Avogadro's Law Examples: 45 Problems with Answers

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Avogadro's Law.

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

Concept Recap

Avogadro's law states that equal volumes of gases at the same temperature and pressure contain equal numbers of particles.

More gas particles need more space if temperature and pressure stay the same.

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: Avogadro's Law starts by listing pressure, volume, temperature, amount, and which are held constant.

Common stuck point: Students often know a formula related to avogadro's law but skip the recognition step: Am I comparing gas variables with units and temperature in kelvin, while holding the stated variables constant? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I comparing gas variables with units and temperature in kelvin, while holding the stated variables constant?

Worked Examples

Example 1

medium

A tire holds

0.80\text{ mol}

of air in

24\text{ L}

. After pumping more air in at constant

T,P

the volume becomes

30\text{ L}

. How many moles of air are now inside?

Answer

n_{2} = 1.00 mol

First step

\frac{V_1}{n_1} = \frac{V_2}{n_2}

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Example 2

medium

Two flasks at the same

T,P

: flask A contains

1.0\text{ L}

He (

M=4

), flask B contains

1.0\text{ L}

CO_2

(

M=44

). Compare (a) moles and (b) mass.

Example 3

medium

At STP,

11.0\text{ g}

CO_2

(

M=44

) occupies what volume?

Example 4

hard

N_2 + 3H_2 \rightarrow 2NH_3

(all gases, same

T,P

), what total change in volume occurs when

10\text{ L}

N_2

reacts completely with stoichiometric

H_2

Example 5

hard

At STP, what is the density of

O_2

(

M=32

Example 6

hard

Two gas samples at the same

T,P

: sample 1 is

5\text{ L}

holding

x

moles; sample 2 is

12\text{ L}

holding

(x+0.35)

moles. Find

x

Practice Problems

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

Example 1

medium

At fixed T and P,

0.40\text{ mol}

of gas fills

9.6\text{ L}

. What volume holds

0.60\text{ mol}

at the same conditions?

Example 2

easy

Two flasks at the same temperature and pressure each hold

1\text{ L}

of gas: one neon, one argon. Which holds more particles?

Example 3

easy

A gas sample of

1\text{ mol}

occupies

5\text{ L}

. What volume does

2\text{ mol}

occupy at the same T and P?

Example 4

easy

Does Avogadro's law describe volume versus number of particles, or volume versus pressure?

Example 5

easy

3\text{ mol}

of gas fills

9\text{ L}

, what is the volume per mole?

Example 6

easy

At the same T and P,

0.5\text{ mol}

of gas occupies

4\text{ L}

. What volume does

1\text{ mol}

occupy?

Example 7

easy

Two balloons at identical T and P contain equal numbers of molecules. How do their volumes compare?

Example 8

easy

Is the relationship between gas volume and moles direct or inverse (at constant T and P)?

Example 9

easy

A gas at

2\text{ mol}

occupies

44.8\text{ L}

at STP. What is the molar volume?

Example 10

medium

At constant T and P a gas expands from

6\text{ L}

15\text{ L}

when more gas is added. If it started with

0.4\text{ mol}

, how many moles are now present?

Example 11

medium

How many liters does

0.25\text{ mol}

of any ideal gas occupy at STP?

Example 12

medium

In the reaction

N_2 + 3H_2 \rightarrow 2NH_3

, how many liters of

H_2

react with

4\text{ L}

N_2

at the same T and P?

Example 13

medium

3.2\text{ L}

gas sample at fixed T and P contains

0.20\text{ mol}

. How many moles are in

8.0\text{ L}

at the same conditions?

Example 14

medium

2H_2 + O_2 \rightarrow 2H_2O

, what volume of

O_2

is needed to react with

10\text{ L}

H_2

(gases at same T and P)?

Example 15

medium

A gas occupies

11.2\text{ L}

at STP. How many moles is this?

Example 16

medium

Equal volumes of

CO_2

and

H_2

are compared at the same T and P. Which sample has more mass, and which has more molecules?

