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Unit 3 Stoichiometry: Calculations with Chemical Formulas and Equations

Unit 3 starts with chemical equations, which are used to write down what happens in a chemical reaction. Chemical equations specifically state the ratios of reactants and products; that is, how much of each is consumed or produced, in terms of actual numbers of atoms or molecules.

Because different atoms, molecules, and compounds have different atomic weights, the coefficients (variables in front of the atoms or molecules; e.g., the first “2” in 2H2O) do not translate directly to the masses that are measured in the laboratory. This is where the concept of the mole comes in—it is used to express the actual number of molecules in a given mass of a pure substance.

Stoichiometry is all about converting masses into moles and moles into masses, usually in the context of chemical equations. It is a fundamental operation that is used daily in the chemical laboratory. It allows us to predict how much of a substance we need to react and how much of a given product we can expect to be formed during a chemical reaction. Stoichiometry can also be used to deduce empirical and molecular formulas.

Objectives

After completing this unit, you should be able to

1. balance chemical equations.
2. write a balanced equation, given the formulas or names of reactants and products.
3. write a balanced equation for a combustion reaction of a compound containing only carbon, hydrogen, and oxygen, given its formula.
4. calculate the formula weight or molecular weight of any substance, given its name or formula.
5. calculate the percentage composition of any substance, given its name or formula.
6. interconvert among number of moles, mass, and number of atoms, ions, molecules or formula units, for any given substance.
7. determine the empirical formula of a compound, given its mass composition or data from combustion analysis.
8. determine the molecular formula of a compound from its empirical formula and its molar mass.
9. calculate the mass or number of moles of a substance used or produced in a chemical reaction.
10. identify limiting and excess reactants in a chemical reaction.
11. calculate the amount of a substance produced in a chemical reaction, when there is a limiting reactant.
12. calculate the percent yield of a product of a chemical reaction, given appropriate experimental data.