The student will:
- explain the meaning of the term “stoichiometry.”
- interpret chemical equations in terms of molecules, formula units, and moles.
- formula unit
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
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.
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:
N2O3+ H2O→2 HNO3
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 CuSO4+4 KI→2 CuI+4 K2SO4+I2
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.
2 C2H6+7 O2→4 CO2+6 H2O
KBrO3+6 KI+6 HBr→7 KBr+3 I2+3 H2O
- Two molecules of C2H6 plus seven molecules of O2 yields four molecules of CO2 plus six molecules of H2O.
- Two moles of C2H6 plus seven moles of O2 yields four moles of CO2 plus six moles of H2O.
- One formula unit of KBrO3 plus six formula units of KI plus six molecules of HBr yields seven formula units of KBr plus three molecules of I2 and three molecules of H2O.
- One mole of KBrO3 plus six moles of KI plus six moles of HBr yields seven moles of KBr plus three moles of I2 and three moles of H2O.
- 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.
- Distinguish between formula unit, molecule, and mole. Give examples in your answer.