Practice Percent Composition in Chemistry

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

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Quick Recap

Percent composition is the percentage by mass of each element in a chemical compound.

What fraction of the compound's total weight is made up by each element inside it.

Showing a random 20 of 50 problems.

Example 1

challenge
A 200 g sample of an unknown contains 32 g of oxygen. If oxygen's percent composition is 16%, verify the sample mass is consistent.

Example 2

medium
Calculate the percent by mass of nitrogen in ammonium nitrate (NH4NO3\text{NH}_4\text{NO}_3).

Example 3

medium
Find the percent by mass of Fe in Fe2O3 (Fe=56, O=16).

Example 4

easy
Calculate the percent composition by mass of each element in H2O\text{H}_2\text{O}.

Example 5

easy
Find the percent by mass of C in CO2 (C=12, molar mass 44).

Example 6

hard
How many grams of nitrogen are in 200 g of NH4NO3\text{NH}_4\text{NO}_3, which is 35.0% N by mass?

Example 7

medium
Find the percent by mass of P in Ca3(PO4)2\text{Ca}_3(\text{PO}_4)_2. (Ca = 40, P = 31, O = 16)

Example 8

easy
Find the percent by mass of O in H2O (O = 16, molar mass 18).

Example 9

challenge
A fertilizer is 35% K2O by mass, and K2O is 83% K (K=39, O=16). What percent of the fertilizer is potassium?

Example 10

medium
Find the percent by mass of S in SO3\text{SO}_3. (S = 32, O = 16)

Example 11

challenge
Which fertilizer delivers more nitrogen per 100 g: ammonium nitrate NH4NO3\text{NH}_4\text{NO}_3 (35.0% N) or urea CO(NH2)2\text{CO(NH}_2)_2 (46.7% N)?

Example 12

challenge
Which has a higher percent of nitrogen by mass: NH3 (N=14,H=1) or N2O (N=14,O=16)?

Example 13

medium
Find the percent by mass of O in N2O5\text{N}_2\text{O}_5. (N = 14, O = 16)

Example 14

medium
Find the percent by mass of O in CaCO3 (Ca=40, C=12, O=16).

Example 15

easy
Find the percent by mass of Mg in MgO\text{MgO} (Mg = 24, molar mass = 40).

Example 16

easy
What is the molar mass of MgO\text{MgO}? (Mg = 24, O = 16)

Example 17

medium
A 25 g sample of CaCO3\text{CaCO}_3 (40% Ca by mass) contains how many grams of calcium?

Example 18

medium
Find the percent by mass of N in N2O5\text{N}_2\text{O}_5. (N = 14, O = 16)

Example 19

easy
In H2S\text{H}_2\text{S}, what is the total mass contribution of hydrogen per mole? (H = 1)

Example 20

hard
A 75 g sample of Fe2O3\text{Fe}_2\text{O}_3 is 70% Fe by mass. How many grams of Fe and how many grams of O does it contain?
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