Atomic Mass Examples in Chemistry

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

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

Concept Recap

The weighted average mass of all naturally occurring isotopes of an element, expressed in atomic mass units (amu), where each isotope's mass is weighted by its relative natural abundance.

The number under each element on the periodic table—a weighted average of all its isotopes.

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: Atomic Mass 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 atomic mass 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

Define atomic mass and explain why the atomic mass of carbon is listed as

12.01\,\text{amu}

rather than exactly

12\,\text{amu}

on the periodic table.

Answer

\text{Atomic mass of C} = 12.01\,\text{amu (weighted average of isotopes)}

First step

Atomic mass is the weighted average of the masses of all naturally occurring isotopes of an element, measured in atomic mass units (amu).

Full solution

2
Carbon has two stable isotopes: ${}^{12}\text{C}$ (mass 12, abundance 98.9%) and ${}^{13}\text{C}$ (mass 13, abundance 1.1%).
3
Weighted average: $12 \times 0.989 + 13 \times 0.011 = 11.87 + 0.14 = 12.01\,\text{amu}$ .

The atomic mass on the periodic table reflects the natural isotope distribution. Since

{}^{12}\text{C}

is by far the most abundant isotope, the average is very close to 12 but slightly above due to the small contribution of

{}^{13}\text{C}

Example 2

medium

Silicon has three stable isotopes:

{}^{28}\text{Si}

(92.23%, mass

27.977

{}^{29}\text{Si}

(4.67%, mass

28.976

), and

{}^{30}\text{Si}

(3.10%, mass

29.974\,\text{amu}

). Calculate the atomic mass of silicon.

Example 3

medium

An element has two isotopes:

40\,\text{amu}

80\%

abundance and

42\,\text{amu}

20\%

. Find the atomic mass.

Example 4

medium

Element X has two isotopes:

63\,\text{amu}

(

x\%

) and

65\,\text{amu}

(

1-x\%

). The atomic mass is

63.55\,\text{amu}

. Find each percent abundance.

Example 5

hard

An element has three isotopes:

20\,\text{amu}

(

90.48\%

21\,\text{amu}

(

0.27\%

22\,\text{amu}

(

9.25\%

). Find atomic mass to two decimals.

Example 6

hard

An element has isotopes

185\,\text{amu}

(

37.40\%

) and

187\,\text{amu}

(

62.60\%

). Find atomic mass.

Practice Problems

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

Example 1

medium

Chlorine has two stable isotopes:

{}^{35}\text{Cl}

(mass

34.97\,\text{amu}

, 75.77%) and

{}^{37}\text{Cl}

(mass

36.97\,\text{amu}

, 24.23%). Calculate its atomic mass.

Example 2

hard

An element has two isotopes. Isotope A has mass

10.01\,\text{amu}

and isotope B has mass

11.01\,\text{amu}

. The atomic mass listed on the periodic table is

10.81\,\text{amu}

. Calculate the percent abundance of each isotope.

Example 3

medium

An element has isotopes

39\text{ amu}

(

93\%

) and

41\text{ amu}

(

7\%

). Find its atomic mass to two decimals.

Example 4

easy

An element has two isotopes:

50\%

10\text{ amu}

and

50\%

12\text{ amu}

. What is its atomic mass?

Example 5

easy

Is atomic mass a whole number or typically a decimal? Why for chlorine (

35.5\text{ amu}

Example 6

easy

What is the difference between atomic mass and mass number?

Example 7

easy

Element X has isotopes of

20\text{ amu}

(

75\%

) and

22\text{ amu}

(

25\%

). Set up the atomic-mass expression.

Example 8

easy

The atomic mass of carbon is

12.01\text{ amu}

. Which isotope dominates its abundance?

Example 9

easy

Atomic number counts protons; what does atomic mass primarily reflect?

Example 10

easy

An element has one isotope only, at

19\text{ amu}

. What is its atomic mass?

Example 11

easy

Why is the atomic mass used (not mass number) when computing molar mass for a real sample?

Example 12

medium

Element X has isotopes

24\text{ amu}

(

60\%

) and

26\text{ amu}

(

40\%

). Find its atomic mass.

Example 13

medium

Chlorine has

Cl

35

(

75\%

) and

Cl

37

(

25\%

). Compute the atomic mass.

