Question

An aqueous solution of hydrofluoric acid is 30.0+ HF, by mass, and has a density of 10101 g cm^-3. What are the molality and molarity of HF in this solution

Answer 1

1. In 100 g solution, 30g is HF and 70g is water as it is a 30% solution of HF in water.

2. Molality:

Number of moles of HF = $\frac{30}{20}=1.5moles$
Wt. of water in kg = $\frac{70}{1000}=0.07kg$

Molality = $$\begin{gathered} \frac{NumberofmolesofHF}{Wtofwaterinkg} \\ =\frac{1.5}{0.07}=21.4molal. \end{gathered}$$

3. Molarity:

Volume of solution = $$\begin{gathered} \frac{Mass}{Density} \\ =\frac{100}{1.101}=90.83mL \\ Molarity=\frac{NumberofmolesofHF}{VolumeofHFinL} \\ =\frac{1.5}{0.09083}=16.58M \end{gathered}$$