Factor-Label Method

• Math technique used for conversion problems.

• Not the same as the proportion technique.

• More powerful in multistep problems.

Changing Units

• In the factor-label method, you use a conversion factor to express the quantity in the desired units.

• Set up your calculation so the units cancel out.

• If you have 8 dozen eggs, how many eggs do you have?

8 Doz eggs X 12 eggs = 96 eggs1 Doz eggs

That’s the conversion factor.That’s the conversion factor.

That’s the given information.That’s the given information.

That’s the That’s the answer.answer.

Another Example of the Factor-Label Method

• How many quarts are in 3.5 gallons?

• Equality you know: 1 gallon = 4 quarts.

• Format = Given X Conversion Factor = AnsFormat = Given X Conversion Factor = Ans

• Conversion Factor is a ratio built from the equalities. You put it together so the units cancel out. The value of the ratio is “one.”

Gallons to Quarts

• 3.5 Gallons X 4 Quarts = 14 Quarts1 Gallon

Identify the given, the conversion factor, and the answer.

Number of Moles in Sample

• The mass of an iron bar is 27.9 g. How many moles of Fe are in the sample?

• What equalities do you know?– 1 mole of Fe =– 55.8 g Fe =– 6.02 X 102323 atoms Fe

formula mass

6.02 X 1023 22.4 L/mol

Factor-Label Method

• Format = Given X Conversion Factor = Ans

• 27.9 g Fe X 1 Mole Fe = 0.5 Mole Fe

55.8 grams Fe

Formula Mass of a Compound

• Sum of the atomic masses for all the atoms present.

• CO2 has a formula mass of:

• 12.0 amu + 16.0 amu + 16.0 amu = 44.0 amu

Molar Mass of any Compound

• = Formula mass expressed in grams.

• 1 mole of NaCl = 58.5 grams NaCl

• 1 mole of CO2 = 44.0 grams of CO2

• 1 mole of H2O = 18.0 grams of H2O

• 1 mole of CaCO3 = 100. grams of CaCO3

• 1 mole of LiF = 25.9 grams of LiF

Mole Mass Conversions

• Conversion factor comes from an equality you build from the periodic table.

Mole - # of particle Conversions

• 1 equality, so it’s easier.

• 1 mole of anything = 6.02 X 1023 of that thing

Mole - # of Particle Conversions

• How many particles are in 3 moles of He?

Given = 3 moles of He

Equality: 1 mole of He molecules = 6.02 X 1023 molecules of He

Format: Given X Ratio = Ans

3 moles He molecules X 6.02 X 1023 molecules =1 mole of He molecules

18 X 1023 molecules!

# of Particles to Moles

• How many moles are in 3.01 X 1023 particles of Ar?

Given: 3.01 X 1023 particles of Ar

Equality: 1 mole = 6.02 X 1023 particles

3.01 X 1023 particles X 1 mole =6.02 X 1023 particles

0.5 moles!