Question

A solution of methanol in water is $20\%$ by volume. If the solution and pure methanol have densities of $0.964 \; kg \; L^{-1}$ and $0.793 \; kg \; L^{-1}$, respectively, find the percent of methanol by weight?

• A. 15.8
• B. 16.45
• C. 20
• D. 14.8
• E. 17.6

Answer 1

Answer: (B)

Given, methanol $=20\,mL$

Water $=8\,mL$

density of methanol $=0.793\,kg L ^{-1}$ or $0.793\,gmL ^{-1}$

density of solution $=0.964\,kg L ^{-1}$ or $0.964\,gmL ^{-1}$

As, density $=\frac{\text { mass }}{\text { volume }}$

$\therefore $ mass/weight of methanol

$=0.793\,g \,mL ^{-1} \times 20 \,mL =15.86 \,g$

Similarly weight of solution

$=0.964\,g \,mL ^{-1} \times 80 \,mL =96.4 \,g$

We know that,

$\%$ by weight $=\frac{\text { weight of solute }}{\text { weight of solution }} \times 100$

$=\frac{15.86 g }{96.4\,g } \times 100=16.45$