States of Matter · Question 1

At the sea level, the dry air mass percentage composition is given as nitrogen gas: 70.0, oxygen gas: 27.0 and argon gas: 3.0. If total pressure is 1.15 atm, then calculate the ratio of following respectively: (i) partial pressure of nitrogen gas to partial pressure of oxygen gas (ii) partial pressure of oxygen gas to partial pressure of argon gas (Given: Molar mass of N, O and Ar are 14, 16 and 40 g mol-1 respectively.)

1. 5.46, 17.8
2. 2.96, 11.2
3. 4.26, 19.3
4. 2.59, 11.85

Answer

(2) 2.96, 11.2

Solution

Take 100 g of dry air:

nN2 = 70.028 = 2.5 mol

nO2 = 27.032 = 0.84375 mol

nAr = 3.040 = 0.075 mol

pi = xiPtotal, the ratio of partial pressures = ratio of moles (the total pressure cancels).

(i) pN2pO2 = 2.50.84375 = 2.96

(ii) pO2pAr = 0.843750.075 = 11.25 11.2

Answer: (2)