Introduction to Stoichiometry

Warm-up: 1. You just got hired at Burger King. It’s the end of the night and you are running low on some ingredients. The last customer pulls up to the drive-thru window and orders 5 cheese burgers. You are worried that you might not have enough cheese. To determine how much cheese you will need you quickly multiply the number of burgers by the amount of cheese you need for each burger.

Using the following recipe, complete the following calculations.

1 patty + 2 cheese + 2 tomato + 1 bun + 3 ketchup packets + 5 pickles + 1 lettuce 1 Cheeseburger

1. How many ketchup packets will you need to make 12 cheeseburger?2. How many slices of cheese would you need to make 8 cheeseburger?3. For each slice of lettuce, how many tomatoes do I need?4. What are the reactants?

1 patty + 2 cheese + 2 tomato + 1 bun + 3 ketchup packets + 5 pickles + 1 lettuce 1 Cheeseburger

1. How many ketchup packets will you need to make 12 cheeseburger?

2. How many slices of cheese would you need to make 8 cheeseburger?

3. For each slice of lettuce, how many tomatoes do I need?

4. What are the reactants?

Although you might not realize it, you make these kind of predictions every day (your warm-up for example).

What do these types of predictions all have in common?

A Ratio!

Each ratio consists of types of things you need in the equation/recipe and their coefficients (how many of each you need). The “wanted” item is always in the numerator, and the “given” term is always the denominator.

Numerator: # on top Denominator: # on bottom

Examples: 2 Cheese 5 pickles 1 Cheeseburger 1 Cheeseburger

Stoichiometry: making predictions about the amounts of reactants and products involved in a chemical reaction

Practice:1. If a recipe for 1 cake calls for 2 cups of flour, ½ cup of sugar, 1 cup of milk, and 3 eggs, how many eggs does it take to make 50 cakes.

2. A peanut butter and jelly sandwich consists of 2 slices of bread, 3 spoons of peanut butter, and 4 spoons of jelly. Calculate the number of spoons of peanut butter needed to make a half a sandwich.

• The ratios we used with the cheeseburger analogy has the same form as the Mole Ratio that we use in chemistry. We get the mole ratio from a balanced chemical equation. So you can say a balanced chemical equation is a mole recipe.

A mole ratio is a mathematical expression that shows the relative amounts of two items involved in

a chemical change. Each mole ratio:

- Comes from a balanced chemical equation- Shows the number of moles of each substance - Show the mole coefficient for the “wanted”

substance in the numerator and the mole coefficient for the “given” species in the denominator.

What is a Mole Ratio?

Write the mole ratio needed to predict the amount of hydrogen that will react with 6.5mol of oxygen gas to produce water.

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

M.R. 2 : 1 → 2

Example

Write the mole ratio needed to solve the problems below. Don’t worry about solving the problem just yet. Just tell me the mole ratio.

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

1.How many moles of oxygen are needed to react with 4.8 mol of hydrogen?

2.How many moles of water can be produced from the reaction of 8.0 mol of hydrogen?

3.How many moles of hydrogen are needed to produce 10 mol of water?

Practice

Write all the possible mole ratios for the following balanced chemical equation.

Practice 2

N2(g) + 3H2(g) 2NH3(g)

The steps to perform a mole-mole calculation:

1. Write a balanced chemical equation for the rxn

2. Identify the “given” and “wanted” substances and list relevant information under each substance in the equation.

3. Multiply the given amount by the mole ratio

Mole to Mole Stoichiometry

Calculate the amount of oxygen gas required to burn 287 mol hydrogen.

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

Example

1. Carbon disulfide is a colorless, foul-smelling, flammable, poisonous liquid. It is a widely used solvent for rubber in manufacturing processes. It can be prepared by a direct reaction of carbon with sulfur.

4 C(s) + S8(s) 4 CS2(l)

Calculate the number of moles of carbon needed to react with 346 mol of sulfur to produce carbon disulfide.

Practice:

2. Hydrogen gas can be produced by reacting zinc metal with hydrochloric acid.

Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq)

Calculate the moles of hydrogen gas that can be produced from the reactions of 45.8 mol of hydrochloric acid.