where nis the number of moles of gas and bis a different constant for each gas. The larger
the gas particles, the more volume they occupy and the larger the bvalue.
The attraction of the gas particles for each other tends to lessen the pressure of the gas,
because the attraction slightly reduces the force of gas particle collisions with the container
walls. The amount of attraction depends on the concentration of gas particles and the mag-
nitude of the particles’ intermolecular force. The greater the intermolecular forces of the
gas, the higher the attraction is, and the less the real pressure. Van der Waals compensated
for the attractive force with:
corrected pressure =P+an^2 /V^2
where a is a constant for individual gases. The greater the attractive force between the mol-
ecules, the larger the value of a. The n^2 /V^2 term corrects for the concentration. Substituting
these corrections into the ideal gas equation gives van der Waals equation:
(P+an^2 /V^2 )(V– nb) =nRT
The larger, more concentrated, and stronger the intermolecular forces of the gas, the
more deviation from the ideal gas equation one can expect and the more useful the van der
Waals equation becomes.Experimental
Gas law experiments generally involve pressure, volume, and temperature measurements. In
a few cases, other measurements such as mass and time are necessary. You should remem-
ber that ΔP, for example, is NOT a measurement; the initial and final pressure measure-
ments are the actual measurements made in the laboratory. Another common error is the
application of gas law type information and calculations for non-gaseous materials. Typical
experiments involving these concepts are numbers 3 and 5 in the Experimental chapter.
A common consideration is the presence of water vapor, H 2 O(g). Water generates a
vapor pressure, which varies with the temperature. Dalton’s law is used in these cases to
adjust the pressure of a gas sample for the presence of water vapor. The total pressure
(normally atmospheric pressure) is the pressure of the gas or gases being collected and the
water vapor. When the pressure of an individual gas is needed, the vapor pressure of water
is subtracted from the total pressure. Finding the vapor pressure of water requires measur-
ing the temperature and using a table showing vapor pressure of water versus temperature.
In experiments on Graham’s law, time is measured. The amount of time required for a
sample to effuse is the measurement. The amount of material effusing divided by the time
elapsed is the rate of effusion.
Most gas law experiments use either the combined gas law or the ideal gas equation.
Moles of gas are a major factor in many of these experiments. The combined gas law can gen-
erate the moles of a gas by adjusting the volume to STP and using Avogadro’s relationship of
22.4 L/mol at STP. The ideal gas equation gives moles from the relationship n=PV/RT.
Two common gas law experiments are “Determination of Molar Mass by Vapor
Density” and “Determination of the Molar Volume of a Gas.” While it is possible to use
the combined gas law (through 22.4 L/mol at STP) for either of these, the ideal gas equa-
tion is easier to use. The values for P, V, T, and nmust be determined.
The temperature may be determined easily using a thermometer. The temperature
measurement is normally in °C. The °C must then be converted to a Kelvin temperature
(K =°C +273).
Pressure is measured using a barometer. If water vapor is present, a correction is needed
in the pressure to compensate for its presence. The vapor pressure of water is found in112 Step 4. Review the Knowledge You Need to Score High
STRATEGYHINT: Make sure
the conditions are
STP before using
22.4 L/mol.