Chem 20 – Unit 2: Gas Laws SUMMARY
1) BASIC FORMULAS:
Density is a measure of how closely packed the particles of a substance are relative to one
another.
density 
mass
volume
D
m
V
Density unit: g/cm3 (for solids) or g/mL (for fluids).
Liquid water has a density of 1.0 g/mL (at 15 oC).
Moles
n




m
M
n = moles (mol)
m = mass (use g)
M = Molar Mass (g/mol)
Significant Digits: Know how to express in (and round) significant digits (5 or greater round up, 4
or less round down).
Ex. 0.005845858 moles in 3 significant digits = 0.00585 moles or 5.85 x 10 -3 moles
Scientific Notation:
Ex. 23580 kPa in scientific notation is 2.3580 x 104 kPa (the decimal always goes directly
behind the 1st digit)
Unit Conversions: Know how to convert all units (refer to pg. 3 of data booklet)
Ex. 100 kg = 100,000 g
Calculating # of molecules:
n x 6.02 x 1023 /mol
2) CONCEPTS
Properties of Gases
Compressibility/Expansion
Viscosity/Density
Shape/Volume
Kinetic Molecular Theory
Kinetic Energy
Ideal Gases
Point Masses
Elastic Collisions
Standard Temperature and Pressure (STP)
0oC (273.15K) 101.325 kPa (or 1 atm)
Standard Ambient Temperature and Pressure (SATP)
25oC (298.15K) 100 kPa
Pressure Conversions:
1 Pa = 1 N/m2
1 atm = 760 mm HG = 760 torr = 101.325 kPa = 1.01325 bar
Boyle’s Law: As the pressure on a gas increases, the volume of the gas decreases
proportionally, provided that the temperature and chemical amount of the gas remain constant.
**constant temperature and closed system**
P
V
P1 V1 = P2 V2
Charles’ Law
As the average velocity of the molecules decreases, the volume of the sample must decrease in order to
keep the frequency of the collisions constant. Ex. as T  the V will  or both will increase.
V1 V2

T1 T2
1) Constant Pressure
2) Closed System
3) Calculate in Kelvin
Volume (L)
Gas A
Charles determined
that all of his gases
would have a
theoretical zero
volume at a
temperature of
– 273.15oC.
Gas B
Gas C
T (oC)
Gay – Lussac’s Law



When a sample of a gas has its temperature changed at a constant volume, its pressure will also
change.
Usually when the gas is in a rigid container such as a steel tank.
As the temperature increases, the velocity of the gas molecules will increase. This will lead to a
greater frequency of collisions and an increased pressure. Ex. as T , the P will also  or vice versa.
P
T
P1 P2

T1 T2
1) Constant Volume
2) Closed System
3) Use Kelvin
Combined Gas Law

The Combined Gas Law can be used for any closed system going through a change in P, T, and V
conditions. As usual, temperature must be in the Kelvin scale.
P1 V1 P2 V2

T1
T2
Law of Combining Volumes

When gases react, the volumes of the gaseous reactants and products, measured at the same
conditions of temperature and pressure, are always in whole-number ratios.
Example: Reaction #1: Formation of water vapour
hydrogen gas + oxygen gas
water vapour
100 mL
+
50 mL
100 mL
50 mL
+
25 mL
50 mL
8 mL
+
4 mL
8 mL
Avogadro’s Law

Equal volumes of all ideal gases at the same temperature and pressure contain the same number of
molecules.
n1 n 2

V1 V2
Molar Volume

According to Avogadro’s law, the volume of one mole of any gas should be the same as the volume
of one mole of any other gas, if the conditions of temperature and pressure are the same.

molar volume: the volume of exactly one mole of a gas
ν
V
n
-At STP, the molar volume of any gas will be 22.4 L/mol.
-At SATP, the molar volume of any gas will be 24.8 L/mol.
Ideal Gas Law
P V = n R T **By rearranging PV = nRT with D = mV and n = mM

PM
, we can get D = RT
Universal Gas Constant  R = 8.314 L ● kPa / mol ● K for all gases
Dalton’s Law of Partial Pressures

In a mixture of gases, the total gas pressure is the sum of the partial pressures of each type of gas.
Ptot = P1 + P2 + P3 …

The greater the amount of a particular gas, the greater its contribution to the total pressure will be.
When a gas is collected over water, the water vapour pressure must be accounted for.
Ptotal = Pgas + Pwater vapour
(Partial Pressure of water vapour given in a data table)
Calculating Experimental Error
Percent error 
theoretica l result  experiment al result
x 100%
theoretica l result