Question

A solution of A (MM=20) and B(MM=10),(mole fraction of XB=0.6) having density 0.7gm/ml then molarity and molality of B in the solution will be

Answer 1

given, molar mass of A, $M_A$ = 20g/mol.

and molar mass of B, $M_B$ = 10g/mol.

density of solution , d = 0.7 g/ml

mole fraction of B, $x_B$= 0.6

so, mole fraction of A , $x_A$= 1 - 0.6 = 0.4

using formula, $M=\frac{x_Bd\times1000}{M_A.x_A+M_B.x_B}$

= 0.6 × 0.7 × 1000/(20 × 0.4 + 10 × 0.6)

= 420/14

= 30 molar

hence, molarity of the solution is 30M

use formula, $m=\frac{x_B\times1000}{M_A(1-x_B)}$

where, m is molality of the solution.

so, m = 600/(20 × 0.4)

= 600/8

= 75 molal

hence, molality of the solution is 75m