Page 177 - Chemistry ICSE Class IX

Study of Gas Laws 165
In chemistry, according to the IUPAC recommendations, the amount
of a substance is expressed in terms of the number of moles (n). The
number of moles can be obtained from the mass of the gas by using the
relationship,
Mass of the gas in grams (m)
Number of moles (n) = (7.1)
–1
Molar mass of the gas in g mol (M)
Volume (V)
Gases ll the container in which they are placed. So the volume of gas is
equal to that of its container. Volumes can be expressed in litre (L), millilitre
3
3
(mL) or cubic centimetre (cm ). The SI unit of volume is cubic metre (m ).
3
In common practice, however, smaller units such as cubic decimetre (dm )
3
and cubic centimetre (cm ) are more frequently employed. These are related
to each other as,
3
6
3
3
3
1 m = 10 dm = 10 cm
The commonly used unit, litre (L) has now been rede ned so that,
1 L = 1 dm 3
and 1 mL = 1 cm 3
The volume (V) of any gas depends upon its amount, temperature
and pressure. Mathematically, volume of a gas is a function of amount,
temperature and pressure, i.e.,
V = f (amount, temperature, pressure)
or V = f (n, T, P)

Pressure (P)
Pressure is force per unit area: Gases exert pressure uniformly in all the
directions. In laboratory, the atmospheric pressure is measured with the
help of a barometer, in terms of height of mercury column in the barometer
tube. The atmospheric pressure (P) is expressed as,
P = h U g (7.2)
where, h is the height of mercury column in the barometer
U is the density of mercury
g is the acceleration due to gravity at the place of measurement
dŚĞ atmosphere unit ŝƐ ŶŽƚ Ă ^/ ƵŶŝƚ͘
Normal pressure of one atmosphere (1 atm) is de ned as the ,ŽǁĞǀĞƌ͕ ŝƚ ŝƐ ƌĞƚĂŝŶĞĚ ĚƵĞ ƚŽ ŝƚƐ
pressure exerted by exactly 76 cm of mercury column at 0°C, (density of ǁŝĚĞƐƉƌĞĂĚ ƵƐĞ͘
–2
–3
Hg = 13.5951 g cm ) and a standard gravity of 981 cm s . ϭ WĂ с ϭ Eŵ –2
ϱ
The SI unit of pressure is pascal (Pa). It is de ned as the pressure ϭ Ăƚŵ с ϭ͘ϬϭϯϮϱ п ϭϬ Eŵ –2
2
exerted when a force of 1 newton (N) acts upon an area equal to 1 m . The ϭ Ăƚŵ с ϭ͘ϬϭϯϮϱ ďĂƌ
pascal unit is related to atm unit by the relationship, ϭ Ăƚŵ с ϳϲϬ dŽƌƌ
5
1 atm = 1.01325 × 10 Pa
Another non-SI unit generally employed in reporting pressure values
is termed as Torr. 1 Torr is equal to the pressure exerted by 1 mm of mercury
–2
column at 0°C, and a gravity of 981 cm s . Thus,
1 atm = 760 Torr
Thus,
5
1 atm = 76 cm Hg = 760 mm Hg = 760 Torr = 1.01325 × 10 Pa