Question

the density of a 2M solution is 1.2 gram per ml. If the molecular mass of the solute is 100g/mol, then molality of the solution is how much?

Answer 1

Quick and easy method used during jee exams

1000×m

_________

1000×d-m×mb

Where m is the polarity

D is the density

mb is molecular mass

Answer 2

Answer : The molality of the solution is, 2 mole/Kg

Explanation :

The relation between the molarity, molality and the density of the solution is,

$d=M[\frac{1}{m}+\frac{M_b}{1000}]$

where,

d = density of solution  = 1.2 g/L

m = molality of solution  = ?

M = molarity of solution  = 2 mole/L

$M_b$ = molar mass of solute = 100 g/mole

Now put all the given values in the above formula, we get:

$1.2g/L=2mole/L[\frac{1}{m}+\frac{100g/mole}{1000}]$

$m=2mole/Kg$

Therefore, the molality of the solution is, 2 mole/Kg