacity (intensive)

The capacity of a substance to block (usually limited to visible light) electromagnetic waves. Opacity ranges from transparent (light passes without diffusion), to translucent (some light passes but is diffused) to opaque (no light passes through).

While opacity is frequently used to discuss visible light, the same term applies to other types of radiation as well.

Mass (extensive)

The total amount of matter present, the sum of all the electrons, protons, and neutrons within the sample. 

Volume (extensive)

The total amount of space occupied by the sample.

Denisty (D) is an intensive property of matter that is the ratio of mass (m) to volume (V) found by:

D = m/V

Monday, May 20, 2019

Energy of Chemical Bonds

Lead Author: Dr. Anne Gull

Chemical energy is a type of energy that is stored in the bonds of compounds. When compounds are formed, some energy is required to "shove" the atoms together. When those bonds are broken, that energy is released.

In typical chemical reaction, some bonds break (and give off energy) and other bonds form (taking in energy. If the total energy given off exceeds the energy need, the excess energy is given off as heat and/or light. When a reaction gives off energy, it is exothermic. Energy exits the reaction. When a reaction takes in energy, it is endothermic. Energy enters the reaction.

Every combination of atoms has a specific bond energy. Knowing the bond energy allows the calculation of energy given off.

For example, the bond between carbon and hydrogen stores 413,000 joules for every mole. That means that one mole of those bonds will give off that much energy.

Chemical energy is stored in bonds, so it’s a type of potential energy that can be released when a chemical reaction occurs. Each type of bond has a unique amount of energy. Some of the values are shown in the chart below. These energy amounts are measured for 1 mole.

The figure below shows the energy stored in several common bonds:

413000 J
347000 J
358000 J
799000 J
467000 J
146000 J
* for CO2
For other C=O bonds, see the link at bottom of page.

In order to figure out how much potential energy is inside a chemical substance, you need to look at the structure to see the bonds (the lines between elements represent the bonds). 

The dash or single line (-) is a single bond. The double line (=) is a different kind of bond, a double bond. There is also triple bonds (≡) three lines.

The energies for the different bonds are different. It is important to recognize the difference and make sure you are using the correct bond energy from the table.

More Bond Energies

For more information and the energies of other types of bonds, see this link:

Click Here

NOTE: The energies on the linked page are given in kilojoules, so multiply by 1000 to get joules per mole.

Example #1 Methane

In the molecule above, there are 4 C-H bonds, so the potential energy stored in this molecule is 4 times the value of each C-H (413000). So there is 4(413000) = 1652000 J of potential energy in one mole of this type of molecule.There are 3 C-H bonds, 1 C-O bond and 1 O-H bond so the potential energy stored in each bond would be added up.

Example #2 Methanol

There are 3 C-H bonds, 1 C-O bond and 1 O-H bond so the potential energy stored in each bond would be added up.

3 C-H 3(413000) J
1 C-O 1(358000) J
1 O-H 1(467000) J
Total 2,064,000 J

Wednesday, May 15, 2019

Light and Optics: Introduction


Light is one form of electromagnetic (EM) energy. Light is defined within a narrow range of electromagnetic frequencies which stimulate the receptors of the eye.  Included in light is the frequencies just beyond what humans can see. Infrared light has a slightly too low of a a frequency for humans to see. Ultraviolet light has a slightly too high frequency for humans to see.

All forms of electromagnetic energy are affected by interacting with matter. When light interacts with matter, it either reflects off of it,  refracts around it, or passes through.

One of the properties of light (and other EM types) is that it will propagate at different rates through different mediums. Though the difference is generally slight, it makes a difference.


Light, because it can be easily observed, provides many opportunities to EM radiation.

Notice how the pencil seems to bend. This occurs because
light travels at different rates through air and water (and glass).
The difference between the speed of light in a vacuum and the speed of light in air is very, very minimal (on a scale of X 108). However, the difference in speed between light in air and light in water water results in light bending at the waters surface.

The differences in speed also explain how lenses can reshape light. Glass can bend the light in one of two ways.

Focusing is the process of taking light that is moving in a straight line and concentrates it into a smaller area. The rays of light (particle model) are bent such that they converge (align into a tighter, smaller area).

Light can also be "un focused." Light that is traveling in a straight line can be dispersed into a wider area. The rays of light (particle model) are bent such that they diverge (spread out over a wider area).

