Percent Composition by Mass Worksheet

Question 1

What belongs in the denominator of a percent-composition calculation for oxygen in water?

1. The molar mass of oxygen alone
2. The molar mass of water
3. The number of atoms in a liter of water
4. The mass of an empty container

Question 2

Calculate the percent by mass of hydrogen in H₂O. Show how the subscript affects the calculation.

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Question 3

Which expression gives the oxygen mass contribution in one mole of CO₂?

1. 1 × 16.00 g
2. 2 × 16.00 g
3. 3 × 16.00 g
4. 12.01 g

Question 4

Explain why the percentages of all elements in a pure compound sum to about 100 percent.

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Question 5

Which compound has the greatest percentage of carbon by mass?

1. P
2. Q
3. R
4. All have the same percent carbon.

Question 6

Calculate the carbon percentage in sample P. Use the atomic mass of carbon from the periodic table.

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Question 7

Which formula has the lowest carbon percentage in the table?

1. CaCO₃
2. MgCO₃
3. Na₂CO₃
4. All have the same percent carbon.

Question 8

Explain why one carbon atom per formula unit does not imply the same carbon percentage for all three compounds.

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