Stoichiometry – Ch. 9

Stoichiometry

Stoichiometry problems consist of predicting the

amount of grams of product that will form from a

reaction if you know how many grams of

reactant you have, or predicting the number of

grams of reactant that will be needed to produce

a certain amount of product.

Proportional Relationships

I have 5 eggs. How many cookies can I make?

3/4 c. brown sugar

1 tsp vanilla extract

2 eggs

2 c. chocolate chips

Makes 5 dozen cookies.

2 1/4 c. flour

1 tsp. baking soda

1 tsp. salt

1 c. butter

3/4 c. sugar

5 eggs 5 doz.

2 eggs= 12.5 dozen cookies

Ratio of eggs to cookies

Proportional Relationships

Stoichiometry

• mass relationships between substances in a chemical reaction

• based on the mole ratio

Mole Ratio

• indicated by coefficients in a balanced equation

2 Mg + O2 2 MgO

Mole Ratio

• A mole ratio is a ratio between the numbers of moles of any two substances in a balanced equation. Refer to coefficients in the balanced equation to determine the mole ratio.

Stoichiometry Steps

1. Write a balanced equation.

2. Identify known & unknown.

3. Line up conversion factors.

• Mole ratio - moles moles

• Molar mass - moles grams

• Molarity - moles liters soln

• Molar volume - moles liters gas

Core step in all stoichiometry problems!!

• Mole ratio - moles moles

4. Check answer.

C. Conversion Factors

of the Mole

Molar Mass(g/mol)

6.02 1023

particles/mol

MASS

IN

GRAMS

MOLES

NUMBER

OF

PARTICLES

LITERS

OF

SOLUTION

Molar Volume(22.4 L/mol)

LITERS

OF GAS

AT STP

Molarity (mol/L)

Grams-Mole Problems

How many grams of Copper are

needed to produce 5 moles of

copper oxide?

2Cu + O2 2CuO

5 mol CuO

2 mol CuO

2 mol Cu

1 mol Cu

63.55 g Cu

1 mol of a gas=22.4 Lat STP

C. Molar Volume at STP

Standard Temperature & Pressure0°C and 1 atm

Limiting and Excess Reactants

Why do reactions stop?

• Reactions proceed until one of the reactants is used up and one is left in excess.

• The limiting reactant limits the extent of the reaction and, thereby, determines the amount of product formed.

• The excess reactants are all the leftover unused reactants.

Calculating the Product when a Reactant

is Limiting

• S8 + 4Cl2→ 4S2Cl2

• Given the following equation and reactant amounts of 200.0g S8 and 100.0g Cl2

• Determine which is the limiting reactant

• Determine which reactant is in excess and how much excess you have

• Determine how much product would be made

Percent Yield

• The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant.

• ***This comes from a stoichiometry calculation of the limiting reactant

• The actual yield is the amount of product actually produced when the chemical reaction is carried out in an experiment.

• The percent yield of a product is the ratio of the actual yield expressed as a percent.

• This tells you how close you were!!!!