4.5 Stoichiometry & Calculations

In the previous section, we looked at qualitatively analyzing chemical reactions. Now, we’ll learn how to quantitatively analyze reactions using stoichiometry! It may seem like a lot of math at first, but once you do more practice, you’ll be more confident and become a stoichiometry master 👨‍🏫

In earlier units, you learned about moles, molar mass/molar volume, molarity, and Avogadro's number. Stoichiometry is all about using mole ratios and these measurements and manipulating them to get to our desired unit. For this reason, it’s important to comfortably know the relationship between these ideas as we do mole ratios. Below is a list of these measurements: 1 mole = molar volume (gas) (At STP, this is 22.4 L/mol. That number is also on your AP equation sheet)

1 mole = 6.022 x 10^23 particles (This is Avogrado’s number, which is on your AP equation sheet!)

1 mole = molar mass (Each element’s is on the periodic table. Hydrogen’s is 1.008 g/mol) You can find mole ratios by looking at a chemical reaction🧪. The coefficients of the reaction represent the ratio of molecules necessary in order for the reaction to complete. In the reaction below, 1 mole of C2H5OH (ethanol) and 1 mole of oxygen gas are needed to produce 2 moles of carbon dioxide and 3 moles of water. A sample mole ratio would be 1 mole of O2 to 2 moles of CO2. Keep in mind that 2 moles of CO2 to 1 mole of O2 would mean the same thing.

Write the chemical reaction, if it’s not given. Be sure to balance it. 2K (s) + 2H2O (l) --> 2KOH (aq) + H2 (g)

Identify the known measurement. In this case, we’re given 11.6 moles of water💧.

Since our given value is already in moles, we just need to write the appropriate mole ratio. We want a ratio that cancels❌ out the known measurement units (moles of H2O) and brings in the unit we want (moles of K).

Looking at the chemical equation, for every 2 moles of H2O, we need 2 moles of K. 

💡Note: Writing out the units and the molecule the value belongs to will help you exponentially. Please do it!

One way to make sure you’re writing the right mole track of the units is to cross out whatever cancels out and circle the unit you’re left with, like so:

Write the chemical equation and balance it.

C2H5OH (aq) + 3 O2 (g) --> 2 CO2 (g) + 3 H2O (g)

Identify the known measurement: 105.2 g of ethanol.

Since our given value is in grams, we need to convert it to moles. To do this, we will calculate the molar mass (g/mol) of C2H5OH. 

2(12.01g) + 6(1.008g) + 16.00g = 46.07 g/mol

Using this value, we can make a mole ratio.

105.2 g of C2H5OH * 1 mole C2H5OH/46.06 g of C2H5OH

Uh oh!😩 We don’t want moles of C2H5OH--we want the volume of carbon dioxide. Now that we have moles, though, we can go over to CO2 with a mole ratio. Looking at the equation, for every 1 mole of C2H5OH, we can make up to 2 moles of CO2.

Almost there🎉! We have a unit of CO2, but we want it’s volume. Since this reaction occurs at STP, we can use the value of the molar volume at STP.

Good work! With 105.2g of ethanol, we could make 102. L of CO2😮.

You will see stoichiometry used with the ideal gas law (PV=nRT) and molarity (moles/mass or moles/volume, often denoted as M). Check out the practice problems to see how you’ll see it in problems.

Here is a neat list of the general steps for tackling stoichiometry problems:

The following reaction occurs at STP:

How many particles of BrF will be produced with 160.0g of Br2?

Step 1 includes using the molar mass of Br2. Step 2 is the mole ratio of Br2 to BrF. Step 3 uses Avogadro's number to convert to the number of particles of BrF.

🎥 Watch: AP Chemistry - Stoichiometry (Part 2)