Thursday, October 15, 2020

Mole to Mole Stoichiometry

General Chemistry Index

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.

Mole to Mole Stoichiometry
And so it begins! But… this is pretty simple… No, really.

The Law of Conservation of Matter is one of the two Laws of Conservation. One has to do with energy. The other has to do with matter.  Ignoring the fact that matter can become energy and all that… relativity stuff and E=mC2

The Law of Conservation of Matter: matter can neither be created nor destroyed.

So what?

Chemistry… Conservation of Matter… moles… grams… There must be connections!

There are. A couple…

In a chemical reaction, everything that is on the reactant side of the equation moves through to the product side. That's easy enough.

Now, about those compounds… Remember this? Compounds are formed in specific, fixed ratios.

This is when that kicks in and becomes useful!

The balanced chemical reaction can be thought of as a recipe for compounds. It tells the ratio of the parts (reactants) that go together to form the product.
The concept is easily understood with the analogy of s'mores. 

According to the Hershey's official website, a s'more is formed by combining a specific, fixed ratio of graham crackers, marshmallows, and chocolate. 
That ratio is:


2 Crackers + 1 Marshmallow + 6 Chocolate Segments --> 1 S'more

So, if you want to have 4 s'mores you need 8 crackers. If you want 10 S'mores, you need 10 Marshmallows.

The ratio for crackers, marshmallows, chocolate and s'mores is

2:1:6 --> 1 

Pretty much anyone can figure out what is needed to make 8 s'mores. I'm not even going to type that out!

The same thing is true for chemical compounds. The coefficients tell the ratios of the reactants and products.

2Li + S --> Li2S

Thus, the ratio is (confused only because we don't write coefficients of 1) is 

2:1 --> 1

If we know how many (or what fraction of a) mole/s of either reactant we have, we can easily find the other amounts (in moles).

The Math: Ratios

I'm going to propose this language usage: The coefficients of a balanced chemical reaction are the basic ratio of reactants and products.

So, if you have a basic ratio of

2:1 --> 2

then knowing how many moles of any part will allow you to convert to the other parts. How?

Consider the reaction of water…

2H2 + O2 --> 2H2O

The basic ratio is 

2 : 1 --> 2

So, if we have 4 moles of hydrogen, we have 2 times as much hydrogen as we need for the basic ratio. Therefore, we can multiply ALL of the numbers by 2 to find out what we need and how much we will make.

2 : 1 --> 2  

          X 2


 4 : 2 --> 4

If we begin with 4 moles of hydrogen, we need 2 moles of oxygen and will make 4 moles of water. Bam!

Okay, let's talk about that red number! Divide how many moles you have of the thing by how much is called for in the basic ratio.

Let's just make up a ratio…

4 3 -- >  2

Let's say we have 2 moles of the first thing (we needed 4).

2 ÷ 4 = 0.5

So, we have half as much of the orange thing as we need. We multiply ALL of the ratio by 0.5 to get the moles that need to be actually present (or that will be produced):

4 3 -- >  2

         X 0.5


2 1.5 --> 1


It is easy to "see" how this works with nice numbers, but it will work with any ugly decimal as well.

Going with 4 3 -- >  2 again…

Suppose we have 0.23 moles of the thing we needed 4 moles of…

0.23 ÷ 4 = 0.0575

This means we have only a small fraction of what we need (the orange thing) for the basic ratio. But… we just multiply that number through the whole ratio:

 3 -- >  2

                  X 0.0575


.23 : 0.1725 --> 0.115

No comments:

Post a Comment