5 Steps to a 5 AP Chemistry 2019

Gases ❮ 107

KEY IDEA

Volume–Pressure Relationship: Boyle’s Law

Boyle’s law describes the relationship between the volume and the pressure of a gas when the

temperature and amount are constant. If you have a container like the one shown in Figure 8.3

and you decrease the volume of the container, the pressure of the gas increases because the

number of collisions of gas particles with the container’s inside walls increases.

Mathematically this is an inverse relationship, so the product of the pressure and

volume is a constant: PV = kb.

If you take a gas at an initial volume (V 1 ) and pressure (P 1 ) (amount and temperature

constant) and change the volume (V 2 ) and pressure (P 2 ), you can relate the two sets of

conditions to each other by the equation:

P 1 V 1 = P 2 V 2

In this mathematical statement of Boyle’s law, if you know any three quantities, you

can calculate the fourth.

Volume–Temperature Relationship: Charles’s Law

Charles’s law describes the volume and temperature relationship of a gas when the pressure

and amount are constant. If a sample of gas is heated, the volume must increase for the

pressure to remain constant. This is shown in Figure 8.4.

Remember: In any gas law calculation, you must express the temperature in kelvin.

There is a direct relationship between the Kelvin temperature and the volume: as one

increases, the other also increases. Mathematically, Charles’s law can be represented as:

V/T = kc

where kc is a constant and the temperature is expressed in kelvin.

Again, if there is a change from one set of volume–temperature conditions to another,

Charles’s law can be expressed as:

V 1 /T 1 = V 2 /T 2

Figure 8.3 Volume–pressure relationship for gases. As the volume decreases, the

number of collisions increases.

KEY IDEA

KEY IDEA