Question

50 cc of oxygen is collected in an inverted gas jar over water. The atmospheric pressure is 99.4 kPa and the room temperature is 27°C. The water level in the jar is same as the level outside. The saturation vapour pressure at 27°C is 3.4 kPa. Calculate the number of moles of oxygen collected in the jar.

Answer 1

Here , 

Atmospheric pressure, `"P"_0 = 99.4 xx 10^3  "Pa"`

SVP at `27^circ"C"  "P"_ω = 3.4 xx 10^3 "Pa"`

T = 300K 

`V = 50 xx 10^-6"m"^3`

Now,

Pressure inside the jar = Pressure outside the jar                                  [∵ Level of water is same inside and outside of the jar]

Pressure outside the jar = Atmospheric pressure

Pressure inside the jar = VP of oxygen + SVP of water at `27^circ`C

⇒ `P_0 = P + Pω` 

⇒ `P = P_0 - Pω = 99.4 xx 10^3 - 3.4 xx 10^3 = 96 xx 10^3`

Applying equation of state , we get 

PV = nRT 

⇒ `96 xx 10^3 xx 50 xx 10^-6 = n xx 8.3 xx 300`

⇒ `n = 1.9277 xx 10^-3 "mol" ≈ 1.93 xx 10^-3 "mol"`