CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 10. The Mole

Practice Problems

1. A sample of a given compound contains 13.18 g of carbon and 3.32 g of hydrogen. What is the percent
   composition of this compound?


2. 5.00 g of aluminum is reacted with 7.00 g of fluorine to form a compound. When the compound is isolated,
   its mass is found to be 10.31 g, with 1.69 g of aluminum left unreacted. Determine the percent composition
   of the compound.

Percent Composition from a Chemical Formula

The percent composition of a compound can also be determined from its chemical formula. The subscripts in the
formula are first used to calculate the mass of each element found in one mole of the compound. That value is then
divided by the molar mass of the compound and multiplied by 100%.

% by mass=mass of element in 1 molmolar mass of compound×100%

The percent composition of a given compound is always the same as long as the compound is pure.

Sample Problem 10.12: Percent Composition from a Chemical Formula

Dichlorine heptoxide (Cl 2 O 7 ) is a highly reactive compound used in some synthesis reactions. Calculate the percent
composition of dichlorine heptoxide.

Step 1: List the known quantities and plan the problem.

Known

• mass of Cl in 1 mol Cl 2 O 7 = 70.90 g


• mass of O in 1 mol Cl 2 O 7 = 112.00 g


• molar mass of Cl 2 O 7 = 182.90 g/mol

Unknown

• percent Cl =? %


• percent O =? %

Calculate the percent by mass of each element by dividing the mass of that element in 1 mole of the compound by
the molar mass of the compound and multiplying by 100%.

Step 2: Calculate.

% Cl=

70 .90 g Cl

182 .90 g

×100%= 38 .76% Cl

% O=

112 .00 g O

182 .90 g

×100%= 61 .24% O

Step 3: Think about your result.

The percentages add up to 100%.