Percent Composition

1
Percent Composition

• Percent part / whole

Example MgO Find molar mass of whole compound
MgO 24.3 16.00 40.3 grams Mg (by
mass) mass of Mg 24.3 g x
100 60.3
molar mass of MgO 40.3 g O (by
mass) mass of O 16.0 g x
100 39.7
molar mass of MgO 40.3 g
2
Empirical Formulas

• Gives the lowest whole-number ratio of elements
  and compounds in a formula

• 4 Steps
• Change the percent to grams
• Convert grams to moles
• Divide each of the moles by the smallest number
  to find the ratio
• Round ratio to a whole number. If the ratio is
  not a whole number (ex. 1.5) multiply each
  element by 2 to get a whole number

3
Examples

• A compound contains 94.1 Oxygen and 5.9
  Hydrogen. What is its empirical formula?

• 94.1 94.1g O
• 5.9 5.9g H

5.9g H
1 mol H
1 mol O

• 94.1g O

5.9 mol H
5.88 mol O
16 g O
1 g H
5.9 5.88

• 5.88
• 5.88

1.003 (can round to 1.0)
1.0
4) Ratio 11 Formula OH
4
Examples

• A compound contains 67.6 Mercury and 10.8
  Sulfur and 21.6 Oxygen. What is its empirical
  formula?

• 67.6 67.6g Hg
• 10.8 10.8g S
• 21.6 21.6g O

67.6g Hg 0.336 mol Hg 10.8g S 0.338 mol
S 21.6g O 1.35 mol O

• 0.336
• 0.336

0.338 0.336
1.35 0.336
1.00 Hg
1.00 S
4.02 O
4) Ratio 114 Formula HgSO4
5
Examples

• What is the empirical formula for a compound
  containing 70.0 Fe and 30.0 O?

• 70.0 70.0g Fe
• 30.0 30.0g O

70.0g Fe 1.25 mol Fe 30.0g O 1.875 mol O
1.875 1.25

• 1.25
• 1.25

1.5 O
1.00 Fe
4) Ratio 1 1.5 5) Mult ratio by 2 2 3
Formula Fe2O3
6
Molecular Formulas

• Same as empirical formula, or a simple
  whole-number multiple of it

• Steps
• Calculate the mass in grams of the empirical
  formula provided
• Divide the molar mass by the mass of the
  empirical formula
• Multiply this whole number ratio by the empirical
  formula

7
Examples

• Calculate the molecular formula of the compound
  whose molar mass is 60.0g and empirical formula
  is CH4N.

1) CH4N 12 4(1) 14 30.0 g

• 60.0g
• 30.0g

2.0
3) 2.0 (CH4N)
C2H8N2
8
Examples

• What is the molecular formula of ethylene glycol
  (CH3O), used in antifreeze. It has a molar mass
  of 62 g/mol.

1) CH3O 31.0 g
3) 2.0 (CH3O)
C2H6O2

• 62.0g
• 31.0g

2.0
Find the molecular formula of C3H2Cl, which is
mothballs. Its molar mass is 147 g/mol.
1) C3H2Cl 73.0 g
3) 2.0 (C3H2Cl)
C6H4Cl2

• 147.0g
• 73.0g

2.0