Practice Atomic Mass in Chemistry

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

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Quick 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.

Showing a random 20 of 50 problems.

Example 1

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 2

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 3

medium

How many grams are in

2.50\,\text{mol}

of an element whose atomic mass is

32.07\,\text{amu}

Example 4

easy

The atomic mass of carbon is

12.01\text{ amu}

. Which isotope dominates its abundance?

Example 5

easy

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

Example 6

easy

True or false: a single

^{12}\text{C}

atom has a mass of exactly 12 amu by definition.

Example 7

medium

An element has isotopes

39\text{ amu}

(

93\%

) and

41\text{ amu}

(

7\%

). Find its atomic mass to two decimals.

Example 8

hard

An element has two isotopes

79\,\text{amu}

and

81\,\text{amu}

, atomic mass

79.90\,\text{amu}

. Find each abundance.

Example 9

easy

Bromine's atomic mass is

79.9\,\text{amu}

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

Example 10

easy

What is the approximate atomic mass of hydrogen?

Example 11

easy

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

Example 12

easy

An element has one isotope only, at

19\text{ amu}

. What is its atomic mass?

Example 13

challenge

An element has isotopes

28

29

30\text{ amu}

92\%

5\%

3\%

. Compute the atomic mass to two decimals.

Example 14

easy

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

35.5\text{ amu}

Example 15

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 16

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 17

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.

Example 18

medium

Magnesium's atomic mass is

24.31\,\text{amu}

. Estimate the mass of

0.500\,\text{mol}

of magnesium.

Example 19

easy

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

Example 20

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.

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Related Concepts

Isotope Molar Mass