Both lenses and mirrors can converge (focus) and diverge light. First… There are these three words you need to understand.

From Google
Concave: A shape that has a recessed center relative to the outer edges. The center caves in.

Flat: The center and the edges are in the same plane. (A window)

Convex: A shape that has a protruding center relative to the outer edges. NOT like a cave (see above).

So, which types of lens and mirror do what?

  • Focus occurs with convex lenses and concave mirrors.
  • Dispersion occurs with concave lenses and convex mirrors.

STOP - NOTICE!Throughout the vast domain of information on optics, lenses, and light, there exists a variety of sets of variables that identify different measures. The "distance to the object from the lens," for example can be found as… 
d… do… D… Do… u… o… 
This variation is very frequent. This discussion reflects that with examples and images used from other sources.

Focal Length

Every lens has a focal length. That is the distance from the lens to the point where parallel light will be focused:

An example of a convex mirror which spreads the light
making objects appear smaller than they actually are.
Convex lenses have a positive focal length. The image appears some distance behind the lens. The lens is between the object and the image.

Concave lenses have a negative focal length. No image appears, but the focal point is in front of the lens, between the object and the lens.

Concave mirrors have a positive focal length. The image appears some distance behind the lens. The lens is between the object and the image.

Convex mirrors have a negative focal length. No image appears, but the focal point is in front of the lens, between the object and the lens.

A flat mirror and a flat piece of glass (window) work into this schema, falling between convex and concave. Imagine a lens that is convex. As the radius of the curve increases, it gets closer and closer to a flat piece of glass (a window). Likewise, a mirror can flatten out as well.

The object appears inside the mirror an equal distance, but behind the mirror. More on this to come!

Depending on the substance from which a lens is made as well as the geometry of the lens, there is a relationship between the distance from the object to the lens, o, the focal length of the lens f, and the distance from the lens to the projected image i.

Object ------ o ------ Lens ----- i -------Image

That relationship is:
1/o + 1/i = 1/f

Solving for f… (consult with a math teacher if you have questions!)

(i/i)(1/o) + (o/o)(1/i) = 1/f

(i/io) + (o/io) = 1/f

(i + o)/(i)(o)  = 1/f

f = (i)(o)/(i + o)

So, knowing i and o, we can easily find f:


An object is 2 cm from a lens and the image appears 5 cm from the lens. Find the focal length.
f = (i)(o)/(i + o)
f = (2 • 4)/(2 + 4)
f = 8 / 6
f = 1.333 cm

An interesting property to examine is the relationship of image size to object size in consideration of where the object is located and where the image appears.

Image from

The formula
Hi/Ho = Di/Do ( see eq. 1 above) 
is not difficult to work with. Cross multiplying will quickly reduce the problem to a simple solution, for instance:

Find the height of the image, Hi, when the object has a height of 3 cm and where the object is 8 cm from the lens and the image is 4 cm from the lens.

Hi/Ho = Di/Do
Hi/3cm = 8cm/4cm 
cross multiply to get 
(4cm)(Hi) = 24cm2 
Hi = 6cm

Lastly, we can talk about magnification. How much bigger or smaller is the image compared to the object?.

This one is pretty easy!

M = hi/ho

So… divide…

EXAMPLE (without units, which would be a distance unit for hi and ho):

A lens has a magnification, M, of 1.25. I f the height of the object is 6, what is the height of the image?
M = hi/ho
1.25 = hi/6
(6)(1.25) = (hi/6)(6/1)
7.5 = hi


A very large portion of society has a daily encounter with optics: those who wear glasses or contacts and those take photos (camera or phone!)

While the need to calculate "things" rarely comes up, a fundamental understanding of lenses and light add to the daily life experience.

1 "propagates" is used to describe the motion of waves through a medium. Mechanical waves (such as sound) propagate by transferring energy through collision of molecules. Electromagnetic waves propagate differently and do not rely on matter as a medium. Once upon a time, scientists theorized that the universe was filled with something they called the aether that was responsible for the propagation of electromagnetic waves.

Tuesday, May 14, 2019

Electromagnetic Energy: Introduction

Energy can appear within creation in several forms, and one of those forms is electromagnetic energy. This is a huge topic of physics, and it will be introduced by taking a look at light.

