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14.1: Introduction to Stoichiometry

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Lesson Objectives

The student will:

  • explain the meaning of the term “stoichiometry.”
  • interpret chemical equations in terms of molecules, formula units, and moles.

Vocabulary

  • formula unit
  • stoichiometry

Introduction

You have learned that chemical equations provide us with information about the types of particles that react to form products. Chemical equations also provide us with the relative number of particles and moles that react to form products. In this chapter, you will explore the quantitative relationships that exist between the reactants and products in a balanced equation. This is known as stoichiometry.

Stoichiometry involved calculating the quantities of reactants or products in a chemical reaction using the relationships found in the balanced chemical equation. The word stoichiometry actually comes from two Greek words: stoikheion, which means element, and metron, which means measure.

Molecules and Formula Units

Before proceeding, recall the differences between molecules and formula units. A formula unit is used specifically with ionic compounds and refers to the smallest unit in the ionic compound. For example, the formula for potassium chloride, \text{KCl}. One formula unit of potassium chloride contains one ion of potassium and one chloride ion. Recall from the chapter “Ionic Bonds and Formulas” that ionic compounds are made up of positive and negative ions held together by electrostatic attraction. When we write \text{KCl}, we are only writing the ratio of \text{K}^+ ions to \text{Cl}^- ions involved in a crystal of potassium chloride, not an actual unit that exists by itself.

\text{KCl} \rightarrow \text{K}^+ + \text{Cl}^-

Another example of an ionic compound is copper(II) chloride, \text{CuCl}_2. A formula unit of copper(II) chloride is composed of one ion of copper and two chloride ions. Again, one unit of \text{CuCl}_2 does not exist by itself. Instead, \text{CuCl}_2 is a crystalline structure that has a ratio of one copper ion to two chloride ions.

\text{CuCl}_2 \rightarrow \text{Cu}^{2+} + 2 \text{Cl}^-

In other words, ionic compounds are not composed of molecules. Therefore, we use the term formula unit to represent one unit of an ionic compound.

Covalent compounds are discrete molecules where the atoms join together by sharing electrons. For example, one molecule of carbon tetrachloride, \text{CCl}_4, contains one carbon and four chlorine atoms, not ions, joined together by sharing of electrons. In the figure below, a space-filling model for \text{CCl}_4 is shown on the left, and the Lewis structure is shown on the right.

We can call this a molecule since there is a singular unit consisting of one carbon atom and four chlorine atoms that can act independently from other units or molecules of \text{CCl}_4. Water, \text{H}_2\text{O}, is another example of a covalently bonded compound that exists as discrete molecules.

Interpreting Chemical Equations

Recall from the chapter “The Mole Concept” that a mole is a quantitative measure equivalent to Avogadro’s number of particles. How does the mole relate to the chemical equation? Consider the following reaction:

\text{N}_2\text{O}_3 + \ \text{H}_2\text{O} \rightarrow 2 \ \text{HNO}_3

We have learned that the coefficients in a chemical equation tell us the relative amounts of each substance involved in the reaction. One way to describe the ratios involved in the reaction above would be, “One molecule of dinitrogen trioxide, \text{N}_2\text{O}_3, plus one molecule of water yields two molecules of nitrous acid, \text{HNO}_3.” However, because these are only ratios, this statement would be equally valid using units other than molecules. As a result, we could also say, “One mole of dinitrogen trioxide plus one mole of water yields two moles of nitrous acid.”

We can use moles instead of molecules, because a mole is simply an amount equal to Avogadro’s number, just like a dozen is an amount equal to 12. It is important to not use units that describe properties other than amount. For example, it would not be correct to say that one gram of dinitrogen trioxide plus one gram of water yields two grams of nitrous acid.

Now consider this reaction:

2 \ \text{CuSO}_4 + 4 \ \text{KI} \rightarrow 2 \ \text{CuI} + 4 \ \text{K}_2\text{SO}_4 + \text{I}_2

Here, we can say, “Two moles of copper(II) sulfate react with four moles of potassium iodide, yielding two moles of copper(I) iodide, four moles of potassium sulfate, and one mole of molecular iodine.” Although we can refer to molecules of iodine, \text{I}_2, it is generally not correct to refer to molecules of something like KI. Because KI is an ionic substance that exists as crystal lattices instead of discrete molecules, formula unit is used instead.

Example:

Indicate the ratio of compounds involved in the following balanced chemical equations. Describe the ratios in two ways: a) using the number of formula units or molecules and b) using the number of moles present.

  1. 2 \ \text{C}_2\text{H}_6 + 7 \ \text{O}_2 \rightarrow 4 \ \text{CO}_2 + 6 \ \text{H}_2\text{O}
  2. \text{KBrO}_3 + 6 \ \text{KI} + 6 \ \text{HBr} \rightarrow 7 \ \text{KBr} + 3 \ \text{I}_2 + 3 \ \text{H}_2\text{O}

Solution:

  1. Two molecules of \text{C}_2\text{H}_6 plus seven molecules of \text{O}_2 yields four molecules of \text{CO}_2 plus six molecules of \text{H}_2\text{O}.
  2. Two moles of \text{C}_2\text{H}_6 plus seven moles of \text{O}_2 yields four moles of \text{CO}_2 plus six moles of \text{H}_2\text{O}.
  1. One formula unit of \text{KBrO}_3 plus six formula units of \text{KI} plus six molecules of \text{HBr} yields seven formula units of \text{KBr} plus three molecules of \text{I}_2 and three molecules of \text{H}_2\text{O}.
  2. One mole of \text{KBrO}_3 plus six moles of \text{KI} plus six moles of \text{HBr} yields seven moles of \text{KBr} plus three moles of \text{I}_2 and three moles of \text{H}_2\text{O}.

Lesson Summary

  • Stoichiometry is the calculation of the quantities of reactants or products in a chemical reaction using the relationships found in the balanced chemical equation.

Further Reading / Supplemental Links

This website contains various resources, including PowerPoint lectures, on many topics in chemistry, including one on stoichiometry.

Review Questions

  1. Distinguish between formula unit, molecule, and mole. Give examples in your answer.

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