Question

The density of 2M NaCl solution is 1.117 g ml-1. The molality of the solution is (A)1 m(B)1.5 m(C)1.8 m(D)2 m

Answer 1

Molarity= 2M

Molar mass of NaCl= 58.5

M= mass of solute/ molar mass of solute x volume
mass = 2 x 58.5 
= 117g

density = 1.117 = mass/ volume 
mass = density x volume 
= 1.117 x 1000
= 1117 g

mass of solvent = mass of solution - mass of solute 
= 1117- 117
= 1000g

molarity (m) = mass of solute /molar mass of solute x mass of solvent 
= 117 x 1000/58.5 x 1000kg
= 2m

Answer 2

Answer: The correct answer is Option D.

Explanation:

The equation which shows the relationship between density, molarity and molality is given by:

$\frac{1}{m}=\frac{d}{M}-\frac{M_B}{1000}$

where,

m = molality of the solution

M = molarity of the solution

d = Density of the solution

$M_B$ = Molar mass of the solute i.e. NaCl

We are given:

$d=1.117g/mL\\M=2M\\M_B=[23+35.5]=58.5g/mol$

Putting values in above equation, we get:

$\frac{1}{m}=\frac{1.117}{2}-\frac{58.5}{1000}\\\\m=2m$

Hence, the correct answer is Option D.