Question

Calculate the molality of a salt (molar mass =138 g $mo{l}^{-1}$) solution obtained by dissolving 2.6 g in one litre of the solution. Density of solution is $0.85gc{m}^{-3}$.

• A. 0.214 mol $k{g}^{-1}$
• B. 0.0214 mol $k{g}^{-1}$
• C. 0.1214 mol $k{g}^{-1}$
• D. None of these

Answer 1

Answer: (B)

0.0214 mol $k{g}^{-1}$

molality = mole/ mass of solute

density of substance = mass / volume

then we find the mass of solution

mass of solution = $density\ast volume$ = $0.85\ast 1000=850gram$

mass of solute = $850-2.6=847.4gram$

mole of substance = $2.6/138=0.0188$

molality = 0.0188/0.8474

$=0.214mole/kg$ (B)