Electromagnetic energy (EM) is a category of energy forms that propagates1 through space at the speed of light, c, which is 3 X 108 m/s or 186,000 mph. Electromagnetic energy is categorized by it's wavelength and frequency. Certain form of EM are defined by ranges of wavelength and frequency.

Because EM travels at the speed of light, wavelength, λ, and frequency, f, are reciprocals and conform to the wave equation:

v = fλ

Since v is always the speed of light for all EM forms, as f goes up, λ goes down.

Ranges of frequencies (and thus wavelengths) determine the type of EM in question. Certain ranges are called radio waves (AM and FM have different frequency ranges—take a look at your car radio tuner, perhaps). Other familiar ranges include radar, x-ray, and microwave ovens.

Electromagnetic energy behaves differently in different situation. In many cases, it behaves like a wave that will reflect (bounce off of) from surfaces or refract (bend around) edges or objects. However, in other cases, it behaves like a particle. Thus, EM energy is thought to have both properties that are explained by both wave and particle models. In reality, the complexities of EM energy are not fully explained by either model, but each model is useful for gaining understanding of different principles of EM energy.

1 "propagates" is used to describe the motion of waves through a medium. Mechanical waves (such as sound) propagate by transferring energy through collision of molecules. Electromagnetic waves propagate differently and do not rely on matter as a medium. Once upon a time, scientists theorized that the universe was filled with something they called the aether that was responsible for the propagation of electromagnetic waves.

Wednesday, May 8, 2019

Heat of Fusion and Heat of Vaporization

From kinetic theory, it is understood that as energy enters a substance, the molecules of the substance gain kinetic energy. Temperature is, in fact, defined as the average kinetic energy of the molecules of a substance.

As energy continues to flow into a substance, the kinetic energy of the molecules increase (that is to say the temperature goes up). This is true throughout the range of temperatures of the substance except at a few points: phase changes.

Heat of Fusion

When a substance reaches the melting point (heating up) or freezing point (cooling off), the temperature stops changing. When heating up, the added energy no longer increases the kinetic motion of the molecules but rather begins to break the bonds that hold a solid into a fixed shape and volume.

The amount of energy needed to break the bonds that holds a solid together is related to called the heat of fusion and is related to specific characteristics of the substance. Substances have different values for heat of fusion.

The amount of energy needed to break the bonds is also related to how much of the substance is present. Naturally, the more of something you have, the more energy it takes to melt it.

The total amount of heat needed to convert a substance from a solid to a liquid is determined by the value of the heat of fusion and how much of the substance is present, and can be found using the formula

Q = mΔHf

where Q is the heat (energy needed), m is the mass, and ΔHf is a number specific to each substance. ΔHis often provided in calories per gram, joules per gram, or joules per mole.

The process of going from liquid to solid work similarly, but in reverse. Instead of taking heat to break the bonds, the substance has to give off heat in order for the bonds to form. The amount is the same, and the same equation can be used.

Heat of Vaporization

In a similar way to that a substance goes from a solid to a liquid, the process of going from a liquid to a gas requires energy to break the bonds of the liquid that give a liquid its definite volume. The energy stops increasing the temperature of the substance, but instead go into breaking the bonds.

This can easily be observed with a thermometer in a boiling liquid. No matter how long the pot sits on the heat source, the temperature of the water will only go so high—100° C—and after that, the energy from the heat source converts the water to a gas (steam).

How much heat is needed is, like melting, a function of the type of substance and the amount present. The more of a substance, the more energy needed to convert it all to a gas.

Thus, a formula emerges that is very like that for melting:

Q = mΔHv

where Q is the heat (energy needed), m is the mass, and ΔHv is a number specific to each substance. ΔHv is often provided in calories per gram, joules per gram, or joules per mole.


How much heat is needed to convert 8 grams of ice (solid) entirely to water (liquid) where ΔHf  for water is 333.55 J/g.

Q = mΔHv 
Q = (8 gr )(333.55 J/g) 
Q = (8 gr )(333.55 J/g)   
Q = 2,688.4 J

Values for ΔHand  ΔHv can be found on many, many websites.

Monday, May 6, 2019

Nuclear Energy

In the 21st Century, the concept of nuclear energy is not strange to most people. Generally speaking, people know about nuclear bombs and nuclear power plants. Many people can explain that the sun (that stars) are giant nuclear plants that give off energy.

