Page 188 - Chemistry ICSE Class IX

176 ICSE Chemistry – 9
Equating the value of v from above, one can write,
P V 1 = V T 1
2
1
P 2 T 2
P V P V
or 1 1 = 2 2
T 1 T 2

It may be generalised as,
PV = Constant
T
or PV = Constant × T (7.17)
The constant in Eq. 7.17 is found to depend upon the amount (n) of
the gas.
Therefore, Constant v n
or Constant = nR
where R is a universal constant called the universal gas constant. Then,
one can write,
PV = n RT
This equation is called the ideal gas equation.

What are the applications of the ideal gas equation
ƒ The ideal gas equation can be used for determining the volume of a
certain amount of gas at any speci ed temperature and pressure, if
its value at some other temperature and pressure is known.
ƒ The volume of certain amount of a gas at any temperature and
pressure can be reduced to that under normal temperature and
pressure (NTP) conditions, i.e., at 273.15 K and pressure of 1 atm
(76 cm or 760 mm of Hg).

How to calculate the value of the universal gas constant (R)

For gases, which obey the Boyle’s law, Charles’ law or in general, the gas
equation, we can write
PV = nRT
The universal gas constant R may be expressed as

PV
R =
nT
For one mole of a gas, i.e., n = 1,
PV
R = m (7.18)
T
where, V is the molar volume of the gas at temperature T and pressure P.
m
The magnitude and unit of R depend upon the units in which pressure,
volume and temperature are expressed.
Under NTP conditions, the values of P, V and T in SI units are
m
P = 101325 N m –2

T = 273.15 K
–3
3
V = 22.414 × 10 m
m