Question

A concentrated sodium hydroxide is 50.5% NaOH by mass (100% *mass of solute/mass of solution) and has a density of 1.53 g/mL at 25 degrees celcius. what is the molality of NaOH?

Answer 1

Sol :-

Let mass of the solution = 100 g

So,

Mass of NaOH i.e. Solute = 50.5 g

Now

Mass of solution = Mass of solute i.e. NaOH + Mass of solvent

So,

Mass of solvent = Mass of solution - mass of solute i.e. NaOH = 100 g - 50.5 g = 49.5 g = 0.0495 kg

Also

Number of moles of solute i.e. NaOH = Given mass of NaOH / Gram molar mass of NaOH

Number of moles of solute i.e. NaOH = 50.5 g / 40 g/mol = 1.2625 mol

We know that

Molality = Number of moles of solute / Mass of solvent in kg = 1.2625 mol / 0.0495 kg = 25.5 mol/kg

Hence, Molality of NaOH solution = 25.5 mol/kg