Question

8 gm of NaOH dissolved in water to form 500 ml of its aqueous solution. If density of the solution is 1.2 gm/ml, then find the molality of the solution.

Answer 1

From the given,

Weight of NaOH (solute) = 8 gm

Volume of solution = 500 ml

Density of the total solution = $1.2\quad gm/{ cm }^{ 3 }$

Let's calculate the weight of the solution  

The weight of the solution = volume × density

$Weight\quad =\quad 500\quad \times \quad 1.2\quad =\quad 60\quad gm\quad$

Weight of solvent = Weight of solution - Weight of solvent

Weight of solvent = 600 - 8 = 592 gm  

$Molality\quad =\quad \frac { w }{ M } \times \quad \frac { 1000 }{ W }$}

w = weight of solute = 8 gm

W = weight of solution = 592 gm

Volume of solution = 1000 ml  

M = molar mass of solution = 40 g/mol

$m\quad =\quad \frac { 8 }{ 40 } \quad \times \quad \frac { 1000 }{ 592 } \quad =\quad 0.34$

The molality of the solution is 0.34.