Question

An aqueous solution of ethanol has a density of $1.025g/mL$ and concentration $2M$. What is the molality of this solution?

• A. $1.79m$
• B. $2.14m$
• C. $1.95m$
• D. None of these

Answer 1

The correct option is B $2.14m$

Let the volume of solution be 1000 mL.
Weight of solution $=1.025\times 1000=1025$ g
Molarity of ethanol is 2 M i.e it contains 2 moles per 1000 mL of the solution.
$\text{Mass of ethanol = 2 mol}\times \text{46 g/mol = 92 g}$
$\text{Mass of solvent (water) = mass of solution – mass of solute (ethanol)}$
$\text{Mass of water = 1025 g – 92 g = 933 g}$

$\text{Molality}=\frac{\text{Moles of solute}}{\text{mass of solvent (g)}}\times 1000$
$=\frac{2}{933}\times 1000=2.14m$