Ch. 12 Gas Laws
Outline
Characteristics of Gases
1. Gases expand to fill the volume of whatever container they are in.
2. Gases are mostly empty space - the molecules are relatively far apart.
3. Gases always form homogeneous solutions.
4. Gases are easily compressed.
Substances that exist as gases at room temperature:
- nonmetals with low molecular masses (ex: H2,N2,O2,F2,Cl2)
- noble gases
- molecular compounds with low molecular masses are often gases (ex: CO2,NH3,CH4)
Metallic or ionic substances are NOT gases at room temperature.
vapor - molecules of a normally liquid or solid substance that have entered the gas phase.
Pressure –Force/Area; amount of force per unit of area. Units of pressure: kilopascals (kPa),
atmospheres (atm), millimeters of mercury (mmHg), torr
(a torr and a mmHg are the same thing)
standard pressure - the average atmospheric pressure at sea level.
Standard pressure is defined as: 760 mm Hg = 760 torr = 1 atm = 101.3 kPa
Temperature
Kelvin temperatures must be used when using the gas laws.
Temperature conversion: K = ºC + 273
STP - Standard Temperature and Pressure STP = 0ºC and 1 atm
The Gas Laws
A. Boyle's Law - the volume and pressure of a sample of gas are inversely proportional
(if temperature remains constant). Read section 12-2 pgs 299-303
Boyle's Law equation:
P1V1 = P2V2
(P1 and V1 are initial (beginning) pressure and volume; P2 and V2 are final pressure and volume.)
B. Charles' Law - the volume of a sample of gas is directly proportional to its Kelvin temperature
(if pressure stays constant). Read section 12-3 pages 303-309
Charles' Law equation:
V1/T1 = V2/T2
(V1 and T1 are initial (beginning) volume and temp; V2 and T2 are final volume and temp.)
C. Temperature and Pressure Law - the pressure of a sample of gas is directly proprtional to its Kelvin
temperature (if volume stays constant). Read 12-4 pages 310
Pressure-Temperature law equation:
P1/T1 = P2/T2
Gas Laws
Outline Page 2
E. Combined Gas Law Combines Boyle’s, Charles’, and pressure-temp laws. Sec 12-5 pgs 311-313
F. Ideal Gas Law - relates the pressure (P), volume (V), # of moles (n), and Kelvin temperature (T) of a
sample of gas. Read 12-13 pages 326-328
Ideal Gas Law equation:
PV = nRT
R is the gas constant. The value of R that you use in a problem depends upon what the other units in
the problem are. The most commonly used values of R are:
0.0821 (L*atm)/(mol*K) or 8.31 (dm3*kPa)/(mol*K)
See Molar mass derivation in notes.
G. Dalton’s Law of Partial pressures (sec 12-9,pg318))– The total pressure of a mixture of gases is
equal to the sum of the pressures each individual gas would exert in the same volume.
H. Graham’s Law of Diffusion (Sec12-10,pg 320)– The rate of one gas diffusing compared to another
gas is equal to the inverse of the square roots of their molar masses (or densities).
Read over Section 12-11, pg322-323, Kinetic Theory and the Gas Laws