And so it begins! But… this is pretty simple… No, really.
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.
https://www.hersheys.com/kitchens/en_us/recipes/smores.html |
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'moreSo, 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!
2Li + S --> Li2S
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:
4 : 3 -- > 2X 0.0575
_________________
.23 : 0.1725 --> 0.115
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