JEE Main 2025 · 22 January · evening · Single correct · +4 / −1
Density of 3 M NaCl solution is 1.25 g / mL. The molality of the solution is :
- (a) 2.79 m
- (b) 2 m
- (c) 1.79 m
- (d) 3 m
Answer and solution
Answer (a) 2.79 m
Solution
Chemistry ma’am · 4 steps
Take exactly 1 L of solution, then remove the salt's mass to find how much water is really there.
- Pick 1 litre. 3 M means 3 mol of NaCl in 1 L of solution. With density 1.25 g mL⁻¹, that 1000 mL of solution weighs 1250 g.
- Mass of solute. The molar mass of NaCl is 58.5 g mol⁻¹, so 3 mol weighs 3 × 58.5 = 175.5 g.
- Mass of solvent. Molality needs the mass of solvent only: 1250 - 175.5 = 1074.5 g = 1.0745 kg of water.
m = (3 mol)/(1.0745 kg) = 2.79 mol kg⁻¹
- So. The molality is 2.79 m, which is option (a).
The idea to keep. Going from molarity to molality, assume 1 L of solution, get its mass from density, and subtract the solute's mass. Molality always divides by the solvent's mass alone.
Answer from the NTA final answer key. Solution by PrepWiser.