Ratios

The key to Stoichiometry is setting up ratios correctly and being able to cross cancel information.

Known x Unknown = Unknown

Known* You cross cancel the two things that are known on this problem and

do the math. You would end up with the unknown (answer).

Mole to Mass

Problem: You have 5 mole Na. How many grams is that?

Molar mass of Na= 22.99 g/mol Na

5 mole Na x 22.99 g Na = 114.95 grams 1 mol of Na

Step by Step Explanation

Mole to Mass Explanation

1. You found the molar mass of the element.

2. You set up your ratio.

3. You found the grams in 5 moles of Na.

Molar mass of Na= 22.99 g/mol Na

5 mole Na x 22.99 g Na = 114.95 grams

1 mol of Na

Try a harder one!

Mass to Mole

Problem: You have 5 grams of Cl. How many moles of Cl is this?

Molar mass of Cl= 35.543 grams/per mole

5 g Cl x 1 mol Cl = .0141 moles Cl

35.453 g Cl

Step by Step Explanation

Mass to Mole Explanation

Molar mass Cl=

35.453 g/mol Cl

5 g Cl x 1 mol Cl = .0141 moles Cl

35.453 g Cl

1. You found the molar mass of the element

2. You set up the ratio

3. You cross cancel the information

4. You found the moles in 5 grams of Cl.

Try a harder one!

Gas to Mole

*Fact: 1 mole of ANY gas is 22.4 liters (1 mole= 22.4 liters)

Problem: You have 5 liters of H2O. How many moles of oxygen is this?

5 liters H2O x 1 mol H2O = .2222 moles O

22.44 liters H2O

Step by Step Explanation

Gas to Mole Explanation

• You know the conversion for liters to moles

• You set up the ratio (What do you know? What is unknown?)

• You cross cancel the information

• You find the mole of Oxygen in 5 liters of H2O

22.44 liters= 1 mole

5 liters H2O x 1 mol H2O = .2222 moles H2O

22.44 liters H2O

Try a harder one!

Particle to Particle

*Remember* A mole of something is 6.02x1023 of something (like a dozen is 12 of something)

Problem: You have 2.5x1020 particles of H2O. How much water vapor is that?

2H2 + O2 H2O

2.5x1020 particles O2 x 1 mole O2 x 2 mole H2O x 6.02x1023

6.02x1023 particles O2 1 mole O2 1 mol H2O

Step by Step Explanation 1.505x 1044 particles O2

Particle to Particle Explanation

1. Determine what you know and what’s unknown.

2. Set up the ratio.

3. Cross cancel information.

2.5x1020 particles O2 x 1 mole O2 x 2 mole H2O x 6.02x1023

6.02x1023 particles O2 1 mole O2 1 mol H2O

Try a harder one!