Many students struggle with stoichiometry. The essence of stoichiometry is being able to take known quantities and calculate other quantities, given a chemical equation. As an example, you could be given the following equation and information:
CH4 + 2O2 <-> 2H2O + CO2, CH4 (methane) = 24 grams
Given this information, the question might ask, “How much oxygen, in moles, would be required in order to react with all of the methane completely?” This is a very classic stoichiometry question, and the actual calculations required are very simple, but the logic of what needs to be done is often what students struggle with. With this in mind, I often tell students that “a chemical equation is a recipe, and what’s given are ingredients.”
The Equation is Your Recipe
Think about baking cookies. For the sake of this example, I looked up a simple recipe for sugar cookies (my favourite), but I’m going to write it in a similar form to how we normally see chemical equations:
2 3/4 cups of flour + 1 teaspoon baking soda + 1/2 teaspoon baking powder + 1 cup butter + 1 1/2 cups sugar + 1 egg + 1 teaspoon vanilla = 48 cookies
Now you look in your baking cupboard, and you see that you have a package of 400 grams of flour, and you are wondering how many cookies you can make. How would you figure that out? Well, your recipe has flour measured in cups, so you will first convert your grams of flour to cups. Given that 1 cup is about 236 grams, the number of cups of flour that you have is:
400 grams of flour * 1 cup/236 grams = 1.69 cups
From here, you will want to figure out how many cookies you can make with your 1.69 cups of flour. Well, your recipe tells you that every 2 3/4 cup of flour can make 48 cookies, so:
1.69 cups of flour * 48 cookies/(2 3/4 cups) = 29.58 cookies
This process is fairly intuitive, and something that many of us due on a daily basis. Now let’s apply the same logic to the original question.
Let’s Start Baking!
Let’s go back to the original question asked above, but write it slightly differently:
1 mole of methane + 2 moles of oxygen = 2 moles of water + 1 mole of carbon dioxide
Now you look in your cupboard, and you see that you have 24 grams of methane, and you want to know how much oxygen you will need in order to use all of the methane that you have. How will you figure that out? Well your recipe has methane measured in moles, so you will want to convert your grams to moles. Since 1 mole of methane is equal to 16.05 grams (molar mass of methane), we can calculate the moles of methane:
24 grams of methane * 1 mole/16.05 grams = 1.50 moles of methane
From here, you will want to figure out how many moles of oxygen you will need to use all of your 1.50 moles of methane. Well, your recipe tells you that every mole of methane requires 2 moles of oxygen, so:
1.50 moles of methane * 2 moles of oxygen/1 mole of methane = 3.0 moles of oxygen
This is a fairly simple illustration, but the logic is the same for any stoichiometry question. We are trying to use given quantities to figure out how much of other ingredients we would need, or how much of any product we can make. Often times, the units in the recipe are not the same as the units that are given, but by converting to the unit in the recipe, whether its cups, grams, or moles, we can use the recipe to answer any question about other ingredients or products. So, if you can bake, you can do stoich!