Mastering Exercise 9.1a: Understanding Mole Ratios in Stoichiometry

Stoichiometry is an essential concept in chemistry that helps us understand the relationship between molecules involved in a chemical reaction. It involves using balanced chemical equations to determine the amount of reactants required to produce a specific amount of product. One of the fundamental principles of stoichiometry is understanding mole ratios. This article will help you master Exercise 9.1a and understand mole ratios in stoichiometry.

What are mole ratios?

A mole ratio is a ratio that relates the amount in moles of any two substances in a chemical reaction. It is obtained from the balanced chemical equation and helps determine the number of moles of one substance required to react with a specific number of moles of another substance. Mole ratios are essential in stoichiometry since they help us predict the amount of reactant or product that will be produced in a chemical reaction.

How to calculate mole ratios?

Calculating mole ratios involves using the coefficients in a balanced chemical equation. Consider the balanced chemical equation below:

2H₂ + O₂ → 2H₂O

The coefficients tell us the mole ratio of hydrogen to oxygen, and hydrogen to water. In this equation, the mole ratio of hydrogen to oxygen is 2:1, meaning that two moles of hydrogen are required to react with one mole of oxygen. Similarly, the mole ratio of hydrogen to water is 2:2, which simplifies to 1:1. This means that one mole of hydrogen reacts with one mole of water.

Why are mole ratios important?

Mole ratios are important because they help us determine the limiting reactant, excess reactant, and the amount of product that can be produced in a chemical reaction. Suppose you have 4 moles of hydrogen and 2 moles of oxygen. Using the mole ratio, we can determine that we need only 2 moles of oxygen to react with 4 moles of hydrogen. Therefore, oxygen is the limiting reactant, and only 2 moles of water can be produced.

Examples of problems involving mole ratios in stoichiometry

Let’s consider the following problem:

Calcium oxide reacts with water to produce calcium hydroxide. How many grams of calcium hydroxide will be produced when 5.00 g of calcium oxide reacts with 1.00 g of water? The balanced chemical equation is:

CaO + H₂O → Ca(OH)₂

To solve this problem, we need to find the limiting reactant, which is the reactant that will be consumed completely. Calculating the amount of each reactant in moles, we get:

moles CaO = 5.00 g / 56.08 g/mol = 0.089 mol

moles H₂O = 1.00 g / 18.02 g/mol = 0.055 mol

Using the mole ratio from the balanced chemical equation

1 mol CaO : 1 mol H₂O : 1 mol Ca(OH)₂

We find that the actual mole ratio in the reaction is:

0.089 mol CaO : 0.055 mol H₂O : x mol Ca(OH)₂

Since the mole ratio of CaO to H₂O is 1:1, it means that H₂O is the limiting reactant. Therefore, the amount of Ca(OH)2 produced is determined by the amount of H₂O reacted.

Using the mole ratio from the balanced chemical equation

1 mol H₂O : 1 mol Ca(OH)₂

We find the amount of Ca(OH)₂ produced is:

0.055 mol H₂O x (1 mol Ca(OH)₂/1 mol H₂O) x (74.09 g/mol Ca(OH)₂) = 4.08 g Ca(OH)₂

Conclusion

Understanding the concept of mole ratios is crucial in mastering stoichiometry. They help us calculate the amount of reactant or product required in a chemical reaction. By following the steps outlined in this article, you can achieve a better understanding of mole ratios and their importance in stoichiometry problems.