Page 131 - Chemistry ICSE Class X
P. 131
Mole Concept and Stoichiometry 117
“When gases react, they do so in volumes which bear a simple ratio
to one another and to the volume of the gaseous products provided all
volumes are measured at the same temperature and pressure.”
Or
“Under constant temperature and pressure, the volumes of the
gaseous reactants bear a simple ratio between themselves and also with
the volumes of the gaseous products.”
Illustrating Gay-Lussac’s law of combining volumes
The Gay-Lussac’s law of combining volumes is illustrated through the
examples described below:
Reaction between hydrogen and chlorine gases to form hydrogen
chloride. It has been found experimentally that when one unit volume
of hydrogen reacts with one unit volume of chlorine, two units volume
of hydrogen chloride gas are produced.
H (g) + Cl (g) o 2HCl(g)
2
2
1 vol 1 vol 2 vol
The ratio of volumes of hydrogen, chlorine and hydrogen chloride is
1 : 1 : 2 (a simple ratio). Joseph Louis Gay-Lussac
Reaction between nitrogen and hydrogen to form ammonia gas. One
unit volume of nitrogen reacts with three unit volume of hydrogen to
form two units volume of ammonia gas. Thus,
N (g) + 3H (g) o 2NH (g)
2
3
2
1 vol 3 vol 2 vol
The volume ratio of 1 : 3 : 2 is a simple ratio.
Reaction between hydrogen and oxygen to form water. Two unit
volume of hydrogen react with one unit volume of oxygen to form
two unit volume of water vapour. Thus,
2H (g) + O (g) o 2H O(g)
2
2
2
2 vol 1 vol 2 vol
The volume ratio of 2 : 1 : 2 is a simple ratio.
Reaction between carbon monoxide with oxygen to form carbon
dioxide. Two unit volume of carbon monoxide (CO) react with one
unit volume of oxygen to form two unit volume of carbon dioxide
(CO ).
2
Thus,
2CO(g) + O (g) o 2CO (g)
2
2
2 vol 1 vol 2 vol
The volume ratio of 2 : 1 : 2 is a simple ratio.
From the examples described here, it appears that the Gay-Lussac’s
law of gaseous volumes (or Gay-Lussac’s law of combining volumes) is
actually the law of FGſPKVG RTQRQTVKQPU D[ XQNWOG
Some Examples Based on Gay-Lussac’s Law of
Combining Volumes
EXAMPLE 5.1. 30 mL of methane is mixed with 80 mL of oxygen and burnt. If
all measurements are made at room temperature and constant pressure, what is
the volume of the unreacted oxygen?
Solution: The chemical equation describing the reaction of methane with oxygen is
room temp.
CH (g) + 2O (g) o CO (g) + 2H O(l)
2
4
2
2
1 vol 2 vol 1 vol
30 mL 2 × 30 mL 30 mL
60 mL
