Question

Explain molarity and molality by example.

Hint:

Molarity depends on the volume of solution, hence changes with temperature.
Molality depends on the mass of solvent, hence remains constant with temperature.

Answer 1

Molarity (M):
Molarity is defined as the number of moles of solute present in 1 litre of solution.
\[ M = \frac{\text{moles of solute}}{\text{volume of solution in litres}} \]
Example:
If 58.5 g of NaCl (molar mass = 58.5 g/mol) is dissolved in water to make 1 litre of solution, then:
\[ M = \frac{1}{1} = 1 \; \text{M solution of NaCl} \]
--- Molality (m):
Molality is defined as the number of moles of solute present in 1 kg of solvent.
\[ m = \frac{\text{moles of solute}}{\text{mass of solvent in kg}} \]
Example:
5.0 g of ethanoic acid (\(CH_3COOH\), molar mass = 60 g/mol) is dissolved in 150 g (0.150 kg) of benzene:
\[ n = \frac{5.0}{60} = 0.0833 \, \text{mol} \]
\[ m = \frac{0.0833}{0.150} = 0.5567 \, \text{mol/kg} \]
So, the molality is \(0.557 \, m\).