NCERT Solutions
Class 11 Physics
Kinetic Theory

Q. 12.4
An oxygen cylinder of volume 30 litres has an initial gauge pressure of 15 atm and a temperature of 27 °C. After some oxygen is withdrawn from the cylinder, the gauge pressure drops to 11 atm and its temperature drops to 17 °C Estimate the mass of oxygen taken out of the cylinder (R = 8.31 J mol–1 K–1, molecular mass of O2 = 32 u).
Given:
Volume of oxygen, V1 = 30 litres = 30 × 10–3 m3
Gauge pressure, P1 = 15 atm = 15 × 1.013 × 105 Pa
Temperature, T1 = 27°C = 300 K
Universal gas constant, R = 8.314 J mole–1 K–1
Let the initial number of moles of oxygen gas in the cylinder be n1.
The gas equation is given as:
P1V1 = n1RT1
n1 = =
= 18.276
But, n1= Where, m1 = Initial mass of oxygen
M = Molecular mass of oxygen = 32 g
m1 = (n1M) = 18.276 × 32 = 584.84 g
After some oxygen is withdrawn from the cylinder, the pressure and temperature reduces.
Volume, V2 = 30 litres = (30 × 10–3) m3
Gauge pressure, P2 = 11 atm = (11 × 1.013 × 105) Pa
Temperature, T2 = 17°C = 290 K
Let n2 be the number of moles of oxygen left in the cylinder.
The gas equation is given as:
P2V2 = n2RT2
n2=
=
= 13.86
But, n2 = Where, m2 is the mass of oxygen remaining in the cylinder
m2 = (n2M) = 13.86 × 32
= 453.1 g
The mass of oxygen taken out of the cylinder is given by the relation:Initial mass of oxygen in the cylinder – Final mass of oxygen in the cylinder
= (m1 – m2) = 584.84 g – 453.1 g = 131.74 g = 0.131 kg
Therefore, 0.131 kg of oxygen is taken out of the cylinder.