Question

the density of 3m solution of nacl is 1.25 gml -1 . calculate molality of the solution given atomic mass sodium = 23 chlorine=35.5

Answer 1

For 3M solution , molarity = no. of moles of solute/ volume of solution in L
So, for every 1 L of solution,
No. of moles of NaCl = 3
So, here , mass of solution= volume x density = 1000ml x 1.25 gm/ml = 1250 gm
Furthur mass of solute = mass of 3 moles = 3 x (23 + 35.5) = 58.5 x 3 = 175.5 gm
So, mass of solvent = mass of solution - mass of solute = 1250- 175.5 gm = 1074.5 gm = 1.07 Kg
So, molality = moles of solute / mass of solvent = 3/ 1.07 = 2.8

Relation between molarity and molality :

$$\begin{gathered} \text{m}\left(\mathrm{molality}\right)\text{=}\frac{\mathrm{Molarity}\times 1000}{\mathrm{density}\times 1000-\mathrm{MM}} \\ \text{2.8 =}\frac{\mathrm{Molarity}\times 1000}{1.25\times 1000-58.5} \\ \mathrm{Molarity}=3.3\mathrm{M} \end{gathered}$$