Solving Stoichiometry Problems

Core Concepts

In this tutorial, you will learn what is stoichiometry and the different types of problems involving it. You will go through several examples to practice and master the content!

Topics Covered in Other Articles

Stoichiometry Definition

What is Stoichiometry?

Stoichiometry is math having to do with chemical reactions. There are different types of calculations you can perform; stoichiometry with moles is the most common, but you can also do math with masses and even percentages. Read about the origins of stoichiometry here! Learn what is mole in chemistry.

Stoichiometric Ratio

A stoichiometric ratio comes into play when talking about the relationships of elements or molecules in specific problems. This is the exact ratio between the coefficients of the reactants and products needed for a reaction to proceed normally. Let’s work through some problems you may see when learning about stoichiometry.

Stoichiometry Problems

Balancing Chemical Equations

A very common type of stoichiometric problem is balancing equations. This is an important chemistry skill to have because you have to have the correct ratio of reactants and products in order for a reaction to proceed; this is also an important foundation for organic chemistry. Although we have a tutorial on balancing equations, let’s look at one example.

Balance the following reaction:

$\begin{gather*} {\_\_ C_{2}H_{2} + \_\_ O_{2} \rightarrow \_\_ CO_{2} + \_\_ H_{2}O} \end{gather*}$

The main idea when balancing equations is that there should be the same number of each element on both sides of the reaction. You can balance the carbons and the hydrogens first, then move onto the oxygen. The balanced equation should look like this:

$\begin{gather*} {2C_{2}H_{2} + 5O_{2} \rightarrow 4CO_{2} + 2H_{2}O} \end{gather*}$

Example – Using Stoichiometric Ratio (Moles)

Use the equation below to solve the problem.

$\begin{gather*} {C_{6}H_{12}O_{6} \rightarrow 2C_{2}H_{5}OH + 2CO_{2}} \end{gather*}$

If $2.5 \text{mol}$ of C₆H₁₂O₆ reacts, how many moles of carbon dioxide are produced?

Using dimensional analysis, you can use the stoichiometric ratio to find the solution to this problem. By looking at the coefficients, you can see that for every $1\text{mol}$ of C₆H₁₂O₆, $2 \text{mol}$ of CO₂ are produced. Using this ratio, you can figure out how many moles of carbon dioxide are made from $2.5 \text{mol}$ of glucose.

$\begin{gather*} {2.5\text{mol}C_{6}H_{12}O_{6} \cdot \frac{2\text{mol}CO_{2}}{1\text{mol}C_{6}H_{12}O_{6}} = 5\text{mol}CO_{2}} \end{gather*}$

Example – Using Stoichiometric Ratio (Mass)

Let’s use the same equation as the problem above.

$\begin{gather*} {C_{6}H_{12}O_{6} \rightarrow 2C_{2}H_{5}OH + 2CO_{2}} \end{gather*}$

How many grams of C₆H₁₂O₆ was initially used if $10\text{g}$ of C₂H₅OH was produced?

Similar to the previous problem, by using the stoichiometric ratio of reactant to product, you can find the answer. Dimensional analysis is used to go from grams of C₂H₅OH to molar mass to mole (stoichiometric) ratio and back to grams.

$\begin{gather*} {10\text{g}C_{2}H_{5}OH \cdot \frac{1\text{mol}C_{2}H_{5}OH}{46.068\text{g}C_{2}H_{5}OH} \cdot \frac{1\text{mol}C_{6}H_{12}O_{6}}{2\text{mol}C_{2}H_{5}OH} \cdot \frac{180.156\text{g}C_{6}H_{12}O_{6}}{1\text{mol}C_{6}H_{12}O_{6}} = 19.55\text{g}C_{6}H_{12}O_{6}} \end{gather*}$

Stoichiometry Practice Problems

Problem 1

You mix $2.00\text{mol}$ of CaCl₂ into a solution with excess AgNO₃, hence CaCl₂ is your limiting reactant. Assuming a 100% yield, how much AgCl would you expect to produce, in grams? (AgCl has a molecular weight of $143.32\text{g/mol}$ ).

CaCl₂ and AgNO₃ react according to the following equation:

$\begin{gather*} {CaCl_{2}(aq) + 2AgNO_{3}(aq) \rightarrow Ca\left(NO_{3}\right)_{2}(aq) + 2AgCl(s)} \end{gather*}$

Problem 2

You burn an unknown amount of hexane (C₆H₁₄) and produce $60\text{mol}$ of gas. How much hexane did you burn, in grams? (Hexane has a mass of $86.162\text{g/mol}$ ).