Molality
- Molality (m) can be defined as the ratio of number of moles of solute to the mass of solvent in kg.
- It can also be defined as the number of moles of solute present in unit kilogram of solvent.
- Mathematically, Molality (m) = Moles of Solute / Mass of Solvent in Kg
- For instance, molality of a solution containing 74.5 g (1 mol) of KCl dissolved in 1 kg of water is 1.00 m or 1.00 mol kg–1.
Problem: Calculate the mass of urea (NH2CONH2) required in making 2.5 kg of 0.25 molal aqueous solution.
Solution:
Molar mass of urea (NH2CONH2) = 2 (1 × 14 + 2 × 1) + 1 × 12 + 1 × 16 = 60 g mol - 1
0.25 Molar aqueous solution of urea means 1000 g of water contains 0.25 mol = (0.25 × 60) g of urea = 15 g of urea
(1000 + 15) g of solution contains 15 g of urea.
Therefore, 2.5 kg (2500 g) of solution contains = (15 X 2500) / (1000+15) g = 36.95 g = 37 g of urea (approximately).
Mass of urea required = 37 g
