Question

The mole fraction of acetone in heptane is $0.25$. Its density is $0.895g/mL$. Then which of the following is/are correct:

• A. Molarity of the solution is $2.5M$
• B. Molality of the solution is $3.33m$
• C. Molality of the solution is $2.5m$
• D. Molarity of the solution is $3.33M$

Answer 1

Answer: (A), (B)

Molality of the solution is $3.33m$

Assuming, total moles of the solution $=1mol$

so, moles of acetone $=0.25mol$
molar mass of acetone $=58g/mol$
therefore, mass of acetone $=14.5g$

moles of heptane $=0.75mol$
molar mass of heptane ($C_7{H}_{16}$) $=100g/mol$
therefore, mass of heptane $=75g$

Total mass of the solution $=89.5g$
volume of the solution = $\frac{mass}{density}=\frac{89.5}{0.895}=100mL$

Molarity of the solution= $\frac{\text{moles of solute}}{\text{volume of the solution(L)}}=\frac{0.25}{0.1}=2.5M$

mass of solvent (heptane) $=89.5-14.5=75g$

Molality of the solution= $\frac{\text{moles of solute}}{\text{mass of the solvent (kg)}}=\frac{0.25}{0.075}=3.33m$