In the 1990s, the band, They Might Be Giants produced a song that stated:
The sun is mass of incandescent gas, a gigantic nuclear furnace
Where hydrogen is built into helium at a temperature of millions of degrees

-- They Might Be Giants - "Why Does the Sun Shine? (The Sun is a Mass of Incandescent Gas)"

So, how exactly does nuclear energy work? 

This goes back to the four basic forces that exist. Some people will recall they are:
strong (nuclear)
weak (nuclear)
Nuclear energy is associated with the last two. It takes a lot of energy to hold an atom together. That energy can be released when the atom is split.

Nuclear fission, the splitting of an atom's nucleus, releases a great deal of stored energy by… we'll get to that in a moment.

Another process related to the nucleus (hence the term nuclear) is when two or more nuclei are shoved together. This also results in the release of a great deal of energy!

Nuclear fusion, the joining of two or more atom's nuclei, releases energy by… stand by! Getting to that soon.

Both fission and fusion release energy. But how? And why so much?

Fission and fusion both involve altering the nuclei of atoms. Fission splits a nucleus and fission joins two or more. It is important to note that both fission and fusion result in atoms that have different atomic numbers (number of protons) than the original elements. For instance fusing hydrogen (atomic number 1) results in heliums (atomic number 2).

Something else happens when fission and fusion take place. Those of you who like to break the laws will really like this! (Note: breaking laws is agains the law and should not be done, ever. Unless you are fission or fusion.)

The law of conservation of matter is broken. The mass before and after nuclear reactions is different. The law of conservation of matter says that matter cannot be created or destroyed. Well…

There is also a law of conservation of energy that says that energy cannot be created or destroyed--that it only changes form. Well…

The processes of fission and fusion do not follow these laws. BUT!! The change in mass and the change in energy are related. In fact, they are exactly connected. Not only is the change in mass and energy connected, but they are connected by the most famous physics formula ever!

E = mc2


This is what you've been waiting for ever since you first discovered Einstein's most famous formula!

So what does that mean?

It means that the conversion of matter to energy joins with the laws of conservation to result in a new, combined law:

The total of mater and energy is a constant never changing, but possibly interchanging.

Back to fusion and fission…

In nuclear reactions (both fission and fusion), energy is released (here are the conclusions) …by converting some amount of matter into energy. The amount of energy released is found using the equation

E = mc2

where E is the energy given off in joules, m is the mass in kilograms, and c is the speed of light (also the speed of an electromagnetic wave [energy]) which is a constant:

c = 299 792 458 m / s 
Now, nearly never is c written that way. The normal procedure is to write it in scientific notation as: 
c = 3 X 108 m/s 
So, who doesn't like exponential notation? 

Working with the equation is not difficult. Find c2  then multiply by the mass. Done.


So, a review of finding the square in exponential notation seems in order…

Let's find y where:

y = x2

and x = 200

y = x2
y = 2002
y = 40,000

Now, 200 = 2  X  102

y = x2
y = 2002
y = (200)2
y = (2  X  102)2

Recall that to raise an exponent to the power of an exponent, you multiply exponents. thus…

y = (2  X  102)2
y = 4  X  104

It is fine to leave the equation above as is, but for the sake of proof, we can take it one more step:

y = 4  X  104
y = 40,000


Working with the equation

E = mc2

is not difficult. Find c2  then multiply by the mass. Done.

Thus,  given any mass in kilograms, multiply by c2 to find the amount of energy in joules that is given off. Finding c2 and working out examples is left as an exercise for the reader.


Thursday, March 21, 2019

Math is Communication

In science and engineering, math is a vital part of communicating the ideas related to any observation or situation. Sloppy or incomplete expressions of math only tell part of the story.

To properly tell the math story, it is necessary to 1.) begin with the formula. The formula establishes the relationship between all of the things that are part of the situation. The next thing is to 2.) plug in the values of the various variables. This tells the specific story. Lastly, 3.) simplify and reduce the math (solve the problem).

Scan for video

So, the steps in summary are:

1. Write the formula.
2. Plug in the values.
3. Solve.

It is important to fully accept that the math is a story. Math is a way to communicate the relationship between different quantities and properties. Math is the whole movie.

Just jotting down some numbers and coming up with an answer is like the movie trailer. It doesn't tell the whole story.

