**Where are we going with this?**This page will give the ability to

*demonstrate an understanding of the law of conservation of mass through the use of particle diagrams and mathematical models.*

**How About Calculating Some Moles and Grams?**

*Let's take another look at this stoichiometry thing…*

The essence here is that we are dealing with the quantities of stuff that goes into a chemical reaction. *How much of one thing reacts with another thing and how much of what is produced?*

*(and that you can't Google)*.

And… a problem to work through… Let's do grams given…

If 5.0 grams of FtM2 is completely reacted with LgNHc without leftovers, how many grams of the second substance is needed?

Copy this page onto paper, filling in the boxes and doing the calculations…

Copy this page onto paper, filling in the boxes and doing the calculations…

STEP 1: Balance the reaction by finding the coefficients that result in the conservation of matter. Type the coefficients in the boxes provided.

_{2}+ LgNHc --> Ft(NHc)2 + LgM

Step 2: Find the

__expected molar ratio__from the coefficients. (You'll use this in step 4, below).

Step 3: Molar Masses

Calculate Molar Masses

_{2 }= : LgNHc = --> Ft(NHc)2. = : LgM =

Step 4: Find the actual

__moles__of whatever is given:

*Recall that moles can be found several ways; See Step 3 here.*

In this problem, use the grams given with the molar mass of the substance to find moles:

Moles = grams / molar mass

Moles present of given substance = grams given / molar massMoles present of given substance =

Step 5: Find what proportion of the expected moles is actually given

• So, the coefficients in Step 1 are the moles expected.• The actual number of moles present was calculated in Step 3.

Some proportion of the "recipe" is present.

Ratio Multiplier = moles present / moles expected of the given

Ratio Multiplier =

Step 6: Find the molar ratio of what you actually are working with based on what was given.

Now, we take the expected ratio (coefficients from Step 2) and multiply it by the ratio multiplier to find the ratio we are actually working with (present ratio or working ratio):

*(The numbers below are the SAME as the coefficients above in step 1).*

________________________________________

: --> : Present, Working Molar Ratio

Step 7: Find whatever you are supposed to find from the working molar ratio.

*To find masses…*

: --> : Grams present / needed / produced.

*To find gas volume at STP…*

: --> : liters of gas at STP.

*If working with Molarity in a solution…*

*M*is molarity,

*n*is number of moles, and

*V*is volume in liters

M = n/v

M = present, working molar ratio (Step 6) ÷ V

**What would that look like on paper with a real equation?**

*(As if you can read the handwriting…)*

*Starting with .5 grams of hydrogen gas, how much oxygen gas would be needed and how much water would be produced?*

**________________________**

**Spoiler alert! Answers to the walkthrough follow!**

_{2}+2LgNHc --> 1Ft(NHc)2 + 2 LgM

Step 2: Find the

__expected molar ratio__from the coefficients. (You'll use this in step 4, below).

1: 2 --> 1:2

Step 3: Molar Masses

Calculate Molar Masses

_{2 }= 70 : LgNHc = 32 --> Ft(NHc)2. = 44 : LgM = 45

Step 4: Find the actual

__moles__of whatever is given:

*Recall that moles can be found several ways; See Step 3 here.*

In this problem, use the grams given with the molar mass of the substance to find moles:

Moles = grams / molar mass

Moles present of given substance = grams given / molar massMoles present of given substance = 5 / 70

Moles present of given substance = 0.071

Step 5: Find what proportion of the expected moles is actually given

• So, the coefficients in Step 1 are the moles expected.

1: 2 --> 1:2

• The actual number of moles present was calculated in Step 3.

Some proportion of the "recipe" is present.

Ratio Multiplier = moles of given present / moles if given expectedRatio Multiplier = 0.071 / 1Ratio Multiplier = 0.071

Now, we take the expected ratio (coefficients from Step 2) and multiply it by the ratio multiplier (Step 5) to find the ratio we are actually working with (present ratio or working ratio):

*(The numbers below are the SAME as the coefficients above in step 1).*

1 : 2 --> 1 : 2 Expected Molar Ratio (Coefficients from Step 2)

X 0.071

0.071 : 0.142. --> 0.071 : 0.142Present, Working Molar Ratio

Step 7: Find whatever you are supposed to find from the working molar ratio.

*To find masses…*

FtM_{2 }= 70 : LgNHc = 32 --> Ft(NHc)2 = 44 : LgM = 45Molar Masses

0.071 : 0.142. --> 0.071 : 0.142Present, Working Molar RatioFtM_{2 }= 5. : LgNHc = 4.57 --> Ft(NHc)2. = 3.14 : LgM = 6.43Grams present / needed / produced.

*To find gas volume at STP…*

0.071 : 0.142 --> 0.071 : 0.142Present, Working Molar Ratio

X 22.4

FtM_{2 }= 1.6 : LgNHc = 3.2 --> Ft(NHc)2. = 1.6 : LgM = 3.2liters of gas at STP.

*If working with Molarity in a solution…*

*M*is molarity,

*n*is number of moles, and

*V*is volume in liters

M = n/v

M = present, working molar ratio (Step 6) ÷ V