Question

The mass percent of NaOH in water is 4.7%. What is the mole fraction of NaOH in the solution?

• A. 0.022
• B. 0.221
• C. 0.177
• D. 1.770

Answer 1

The correct option is A 0.022

Let the mass of solution be 100 g
Mass % $=\frac{\text{mass of solute}}{\text{mass of solution}}\times 100$
$4.7=\frac{\text{mass of solute}}{100}\times 100$
Mass of solute = 4.7 g
Mass of solvent = 100 - 4.7 g = 95.3 g
Moles of solute $NaOH$ $=\frac{\text{given mass}}{\text{molar mass}}$
$=\frac{4.7}{40}=0.1175$
Moles of solvent $\left(H_{2}O\right)$ = $\frac{\text{given mass}}{\text{molar mass}}=\frac{95.3}{18}=5.294$
Mole fraction of NaOH $x_{\text{NaOH}}=\frac{\text{moles of NaOH}}{\text{moles of NaOH + moles of}{H}_{2}O}=\frac{0.1175}{0.1175+5.294}=0.022$