Percent Composition Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Percent Composition.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept 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.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Percent Composition starts with the given amount, names the substance, and chooses the conversion factor that cancels the old unit.

Common stuck point: Students often know a formula related to percent composition but skip the recognition step: Am I using a mole bridge, molar mass, formula ratio, or balanced-equation ratio to connect measured amounts? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I using a mole bridge, molar mass, formula ratio, or balanced-equation ratio to connect measured amounts?

Worked Examples

Example 1

easy

Calculate the percent composition by mass of each element in

\text{H}_2\text{O}

Answer

\text{H: } 11.19\%,\quad \text{O: } 88.81\%

First step

Molar mass of

\text{H}_2\text{O} = 2(1.008) + 16.00 = 18.02\,\text{g/mol}

Full solution

2
%H $= \frac{2(1.008)}{18.02} \times 100\% = \frac{2.016}{18.02} \times 100\% = 11.19\%$ .
3
%O $= \frac{16.00}{18.02} \times 100\% = 88.81\%$ .

Percent composition tells us the mass fraction of each element in a compound. The percentages must add up to 100% (within rounding error).

Example 2

medium

Calculate the percent by mass of nitrogen in ammonium nitrate (

\text{NH}_4\text{NO}_3

Example 3

medium

Find the percent by mass of N in

\text{N}_2\text{O}_5

. (N = 14, O = 16)

Example 4

medium

A 25 g sample of

\text{CaCO}_3

(40% Ca by mass) contains how many grams of calcium?

Example 5

hard

Find the percent by mass of water in

\text{MgSO}_4 \cdot 7\text{H}_2\text{O}

. (Mg=24, S=32, O=16, H=1)

Example 6

challenge

Which fertilizer delivers more nitrogen per 100 g: ammonium nitrate

\text{NH}_4\text{NO}_3

(35.0% N) or urea

\text{CO(NH}_2)_2

(46.7% N)?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

What is the percent composition of carbon in

\text{CO}_2

? (C = 12.01, O = 16.00)

Example 2

easy

What percent by mass of calcium carbonate (

\text{CaCO}_3

) is oxygen? (Ca =

40.08

, C =

12.01

, O =

16.00

)

Example 3

easy

In H2O (molar mass 18 g/mol), what mass of hydrogen is present per mole (H=1)?

Example 4

easy

Find the percent by mass of H in H2O (molar mass 18, H total 2 g).

Example 5

easy

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

Example 6

easy

What is the molar mass of CO2 (C=12, O=16)?

Example 7

easy

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

Example 8

easy

In NaCl (Na=23, Cl=35.5), what is the molar mass?

Example 9

easy

Find the percent by mass of Na in NaCl (Na=23, molar mass 58.5).

Example 10

easy

Do the percent compositions of all elements in a compound sum to about 100%?

Example 11

medium

Find the percent by mass of H in CH4 (C=12, H=1).

Example 12

medium

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

Example 13

medium

Find the percent by mass of N in NH3 (N=14, H=1).

Example 14

medium

Find the percent by mass of water in CuSO4 . 5H2O hydrate (CuSO4=160, H2O=18).

Example 15

medium

Find the percent by mass of S in H2SO4 (H=1, S=32, O=16).

Example 16

medium

A 50 g sample of CaCO3 (molar mass 100) is 40% Ca by mass. How many grams of calcium does it contain?

Example 17

medium

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

Example 18

medium

Find the percent by mass of C in CH4 (C=12, H=1).

Example 19

medium

Find the percent by mass of Cl in NaCl (Cl=35.5, molar mass 58.5).

Example 20

challenge

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

Example 21

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 22

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 23

easy

What is the molar mass of

\text{MgO}

? (Mg = 24, O = 16)

Example 24

easy

Find the percent by mass of Mg in

\text{MgO}

(Mg = 24, molar mass = 40).

Example 25

easy

What is the molar mass of

\text{KCl}

? (K = 39, Cl = 35.5)

Example 26

easy

Find the percent by mass of K in

\text{KCl}

(K = 39, molar mass = 74.5).

Example 27

easy

\text{H}_2\text{S}

, what is the total mass contribution of hydrogen per mole? (H = 1)

Example 28

medium

Find the percent by mass of O in

\text{N}_2\text{O}_5

. (N = 14, O = 16)

Example 29

medium

Find the percent by mass of C in glucose

\text{C}_6\text{H}_{12}\text{O}_6

. (C=12, H=1, O=16)

Example 30

medium

Find the percent by mass of O in glucose

\text{C}_6\text{H}_{12}\text{O}_6

. (Use molar mass 180.)

Example 31

medium

Find the percent by mass of H in glucose

\text{C}_6\text{H}_{12}\text{O}_6

. (molar mass 180)

Example 32

medium

\text{Al}_2\text{O}_3

, find the percent by mass of Al. (Al = 27, O = 16)

Example 33

medium

Find the percent by mass of Cu in

\text{CuO}

. (Cu = 63.5, O = 16)

Example 34

medium

Find the percent by mass of S in

\text{SO}_3

. (S = 32, O = 16)

Example 35

medium

Find the percent by mass of P in

\text{Ca}_3(\text{PO}_4)_2

. (Ca = 40, P = 31, O = 16)

Example 36

medium

Find the percent by mass of N in urea

\text{CO(NH}_2)_2

. (C=12, O=16, N=14, H=1)

Example 37

hard

A 75 g sample of

\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?

Example 38

hard

Which has a higher percent of oxygen by mass:

\text{CO}_2

\text{SO}_2

? (C=12, S=32, O=16)

Example 39

hard

Find the percent by mass of N in ammonium sulfate

(\text{NH}_4)_2\text{SO}_4

. (N=14, H=1, S=32, O=16)

Example 40

hard

A compound is 40.0% C, 6.7% H, and 53.3% O by mass. Verify this is consistent with the empirical formula

\text{CH}_2\text{O}

. (C=12, H=1, O=16)

Example 41

hard

How many grams of nitrogen are in 200 g of

\text{NH}_4\text{NO}_3

, which is 35.0% N by mass?

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Related Concepts

Molar Mass Empirical Formula

Background Knowledge

These ideas may be useful before you work through the harder examples.

molar mass