It is noteworthy that telling the math story is very similar to solving a math problem. Telling the story is only different in that it begins with the relationship between the parts, whereas solving a problem begins with the question, then identifies the relationship between what is given and what is asked to be found.

Math is often the most efficient way to explain the relationship between things and to show, when they interact, what happens. Suppose that a situation was described as follows:

A 10 newton force acts on a  5 kg box unopposed. What is the rate of acceleration.

Without math, using only words (and in the style of epic fantasy) here's what we would have:

Long, long ago, when the foundations of the universe were being created, the Creator deemed that there would be a universal relationship between the force acting upon an object, its mass, and the rate at which the force would accelerate it. So it came to pass that the rate of acceleration would be proportional to the force acting upon the object and inversely proportional to the mass of the object.
Some time later it occurred that there was a 5 kg box. Upon this box a force was applied and the magnitude of that force was 10 newtons. The relationship of force and mass resulted in the box changing velocity, accelerating from rest at a rate of 2 meters/second every second.

Now, with math, the same story:
F = ma
10 N = 5 kg • a
10 N / 5 kg = a
2 m/s/s = a
The story is the same, but the telling is different. Math tells the story of science and engineering.

It is vital to note that each line of the solution is a sentence in the math story, and every sentence must have a verb. In the case of math, the verb is the equal sign. Therefore, every line of the solution needs to have BOTH sides of the equation AND the equal sign, or else the story is not being told well.


The math story begins with the formula, the relationship between all of the variables involved., The next part of the story is the insertion of the specific values into the formula. The conclusion of the story is the algebraic / arithmetic solution and reduction.

Tell the whole story!

1. Write the formula.
2. Plug in the values.
3. Solve (making sure to use complete math expressions that include the equal sign).

Monday, March 18, 2019

Grams to Grams Stoichometry

The gist of this concept is fairly simple to understand. But, there are a lot of details between the concept and answering the questions.

The question will take the form of something like…
You have this much of something. How much of something else do you need for a complete reaction without anything being left over.
Simple enough on face value. But, did you catch the part about there being a lot of steps?

Let's do it.

Step One: Start with a balanced equation.

The balanced chemical equation is like a recipe. It tells the ratio of ingredients in the compound. The principle works for all types of reactions, but a simple synthesis reaction makes the simplest example, so we'll look at that.

And, the mention of recipe evokes the idea of food, so let's start there.

(Credit to my colleague, M. Peterson, for this example.)

Let's make some s'mores. You know…

Now, just to be fair, there are two types of s'mores (just as some compounds form in different ratios): there is the single-chocolate layer s'more and the double-chocolate layer s'more.

So, let's look at the balanced recipe for those:

Cracker = C

Marshmallow = M

Ch = Chocolate squares.

With these basic components two different varieties of S'more can be created!

Single Chocolate S'more

M + 2C + 3Ch --> MC2Ch4


Double Chocolate S'more

M + 2C + 6Ch --> MC2(Ch3)2

The coefficients to the balanced recipe tells how much of each thing is needed in a ratio. For the Single Chocolate S'more, the ratio is:

1:2:3 -->1

1 marshmallow : 2 crackers : 3 squares of chocolate --> 1 s'more

The ratio works for individual items, dozens of items, scores of items, bazillions of items, or…

…or moles of items.

With chemistry, the concept of the mole prevails because of its connection to grams through the atomic mass of the elements. A mole of atoms weighs in grams the atomic mass. This is super, super convenient!

Understanding that the balanced chemical reaction gives us a set, fixed ratio of the atoms that must combine is the first, vital concept needed for the grams to grams stoichometry process.

So, let's make some water? And some hydrogen peroxide? Sure…

2H2 + O2 --> 2H2O   (water)
H2 + O2 --> H2O     (hydrogen peroxide)
The balanced reactions above give us the ratio recipe for the molecules in the reaction. Granted, it is a simple synthesis reaction, but, as stated before, the principle that applies here and the method is the same regardless of the complexity of the reaction.

Step 1 is to find the balanced reactions. Done.

Step 2: Extract the ratio recipe from the coefficients.


For the first reaction, the ratio is 2:1 --> 2

For the second reaction, the ratio is 1:1 --> 1

What does that even mean?

This is actually pretty awesome in that chemistry is cool sort of way. (Just nod and follow along. Trust me, it's awesome!)

