Factor-Label Method

1
Factor-Label Method

• Math technique used for conversion problems.
• Not the same as the proportion technique.
• More powerful in multistep problems.

2
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.

3

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

? Thats the given information.
8 Doz eggs X 12 eggs 96 eggs
1 Doz eggs
? Thats the answer.
Thats the conversion factor.?
4
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 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.

5
Gallons to Quarts

• 3.5 Gallons X 4 Quarts 14 Quarts

1 Gallon
Identify the given, the conversion factor, and
the answer.
6
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 1023 atoms Fe

7
formula mass
22.4 L/mol
6.02 X 1023
8
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
9
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

10
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

11
Mole ?? Mass Conversions

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

12
Mole - of particle Conversions

• 1 equality, so its easier.
• 1 mole of anything 6.02 X 1023 of that thing

13
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
14
18 X 1023 molecules!
15
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
16
0.5 moles!