Question

Suppose you have mixture consisting of 20.0 g of compound A and 20.0 g of compound B dissolved in 100 ml of water at 95 C. Suppose further that only 1 g of A are soluble in 100 ml of water at 5 C but 15 g of B are soluble in 100 ml of water at 5 C. What is the mass ratio of A to B in the crystals collected after recrystallization from 100 ml of hot water (95 C) and filtration at 5 C? What would be the mass ratio of A to B if the mixture were dissolved in 100 ml of water at 95 C, and then the water were completely evaporated?

Answer 1

Suppose you have mixture consisting of 20.0 g of compound A and 20.0 g of compound B dissolved in 100 ml of water at 95 C.
Suppose further that only 1 g of A are soluble in 100 ml of water at 5 C but 15 g of B are soluble in 100 ml of water at 5 C.
Q1
What is the mass ratio of A to B in the crystals collected after recrystallization from 100 ml of hot water (95 C) and filtration at 5 C?
Mass of solid A at 5C = 20-1= 19gm
Mass of solid B at 5C = 20-15= 15g
Mass ratio of A to B at 5C = 19/15 = 1.27

Q2
What would be the mass ratio of A to B if the mixture were dissolved in 100 ml of water at 95 C,
and then the water were completely evaporated?
Mass ratio after evaporation of water = 20/20=1.0