Reference: Mole Calculations & Percent Compositions (Page 2 of 2) in pdf

Pre-AP Chemistry

Use empirical formula to find the percent composition.

Percent Composition

1. Use the formula subscript and the mass of one mole to

get the total grams of each element present in the

There are two main types of these problems.

compound.

2. Find the total mass of one mole of the compound.

1. Use the percent composition to find the empirical

3. Divide each individual element mass by the total mole

formula.

mass.

2. Use the empirical formula to find the percent

composition.

Example: Find the percent composition of each element

in carbon dioxide.

à

3. Use a percent composition to find how much of a single

1. CO

element is present.

C=12 grams present

2 × O = 2(16) = 32 grams present.

Most problems that you will see in here are variations of

working in these two directions.

2. CO

= 12 + 2(16) = 44 grams total for one mole

Use Percent Composition to find formula.

3. 12 grams ÷ 44 = 0.273 = 27.3%

32 ÷ 44 = 0.727 = 72.7%

1. Pretend that you have 100 grams of the substance. The

percentages become the mass of each.

Example: Find the percent of oxygen in BaCO

. How

2. Change the gram amounts of each element into moles

much oxygen would be in 22.8 grams of BaCO

3 ?

3. Divide all the moles of each by the smallest of the mole

1. Ba=137.33 grams,

answers to get a relative amount.

C=12.0 grams

3 × O = 3(16) = 48 grams present

4. Sometimes you have to double or triple (or multiple by

some other factor) to get the values to all be whole numbers.

2. CaCO

=40.1+12.0+48 = 197.3 grams for one mole

Example: A compound is 27% carbon and 73% oxygen.

3. 48 grams ÷197.3 = 0.243 = 24.3%

Determine the empirical formula

4. 22.8 grams of compound × 0.243 = 5.55 grams

1. 27 grams of carbon, 73 grams of oxygen.

2. 1 mole

× 27 grams = 2.25 moles of carbon

12.0g

1mole

×73 grams = 4.5625 moles of oxygen

16.0g

3. 2.25 ÷ 2.25 = 1.00

4. 4.5625÷2.25 = 2.03

These two answers round to whole numbers: 1 C to 2 O =

Handout: Mole Calculations

11/20/14

Reference: Mole Calculations & Percent Compositions Page 2

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