Like the s'mores, it is the ratio of "things" needed for a complete reaction without leftovers.

In s'mores, if you have only 2 crackers, you need only 1 marshmallow. Done. Period. Having more than one marshmallow means you'll have leftovers.

In many cases of introductory chemistry, you are looking for complete reactions without leftovers.

So, for the H2O reaction (water), starting with 2 somethings of H2 requires 1 something of O2. The something can be molecules, dozens of molecules, thousands of molecules, or bazillions of molecules. However, counting molecules requires very, very, very small fingers #sarcasm! Counting them a bazillion times is a bazillion times harder!

This is where the mole concept comes to the rescue! The relationship between moles, atomic mass, and grams saves us.

Time to chant (seriously):
Moles to grams you multiply!
Grams to moles divide!
Repeat 10 X
So, if you know the number of moles, multiply by the atomic mass OF THE MOLECULE to find out how many grams you have.

If you know how many grams you have, divide by the atomic mass OF THE MOLECULE to find out how many moles you have.

Now, that we have that figured out, we are ready to go to the next step.

Step 3: Figure out how many moles of the given thing you have.

This is why we chanted.

Say you have 37 grams of O2 and you are doing the water reaction. How many grams of H2 do you need (to react completely without left over).

Okay, start with the balance reaction (step 1)

2H2 + O2 --> 2H2O

If you have 37 grams of Ohow many moles is that?  Okay, back up…

Step 3a: Find the MOLECULAR MASS for the reaction.  Let's put those numbers below the molecules in the reaction.

This is the mass of the molecules Not the mass of the reactants or products. 

2H2 + O2 --> 2H2O
  2 g          32 g              18 g 
A more complete look at the masses…

                    Atom Mass                                 1 g         16 g          2g 16g
                    Balanced Equation                    2H2 + O2 --> 2H2O
                    Molecule Mass                             2 g        32 g            18 g
                    Total Reactant/Product Mass       4 g         32 g            36 g  

Step 3b: Find the moles of what's given.  Chant!

Divide what you have by the mass of one molecule:

37 g / 32 g/mole  =  1.156 moles   (If the units confuse you, just chant again.)

Step 4: Use what you have (Step 3b) and the recipe ratio (Step 2) and find out what you need for the other thing.

That sounds more confusing than it is.

The recipe calls for:

2 : 1 --> 2

We have more than the 1 in the ratio, so we need to figure out what all of the numbers are. Easy.

Multiply the ratio (which is 2 : 1 --> 2)
by what you have (which is 1.156 moles)
divided by what you need (which is 1 mole)


1.156/1 • (2 : 1 --> 2
2.312 : 1.156 --> 2.312
Now what?

That multiplication tells us how much of the other thing we need. In the case above, since we have 1.156 moles of O2, we need 2.312 moles of H2.

Step 5: Convert the moles needed of the other thing to grams.

This is why we chanted.

How much did one mole of H2 weigh? It's up there somewhere!

So, multiply!
2.312 moles • 2 g/mole = 4.624 grams.

Step 6: Bask in satisfaction that you are now done.

That's it. That's the answer. If you have 37 grams of O2 you need 4.624 grams of H2


Let's do the peroxide without all of the discussion.

Starting with .75 grams of H2, how much O2 is needed to completely react without excess?

Step 1: Balance the equation:

H2 + O2 --> H2O

Step 2: Extract the recipe ratio:

1: 1 --> 1

Step 3: How many moles do you have to begin with?

H2 + O2 --> H2O
2 g          32 g             34 g

Grams to moles divide…

.75 g / 2 g/mole = .375 moles of H2

Step 4: Use the ratio to find moles of "other" thing.

.375/1 • ( 1: 1 --> 1 ) 
.375 : .375 --> .375

Step 5: Convert moles needed of the "other" thing to grams.

So, multiply!
.375 moles • 32 g/mole = 12 grams.




The process is long, the concept is sorta complex, but it's really not all that hard. Just more tedious than anything.

Here are the steps:

Step 1: Balance the equation:

Step 2: Extract the recipe ratio:

Step 3: How many moles do you have to begin with?

Step 4: Use the ratio to find moles of "other" thing.

Step 5: Convert moles needed of the "other" thing to grams.

(Chanting is optional.)

Step 6: Bask in satisfaction.