USING THE IDEAL GAS LAW
The concepts in this experiment are also discussed in sections 5.4 – 5.7 of
Principles of Chemistry –
A Molecular Approach
, by Tro.
The purpose of the experiment is to verify the stoichiometry of a chemical reaction between a metal
and an acid using the Ideal Gas Law.
The metal sample will be reacted with an excess of aqueous
solution of hydrochloric acid, which will result in the production of hydrogen gas.
sample is collected over water, and then quantified by applying Dalton’s Law of Partial Pressures
and the Ideal Gas Law.
The mole ratio of metal to hydrogen gas will be compared with the expected
In addition, an unknown metal sample will be supplied and its identity will be determined using
the moles of hydrogen gas, the mass of the metal and information regarding the reaction
Students will work in pairs.
Equal volumes of all gases, measured at the same temperature and pressure, contain equal
numbers of particles.
Amadeo Avogadro proposed this assumption in 1811. Stanislao Cannizzaro
came upon Avogadro’s hypothesis nearly 50 years after it had been proposed. He realized that this
hypothesis could be used to determine the molar masses of gaseous elements and compounds. If
equal volumes of gases contain equal numbers of particles, then the masses of those gas volumes
should be in the same ratio as the masses of their constituent particles.
The volume of gas chosen for comparison was the volume occupied by one mole of a substance.
However, the volume occupied by a mole of gas depends on the temperature and pressure of the
gas. Therefore a standard temperature and pressure were chosen. Standard temperature and
pressure (STP) are 273 K and 101.3 kPa (1.00 atm). At STP the volume occupied by one mole of a
gas is 22.4 L, the
standard molar volume.
Experiments on gases by chemists led to the ideal gas law, PV = nRT, where P is the pressure in
atm, V is volume in liters, n is the moles of gas, and T the temperature in Kelvin.
R is the gas
constant which can be calculated as follows:
The ideal gas law may be used to calculate the number of moles of a gas if we are given the
pressure, volume, and temperature of the gas.
The gas constant, R = 0.0821 atm
used for any pressure, volume, temperature and moles of gas.
For example, a sample of CO
has a pressure of 735 mmHg and occupies a volume of 250.0 mL at a temperature of 20.0
can calculate the moles of the gas but we must convert pressure to units of atm (1.00 atm = 760
mmHg), volume to liters, and tempertaure to Kelvin:
P = 735 mmHg x
V = 250.0 mL x
= 0.2500 L
T = 20.0
C = 20.0 + 273 = 293 K