Question

How many grams of $C{H}_{3}OH$ is needed to make a 0.244 m solution in 400 g of water?

• A. 3.140g
• B. 0.313g
• C. 1639g
• D. 97.6g
• E. 32g

Answer 1

The correct option is A 3.140g

The molality of solution is the ratio of the number of moles of solute present in 1 kg of solvent. The number of moles of solute is the ratio of mass of solute to its molar mass.
The molar mass of methanol is 32 g/mol.
Let W g of methanol is needed.
The number of moles of methanol $=\frac{W}{32}$ mol
Mass of water $=400$ g $=0.400$ kg
(because 1 kg = 1000 g)
The molality of solution $=\frac{W}{32\times 0.400}=0.244$
$W=0.244\times 32\times 0.400=3.140$ g