Question

If the concentration of a sodium hydroxide aqueous solution is 50% by mass, calculate the molality of the solution.

• A. 25.0 m
• B. 0.5 m
• C. 12.5 m
• D. 13.8 m

Answer 1

The correct option is A 25.0 m

Let's assume mass of the solution be 100 g
50% (w/w) means 50 g of $NaOH$ is present in 100 g of the solution.
Mass of the solvent in the solution
= 100 - mass of solute
Mass of the solvent = 50 g
Number of moles of NaOH
= $\frac{\text{given mass}}{\text{molar mass}}=\frac{50}{40}mol$

Molality = $\frac{\text{moles of solute}}{\text{mass of solvent in g}}\times 1000$

Molality = $\frac{50/40}{50}\times 1000$

Molality = 25 m