was first noted by the French scientist Joseph Louis Gay-Lussac and is sometimes
called Gay-Lussac’s Law of Combining Gases. This law states that when only
gases are involved in a chemical reaction, the volumes of the reacting gases and
the volumes of the gaseous products are in small whole-number ratios with each
other. Those small whole numbers are the coefficients in the balanced reaction
equation.
Often reactions between gases do not occur at STP. However, Gay-Lussac’s
Law still applies. The reason is fundamentally due to Avogadro’s Law. That law
shows that the only requirement for the volumes of gases to be related to the
number of particles of those gases is that the temperature and pressure of the
gases be the same. Whether or not the temperature and pressure of the gases are at
STP is inconsequential. Using the coefficients from the balanced reaction equation
as volume ratios between reacting gases in dimensional analysis/stoichiometry
problems is encouraged.
STOICHIOMETRY: MASS-VOLUME OR VOLUME-MASS
PROBLEMS
Reactions often involve gases and other phases of matter. In those reactions, it is
common to know the mass of one substance involved in the chemical process and
the need to determine the volume of a different substance, such as a gas. Likewise,
it is not unusual to know the volume of a gas taking part in a reaction and the need
to determine the mass of another substance, often a solid or liquid. Even if the
reaction is taking place at STP, Gay-Lussac’s Law cannot be taken advantage of
here since both of the substances are not gases and the information desired is not
restricted to just volumes. In other words, Gay-Lussac’s Law applies only when
all the substances being considered are gases.
Example 1
In the reaction below, what mass of magnesium is required to produce 0.250 L of
H 2 at STP?
Mg(s) + 2HCl(aq) → MgCl 2 (aq) + H 2 (g)The solution to this problem uses both molar mass and molar volume. Use
dimensional analysis:
Reactions involving gases and other phases of matter NOT at STP are very
common. Obviously, Gay-Lussac’s Law cannot be used to solve such problems