CALCULATIONS INVOLVING REACTIONS

A.K.A Stoichiometry

Introduction

Chemical reactions provide two important pieces of information.

  1. They tell us the type and number of atoms and molecules that interact and how they arrange.
  2. They tell us the relative number of moles of atoms and molecules that interact and form.

REACTION / 1 H2 / + / 1 Cl2 /  / 2 HCl
The reaction indicates that if we had 1 molecules of H2, then: / 1 molecule of H2 / + / 1 molecules of Cl2 /  / 2 molecules of HCl
These are relative ratios therefore, if we had 10 molecules: / 10 molecules / + / 10 molecules /  / 20 molecules
If we had 1,0000 molecules: / 1,000 molecules / + / 1,000 molecules /  / 2,000 molecules
If we had 1 mole of hydrogen: / 1 mole / + / 1 mole /  / 2 moles

Moles can be converted to masses, volumes or particles, therefore we can use a chemical reaction to calculate how much of a substance will form or react.

Mole Recipes

Balanced equations are like mole recipes. Consider part of a typical recipe for monster cookies.

Ingredients / Butter / Sugar / Oatmeal / Eggs / # of cookies
Amounts Needed / 1/2 cup / 1 cup / 4 cups / 3 / 15 cookies

Questions:

  1. How much butter is needed to make 30 cookies?
  2. If you had 12 cups of oatmeal and enough of the other ingredients,

how many cookies could you make?

  1. How many eggs are needed to make 60 cookies?

Chemical reactions are like the recipes for the cookies. The coefficients in the equations are mole recipes that tell us the # of moles that react and form.

Balanced Equations: / 1 C3H8 / + / 5 O2 /  / 3 CO2 / + / 4 H2O
If we had 2 moles of C3H8 / 2 moles / 10 moles / 6 moles / 8 moles

Notice that all of the values in the table are in the same ratio as the coefficients in the equation!

  • C3H8 : O2 is always 1:5
  • CO2 : H2O is always 3:4
  • We call these ratios, equation factors or mole ratios.
  • The reactants and products in this equation always react and form in these relative ratios.

Moles of  / C3H8 / O2 / CO2 / H2O
Complete the rest of this table using the equation above / 12 moles
2 moles
20 moles
5 moles

EXERCISE A:

1. For the equation:

Zn(s) + 2HCl(aq)  ZnCl2 + H2(g)

a)How many moles of HCl are needed to produce 0.452 moles of Zinc chloride?

b)How many grams of Zn will react with 1.05 moles of HCl?

c)How many molecules of Hydrogen will be formed by the reaction of 2.5 g Zn and HCl?

d)How many moles of HCl are needed to form 6.12 grams of ZnCl2?

2. For the equation:

Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)

a)How many moles of silver are formed when 2.5 moles Cu react with sufficient AgNO3?

b)How many grams of AgNO3 are required to completely react with 2.1 moles of Cu?

c)How many moles of Cu(NO3)2 are produced when 16.2 g Cu react with sufficient AgNO3?

d)How many grams of Ag are formed when 16.2 g Cu react with sufficient AgNO3?

e)How many atoms of Ag are formed when 2.5 moles Cu reacted with sufficient AgNO3?

3. For the equation:

C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l)

a)What mass of CO2(g) is produced by reacting 2.00 mol of O2(g)?

b)If a sample of propane is burned, what mass of H2O (l) is produced if the reaction also produces 50.0 L of CO2(g) at STP?

c)A sample of porous, gas-bearing rock is crushed and 1.25 x 10-6 g of C3H8(g) is extracted from the powdered rock. How many molecules of CO2(g) are produced if the gas sample is burned in the presence of an excess of O2(g)?