Avogadro's Law Formula

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

The Formula

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

When to use: More gas particles need more space if temperature and pressure stay the same.

Quick Example

Doubling the number of moles of gas in a balloon roughly doubles the volume if temperature and pressure are unchanged.

Notation

V

is volume and

n

is amount in moles. At constant temperature and pressure,

V_1/n_1 = V_2/n_2

— volume is directly proportional to moles.

What This Formula Means

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.

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\text{ 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?

Common Mistakes

Changing temperature or pressure and still using Avogadro's law by itself - Fix this by naming the substances or sample, checking "Am I comparing gas variables with units and temperature in kelvin, while holding the stated variables constant?", and attaching units, formulas, states, or evidence to the final statement. - Fix this by naming the substances or sample, checking "Am I comparing gas variables with units and temperature in kelvin, while holding the stated variables constant?", and attaching units, formulas, states, or evidence to the final statement.
Confusing Avogadro's law with Avogadro's number - Fix this by naming the substances or sample, checking "Am I comparing gas variables with units and temperature in kelvin, while holding the stated variables constant?", and attaching units, formulas, states, or evidence to the final statement. - Fix this by naming the substances or sample, checking "Am I comparing gas variables with units and temperature in kelvin, while holding the stated variables constant?", and attaching units, formulas, states, or evidence to the final statement.
Forgetting that the relationship is direct, not inverse - Fix this by naming the substances or sample, checking "Am I comparing gas variables with units and temperature in kelvin, while holding the stated variables constant?", and attaching units, formulas, states, or evidence to the final statement. - Fix this by naming the substances or sample, checking "Am I comparing gas variables with units and temperature in kelvin, while holding the stated variables constant?", and attaching units, formulas, states, or evidence to the final statement.
Using avogadro's law from a keyword alone - Signal words like gas, pressure, volume only point to a possible model; the substances and evidence must match too. - Fix this by naming the substances or sample, checking "Am I comparing gas variables with units and temperature in kelvin, while holding the stated variables constant?", and attaching units, formulas, states, or evidence to the final statement.

Why This Formula Matters

Avogadro's Law helps students reason about gases as particle systems rather than loose formulas. It connects lab measurements to molecular motion and conditions.

Frequently Asked Questions

What is the Avogadro's Law formula?

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

How do you use the Avogadro's Law formula?

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

What do the symbols mean in the Avogadro's Law formula?

$V$ is volume and $n$ is amount in moles. At constant temperature and pressure, $V_1/n_1 = V_2/n_2$ — volume is directly proportional to moles.

Why is the Avogadro's Law formula important in Chemistry?

Avogadro's Law helps students reason about gases as particle systems rather than loose formulas. It connects lab measurements to molecular motion and conditions.

What do students get wrong about Avogadro's Law?

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.

What should I learn before the Avogadro's Law formula?

Before studying the Avogadro's Law formula, you should understand: gas laws, mole.

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Related Concepts

Gas Laws Mole

Related Formulas

Gas Laws Formula Mole Formula