UNIT 3 GASES
IMPORTANT CONCEPTS THAT I SHOULD KNOW:
1. 1 mole of any element is equal to 6.022 x 1023 atoms. 1 mole of an element is equal to its mass (in grams) on
the periodic table. Remember the following conversion factors
1 moles
1 mole
g
6.022 x 10 23 Atoms
and Want
Gives us
or
or
or
Have
1 moles
6.022 x 10 23 Atoms
1 mole
g
2. Gases uniformly fill any container are easily compressed, have no definite shape or volume and mix
completely with any other gas. Gases also exert pressure on their surroundings.
3. The standard pressure of one atmosphere (1 atm) is the pressure required to support 760 mm Hg at 25o C at
sea level. The SI unit of pressure is the pascal (Pa). 1 atm= 101.3 kPa. Use
Want
to convert pressures.
Have
4. Molecules collide with one another and also with the sides of the container. The kinetic Molecular Theory of
Gases is a model that explains the behavior of gases. Postulates of the Kinetic Theory are:
 Gases consist of tiny particles, so small that the volume of the particles is assumed to be zero
 The particles are in constant, random motion colliding elastically with container wall which
causes the pressure exerted by the gas. The particles are assumed not to attract or repel each
other.
 The average kinetic energy of the particles is directly proportional to the Kelvin temperature of
the gas. (The energy an object has because of its motion is kinetic energy.)
5. [Charles’ Law V1/T1=V2/T2] As the TEMPERATURE goes up – the speed of the molecular motion goes up,
so does the kinetic energy (or force) so that the molecules hit the walls harder (with more force). The
VOLUME increasing reduces the number of collisions so that the PRESSURE stays the same.
6. [Boyle’s Law P1V1=P2V2] As the Volume Decreases and the TEMPERATURE remains constant– the
speed of the molecular motion remains constant, so the velocity (kinetic energy) that these molecules hit the
walls does not change. But the number of wall collisions (not the speed of the collisions) increases as the
surface area (and volume) goes down. Hence, the PRESSURE goes up. (same idea if # of moles increases)
7. There is a third law [Gay-Lussac’s law P1/T1=P2/T2] where we keep volume constant. I have skipped this for
simplicity and time. But this was the tanker car implosion and the grenade explosion.
8. Remember that all three laws above are derived from the combined gas law T2P1 V1 = T1P2 V2. For the
combined gas law we don’t keep either T or P constant, and both are allowed to change to alter the volume. The
amount the volume is impacted depends on the magnitudes of change of P & T. The ratio is P/T (i.e. if P
doubles and T quadruples then we would expect the volume to decrease by ½). Moles is only thing constant.
9. A combination of these three laws produces the ideal gas law which is usually written as PV=nRT where R
is the universal gas constant R  8.314
L  KPa
We can use this gas law to determine steady state of the gas.
mol  K
FOR THIS TEST I CAN:





convert between pressure units (atm, mmHg, kPa,)
/ between pressure
(C, K) / moles (grams,atoms)
explain that Kelvin temperature is always used in gas law calculations because 0 K actually means zero
molecular motion (kinetic energy) whereas 0 C is simply the freezing point of pure water
P V PV
identify a problem as P·V, ,
,
, or PV= nRT
T T T
explain what temperature and pressure look like in terms of the Kinetic Molecular Theory
temperature = motion of particles (average KE) ; pressure = collisions with the walls of the container
substitute and evaluate the PV= nRT equation correctly so I can solve the equation for various variables.