  # Gas Laws – The combined gas law

The physical behaviour of a gas is described using four main variables :

1. Temperature
2. Pressure
3. Volume
4. Amount (mol number)

## The combined gas law

By combining Boyle’s, Charles’s and Gay Lussac’s Laws, the following equation is derived:

This is the combined gas law that give the relation between the pressure, the temperature and the volume that are all changing. The mol number of the gas is constant.

## Practice problem – The combined gas law

The volume of a gas-filled balloon is V1=30.0 L at T1 = 313 K and P1= 153 kPa pressure.

What would the volume be at standard temperature and pressure (STP)?

T2 = 273 K   P2 = 1atm = 101.325 KPa

STP represents standards temperature and pressure : The standard temperature is 273 K (0° Celsius), and the standard pressure is 1 atm pressure. P= 1 atm = 101.325 kPa and T=  273 K

## Solution

1- Analyse : List the knowns and the unknown.

V1 = 30.0 L T1 = 313 K P1 = 153 kPa T2 = 273 K (standard temperature)

P2 = 101.325 kPa (standard pressure) V2 = ??

2- Calculate : Solve for the unknown.

State the combined gas law. And rearrange the equation to isolate V2.

Substitute the known quantities into the equation and solve.