Example 14

medium

An element has isotopes

63\text{ amu}

(

69\%

) and

65\text{ amu}

(

31\%

). Find the atomic mass to two decimals.

Example 15

medium

An element has isotopes

6\text{ amu}

and

7\text{ amu}

, with atomic mass

6.94\text{ amu}

. Estimate the abundance of the

7\text{ amu}

isotope.

Example 16

medium

An element has isotopes

10\text{ amu}

(

20\%

) and

11\text{ amu}

(

80\%

). Find the atomic mass.

Example 17

medium

Two isotopes have masses

84\text{ amu}

and

86\text{ amu}

in a

1:3

ratio. Find the atomic mass.

Example 18

medium

If a sample is

90\%

of a

20\text{ amu}

isotope and

10\%

of a

22\text{ amu}

isotope, what is the atomic mass?

Example 19

medium

An element's atomic mass is

24.3\text{ amu}

with isotopes

24

25

, and

26\text{ amu}

. If

24

79\%

and

25

10\%

, what is the

26

abundance, and verify the mass.

Example 20

challenge

An element has isotopes

69\text{ amu}

and

71\text{ amu}

and an atomic mass of

69.7\text{ amu}

. Find each isotope's percent abundance.

Example 21

challenge

A sample of element Z is enriched: isotope

235\text{ amu}

raised to

4\%

and isotope

238\text{ amu}

96\%

. Find the average atomic mass to two decimals.

Example 22

challenge

An element has isotopes

28

29

30\text{ amu}

92\%

5\%

3\%

. Compute the atomic mass to two decimals.

Example 23

easy

Is atomic mass the same for every atom of an element?

Example 24

easy

True or false: a single

^{12}\text{C}

atom has a mass of exactly 12 amu by definition.

Example 25

easy

An element has a single isotope of mass 40 amu. What is the atomic mass?

Example 26

easy

What is the approximate atomic mass of hydrogen?

Example 27

easy

1 mole of an element has a mass in grams equal to what number?

Example 28

easy

Bromine's atomic mass is

79.9\,\text{amu}

. About what is the molar mass in g/mol?

Example 29

medium

An element has isotopes

54\,\text{amu}

(

5.85\%

56\,\text{amu}

(

91.75\%

57\,\text{amu}

(

2.12\%

58\,\text{amu}

(

0.28\%

). Find the atomic mass to two decimals.

Example 30

medium

How many grams are in

2.50\,\text{mol}

of an element whose atomic mass is

32.07\,\text{amu}

Example 31

medium

An element has isotopes

122\,\text{amu}

(

57\%

) and

124\,\text{amu}

(

43\%

). Find the atomic mass.

Example 32

medium

An element with atomic mass close to

24.31\,\text{amu}

has isotopes

24

25

26\,\text{amu}

. Which is the most abundant?

Example 33

medium

Magnesium's atomic mass is

24.31\,\text{amu}

. Estimate the mass of

0.500\,\text{mol}

of magnesium.

Example 34

medium

An element has isotopes

107\,\text{amu}

(

51.84\%

) and

109\,\text{amu}

(

48.16\%

). Find atomic mass.

Example 35

hard

How many moles are in

50.0\,\text{g}

of an element with atomic mass

200.0\,\text{amu}

Example 36

hard

If iron's atomic mass is

55.85\,\text{amu}

, how many iron atoms are in

11.17\,\text{g}

of iron?

Example 37

hard

An element has two isotopes

79\,\text{amu}

and

81\,\text{amu}

, atomic mass

79.90\,\text{amu}

. Find each abundance.

Example 38

hard

A sample is

100\%

enriched to a single isotope of mass

235\,\text{amu}

. What is the atomic mass of this enriched sample?

Example 39

hard

What mass in grams contains

3.011\times 10^{23}

atoms of an element with atomic mass

40.08\,\text{amu}

Example 40

challenge

Atomic mass is

35.45\,\text{amu}

for chlorine with isotopes

35

and

37\,\text{amu}

. Suppose a sample is enriched to

50\%

of each. What is the new sample's atomic mass?

Example 41

challenge

A diatomic element has atomic mass

19.00\,\text{amu}

. What is the molar mass of the

\text{X}_2

molecule?

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Related Concepts

Isotope Molar Mass

Background Knowledge

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

isotope