Question

The density of 3 M solution of NaCI is 1.2 g ${mL}^{-1}$. The molality of the solution is?

• A. 2.79 m
• B. 1.79 m
• C. 3.5 m
• D. 5.58 m

Answer 1

The correct option is A 2.79 m

3 Molar solution means there are 3 moles of $NaCl$ salt in 1 litre.

Molecular weight of $NaCl$ = 55.44

Hence, there are 3 * 58.44 gms in 1 Litre of water.

Density = mass $\times$ volume Mass of 1 litre of solution = 1.25 gms /ml *1000 ml = 1,250 gms

V = volume of water added to make the solution

175.32 gms +1 gm/ml * V = 1,250 gm

V = 1,074.68 ml

1,074.68 ml gms of water is mixed with 3 moles of NaCl to make 3 M solution.

Molaity = mass of solute in number of moles / mass of solvent in kg'

= 3 / 1.07468 kg

= 2.7915 m