Exambusters Study Cards 11 Gases

State Normal atmospheric pressure in five standard units

1 atm

760 torr

101.3 kilopascals

14.7 lbs/square inch

definition: Pressure



An instrument, invented by Torricielli in 1643, which measures atmospheric pressure. An evacuated glass tube in which a column of mercury rises or falls with changing pressure.
An instrument used to measure pressure of gases in a closed container.
State three postulates of the Kinetic-Molecular Theory of Gases

1. Matter is composed of very small particles; a gas is mostly empty space.

2. Particles are in constant random motion.

3. Collisions are elastic.

State the formula for Graham’s Law of Diffusion.

In a mixture of gases A, B:


(RA)(MWA)1/2 = (RB)(MWB)1/2


MW = Molecular Weight

R = Rate of diffusion

Do heavy or light gases diffuse faster?
Light gases diffuse faster.
Charles’ Law

V1/T1 = V2/T2


Volume and temperature have a direct relationship when pressure and moles of gas remain constant.


V = volume

T = Kelvin

Solve this problem:

At 300 K, a gas has a volume of 10 liters.

Find the volume at 400 K

Use Charles’ Law: V1/T1 = V2/T2

V2 = 13.3 L

Boyle’s Law

(P1)(V1) = (P2)(V2)

Pressure and volume have an inverse relationship when temperature and moles of gas remain constant.


P = Pressure

V = Volume

A sample of gas has a volume of 25 L at 800 torr. Find the volume at 400 torr.

Use Boyle’s law: (P1)(V1) = (P2)(V2)


V2 = 50 L

Combined Gas Law

P1V1/T1 = P2V2/T2


Moles of gas constant

P = Pressure

V = Volume

T = Kelvin

Dalton’s Law of Partial Pressures

In a gas mixture, the total pressure equals the sum of the partial pressures of each component.


Ptotal = P1 + P2 + P3 …..

Ideal Gas Law

PV = nRT


P = Pressure (atmospheres)

V = Volume (liters)

n = # of moles

R = 0.0820 (L atm)/(mol K)

T = temperature in K

How many moles of H2 gas are there in 20 L at 4 atm and 400K?

Use the Ideal Gas Law: PV = nRT


n = 2.44 moles

Standard temperature (0 C, 273 K) and standard pressure (1 atm or 760 torr)
Gay-Lussac’s Law of Combining Gas Volumes
When only gases are involved in a reaction, the volumes of reactants and products are in a small, whole number ratio.
Avagadro’s Law (for gases)
Under the same conditions of temperature and pressure, equal volumes of gases contain equal numbers of moles.
What is the volume of 1 mole of any gas at STP?
22.4 Liters

N2 + 3H2 ↔2 NH3

In the above reaction, how many L of NH3 will be produced from 6 L of H2?

According to Gay-Lussac’s Law, the volumes of gases are in direct proportion to the coefficients in the equation.


x = 4 L of ammonia