Molecular Formula Examples: 47 Problems with Answers

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

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 chemical formula showing the actual number of atoms of each element in one molecule of a compound, as opposed to the empirical formula, which shows only their simplest whole-number ratio.

The real count of atoms, not just the ratio — it tells you exactly what the molecule contains.

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: Molecular Formula 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 molecular formula 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

Explain the difference between an empirical formula and a molecular formula. Give an example where they differ.

Answer

{Empirical: CH}_{2} {O. Molecular: C}_{6} H_{12} O_{6} (×6 multiple)

First step

The empirical formula shows the simplest whole-number ratio of atoms (e.g.,

\text{CH}_2\text{O}

Full solution

2
The molecular formula shows the actual number of atoms in one molecule (e.g., $\text{C}_6\text{H}_{12}\text{O}_6$ for glucose).
3
The molecular formula is always a whole-number multiple of the empirical formula: $\text{C}_6\text{H}_{12}\text{O}_6 = (\text{CH}_2\text{O})_6$ .

Some compounds have the same empirical and molecular formula (e.g., water is both

\text{H}_2\text{O}

). Others differ — acetic acid (

\text{C}_2\text{H}_4\text{O}_2

) and glucose (

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

) share the same empirical formula

\text{CH}_2\text{O}

but are completely different compounds.

Example 2

medium

A compound has an empirical formula of

\text{NO}_2

and a molar mass of

92\,\text{g/mol}

. Determine the molecular formula. (N =

14.01

, O =

16.00

)

Example 3

medium

A compound is 40% C, 6.7% H, 53.3% O by mass. Its molar mass is 180 g/mol. Find the molecular formula.

Example 4

medium

A hydrocarbon contains

82.7\%

C and

17.3\%

H by mass, with molar mass

58\,\text{g/mol}

. Find the molecular formula.

Example 5

hard

Combustion of

1.50\,\text{g}

of a hydrocarbon yields

4.62\,\text{g}

\text{CO}_2

and

2.52\,\text{g}

\text{H}_2\text{O}

. Molar mass is

30\,\text{g/mol}

. Find the molecular formula.

Example 6

hard

A compound is

32.0\%

6.67\%

18.7\%

42.6\%

O with molar mass

75\,\text{g/mol}

. Find the molecular formula. (C=12, H=1, N=14, O=16)

Practice Problems

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

Example 1

medium

A compound has empirical formula

\text{CH}

and molar mass

78\,\text{g/mol}

. Find the molecular formula. (C =

12.01

, H =

1.008

)

Example 2

hard

A compound is

85.7\%

C and

14.3\%

H by mass, with a molar mass of

easy

Empirical formula

\text{CH}_3

(mass

15

) has molar mass

30\,\text{g/mol}

. Find

n

and the molecular formula.

Example 24

easy

Empirical formula

\text{CH}_2

has empirical mass

14\,\text{g/mol}

. If

n = 4

, give the molecular formula.

Example 25

easy

Empirical formula

\text{P}_2\text{O}_5

(mass

142

) has molar mass

284

. Find the molecular formula.

Example 26

easy

Empirical formula HO has empirical mass

17

. Molar mass is

34\,\text{g/mol}

. Find the molecular formula.

Example 27

easy

Empirical formula

\text{NO}_2

(mass

46

), molar mass

138\,\text{g/mol}

. Find the molecular formula.

Example 28

medium

A compound is

40.0\%

S and

60.0\%

O by mass with molar mass

80\,\text{g/mol}

. Find the molecular formula. (S=32, O=16)

Example 29

medium

A compound is

43.7\%

P and

56.3\%

O with molar mass

284\,\text{g/mol}

. Find the molecular formula. (P=31, O=16)

Example 30

medium

Empirical formula

\text{CHO}_2

(mass

45

), molar mass

90\,\text{g/mol}

. Find the molecular formula.

Example 31

medium

Empirical formula

\text{NH}_2

(mass

16

), molar mass

32\,\text{g/mol}

. Find the molecular formula.

Example 32

medium

A compound has molecular formula

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

. What is its empirical formula?

Example 33

medium

\text{KCl}

a molecular formula?

Example 34

medium

A compound contains

52.2\%

13.0\%

34.8\%

O. Its molar mass is

46\,\text{g/mol}

. Find the molecular formula. (C=12, H=1, O=16)

Example 35

medium

Empirical formula

\text{CH}_2

(mass

14

), molar mass

84\,\text{g/mol}

. Find the molecular formula.

Example 36

hard

A compound is

24.3\%

4.1\%

71.6\%

Cl with molar mass

99\,\text{g/mol}

. Find the molecular formula. (C=12, H=1, Cl=35.5)

Example 37

hard

A compound contains

54.5\%

9.10\%

36.4\%

O with molar mass

88\,\text{g/mol}

. Find the molecular formula. (C=12, H=1, O=16)

Example 38

hard

Vitamin C is

40.9\%

4.58\%

54.5\%

O with molar mass

176\,\text{g/mol}

. Find the molecular formula. (C=12, H=1, O=16)

Example 39

hard

A compound has empirical formula

\text{C}_3\text{H}_4\text{O}_3

and molar mass

264\,\text{g/mol}

. Find the molecular formula.

Example 40

challenge

A gaseous compound has density

1.964\,\text{g/L}

at STP and contains

80.0\%

C and

20.0\%

H. Find the molecular formula. (At STP, molar volume

= 22.4\,\text{L/mol}

, C=12, H=1)

Example 41

challenge

Caffeine is

49.5\%

5.15\%

28.9\%

16.5\%

O with molar mass

194\,\text{g/mol}

. Find the molecular formula. (C=12, H=1, N=14, O=16)

← Back to Molecular Formula Practice Mode

Related Concepts

Empirical Formula Molar Mass Lewis Structure

Background Knowledge

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

empirical formulamolar mass