Unit G

Mathematics of Chemical Reactions

(Gravimetric Stoichiometry)

G.1 Introduction: Mathematics of Chemical Reactions

This unit studies the use of mathematics in balanced equations to determine the amount of reactant needed or predict the amount of product that will be formed when the reaction is run.

Gravimetric: involves the use of a balance Gravi-: using gravity (mass) -metric: to measure

Stoichiometry (based on two Greek words) Stoicheion: “of the elements” -metry: to measure

G.1 Introduction: Mathematics of Chemical Reactions

Stoichiometry is a review: Must understand the formulas of elements

and ions (Unit B) Must be able to write chemical names and

formulas (Unit D) Must be able to write and balance chemical

equations (Unit E) Must be able to convert mass to moles and

moles to mass (Unit F).

G.1 Introduction: Mathematics of Chemical Reactions

Practical Importance of Stoichiometry1. The gasoline-to-air mixture in a car.

2. The proper proportions of ingredients in recipes.

3. Antacid tablets to neutralize stomach acid may be harmful in excess.

G.2 Mole-to-Mole Stoichiometry Reading Balanced Equations in terms

of Moles

G.2 Mole-to-Mole Stoichiometry

1. Write the balanced chemical equation.2. List given amount under appropriate

substance. Put a (?) with the necessary units under the substance you are looking for.

3. Multiply the given amount by the mole ratio:

coefficient of desired substance? mol desired = # mol given

coefficient of given substance

4. Round final answer using appropriate rules.

Generalized Solving Method

G.2 Mole-to-Mole Stoichiometry

2 H2(g) + O2(g) 2 H2O(g)

? mol 4.13 mol

2

2H 2 2

2

2 mol Hn = 4.13 mol O = 8.26 mol H

1 mol O

Example #1: How many moles of hydrogen are required to react exactly with 4.13 mol of oxygen?

G.2 Mole-to-Mole StoichiometryExample #2: Determine the number of moles of natural gas (methane) that must undergo combustion to produce 1.62 mol of water vapor.

4

4CH 2 4

2

1 mol CHn = 1.62 mol H O = 0.810 mol CH

2 mol H O

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)

? mol 1.62 mol