Elements P and Q form two types of non-volatile, non-ionizable compounds PQ and PQ2. When 1 g of PQ is dissolved in 50 g of solvent 'A', Tb was 1.176 K while when 1 g of PQ2 is dissolved in 50 g of solvent 'A', Tb was 0.689 K. (Kb of 'A' = 5 K kg mol-1). The molar masses of elements P and Q (in g mol-1) respectively, are :
- 25, 60
- 60, 25
- 65, 145
- 70, 110
Answer
(1) 25, 60
Solution
Tb = Kb wB 1000MB wA
MB = Kb wB 1000 Tb wA
For PQ: MPQ = 5 1 10001.176 50 = 500058.8 = 85 g mol-1
For PQ2: MPQ2 = 5 1 10000.689 50 = 500034.45 = 145 g mol-1
P + Q = 85 and P + 2Q = 145
Q = 145 − 85 = 60 g mol-1
P = 85 − 60 = 25 g mol-1
Answer: (1)