Question

calculate the molality of 0.2M nacl solution

Answer 1

NaCl dissociates in a 1:1:1 ratio according to the equation:

NaCl --> Na+ + Cl-

So, based on this ratio if you have 0.2 molar NaCl you will get 0.2 molar Na+ and Cl-. The 1:1:1 ratio means that the molarity of each of the ions will be the same as the molarity of NaCl.

Contrast this to the situation for MgCl2. This dissociates in a 1:1:2 ratio:

MgCl2 --> Mg2+ + 2Cl-

In this case if you have 0.2 molar MgCl2, it will dissociate to give 0.2 molar Mg2+ but 0.4 molar Cl- because there are twice as many Cl- ions as MgCl2 particles.

Answer 2

given mass / mass of solution in litre

0.2 M / 1 litre