Example 17

medium

0.75\text{ mol}

of gas fills a

9\text{ L}

container at fixed T and P, what volume holds

0.25\text{ mol}

at the same conditions?

Example 18

challenge

2CO + O_2 \rightarrow 2CO_2

30\text{ L}

CO

reacts completely with the stoichiometric

O_2

at the same T and P. What total volume of gas (CO + O2 consumed vs CO2 formed) change occurs?

Example 19

challenge

A mixture at fixed T and P contains

0.30\text{ mol}

He in

7.2\text{ L}

. Some He leaks out until the volume is

4.8\text{ L}

. How many moles leaked?

Example 20

challenge

At the same T and P, gas X occupies

V

liters for

n

moles. If both the volume is tripled and

0.6\text{ mol}

is removed, the new amount is

1.5\text{ mol}

. Find the original

n

Example 21

easy

At fixed

T

and

P

0.20\text{ mol}

of gas fills

4.8\text{ L}

. What volume holds

0.50\text{ mol}

at the same conditions?

Example 22

easy

At STP, what volume does

1.0\text{ mol}

of any ideal gas occupy?

Example 23

easy

2.0\text{ L}

flask of

H_2

and a

2.0\text{ L}

flask of

O_2

are at the same

T

and

P

. Which flask has more molecules?

Example 24

easy

0.10\text{ mol}

of He occupies

2.24\text{ L}

at some

T

and

P

. What volume does

0.30\text{ mol}

He occupy at the same conditions?

Example 25

easy

At STP, how many moles of gas occupy

5.6\text{ L}

Example 26

medium

In the reaction

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O(g)

at the same

T

and

P

, what volume of

O_2

reacts with

5.0\text{ L}

CH_4

Example 27

medium

At STP, what is the volume of

3.0\text{ mol}

N_2

Example 28

medium

6.4\text{ g}

O_2

(

M=32

) at STP occupies what volume?

Example 29

medium

0.15\text{ mol}

of gas occupies

3.6\text{ L}

. At the same

T,P

, how many moles fit in

9.6\text{ L}

Example 30

medium

2SO_2 + O_2 \rightarrow 2SO_3

(all gases) at the same

T,P

, what volume of

SO_3

forms from

8\text{ L}

SO_2

(excess

O_2

Example 31

medium

A balloon holds

0.50\text{ mol}

of air at

12\text{ L}

. If

0.30\text{ mol}

more air is added at constant

T,P

, what is the new volume?

Example 32

medium

A gas sample at fixed

T,P

holds

0.60\text{ mol}

15\text{ L}

. How many moles must be removed to shrink the sample to

10\text{ L}

Example 33

medium

Two equal-volume containers at the same

T,P

hold

N_2

and

Ne

. Which has more atoms?

Example 34

hard

At STP, how many molecules are in

11.2\text{ L}

of gas? (Use

N_A = 6.022 \times 10^{23}

Example 35

hard

A mixture at fixed

T,P

has

0.40\text{ mol}

He and

0.10\text{ mol}

Ar in

20\text{ L}

total. If all the He is removed, what new volume do the remaining moles occupy at the same

T,P

Example 36

hard

2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O(g)

, all gases at the same

T,P

. What is the net volume change per liter of

C_2H_6

burned?

Example 37

hard

0.50\text{ mol}

of a gas fills

11.2\text{ L}

. Are these STP conditions?

Example 38

hard

A balloon at

T,P

holds

0.80\text{ mol}

gas in

19.2\text{ L}

. After leaking, it holds

14.4\text{ L}

at the same

T,P

. What fraction of moles escaped?

Example 39

challenge

10\text{ L}

vessel at fixed

T,P

contains

0.30\text{ mol}

N_2

and

0.20\text{ mol}

H_2

. Enough additional

H_2

is added to make the mixture stoichiometric for

N_2 + 3H_2 \rightarrow 2NH_3

. What is the new volume at the same

T,P

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Related Concepts

Gas Laws Mole

Background Knowledge

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

gas lawsmole