Question

What is the molarity of a hydrofluoric acid solution, $H{F}_{\left(aq\right)}$, if $4.0$ g of $HF$ is dissolved in enough water to make $1.0$ L of a solution?

• A. $0.1M$
• B. $0.20M$
• C. $0.40M$
• D. $0.50M$
• E. $0.60M$

Answer 1

Answer: (B)

$0.20M$

Molarity is defined as moles of solute per liter of solution.
$\text{Molarity}=\frac{\text{moles of solute}}{\text{volume of solution (in L)}}=\frac{\text{mass of solute}}{\text{molecular weight of solute}\times \text{volume of solution (in L)}}$
The atomic masses of $H$ and $F$ are 1 g/mol and 18.9 g/mol respectively.
The molar mass of $HF$ is $1+18.9=19.9$ g/mol.
$\text{Molarity}=\frac{4.0}{19.9\times 1.0}=0.20$ M.