Q. 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
Step 1: Consider 1 litre of solution
Molarity = 3 M ⇒ moles of NaCl = 3
Step 2: Density = 1.25 g/mL
Mass of 1 L solution = 1.25 × 1000 = 1250 g
Step 3: Molar mass of NaCl = 58.5 g/mol
Mass of NaCl = 3 × 58.5 = 175.5 g
Step 4: Mass of solvent (water)
= 1250 − 175.5 = 1074.5 g = 1.0745 kg
Step 5: Molality = moles of solute / kg of solvent
= 3 / 1.0745 ≈ 2.79 m