16

Sol. Both the gases before mixing are at the same temperature and pressure. Since the amount of neon is twice as that of helium, it is obvious that

V Ne = 2 V He

Now the volume of gas after mixing will be

V total = V He + V Ne = 3 V He

Each of the two gases suffer entropy change due to volume change. Thus

Δ S He = nR ln = 1 × R × ln 3

Δ S Ne = 2 R ln

or Δ S mixing = R ln 3 + 2R ln 3/2

or Δ S mixing = R ln 3 + R ln 9/4

or Δ S mixing = R ln

or Δ S mixing = R × 2.303 (3 log 3 – 2 log 2)

or Δ S mixing = 8.314 × 2.303 (1.44 – 0.6)

= 16.08 J/K