The density of NaOH solution is 1.2 g cm⁻³. The molality of this solution is _____________ m. (Round off to the Nearest Integer)
[Use : Atomic mass : Na : 23.0 u, O : 16.0 u, H : 1.0 u, Density of H₂O : 1.0 g cm⁻³]

Solution

Chemistry ma’am · 4 steps

Take 1 L of solution and treat its volume as the water's volume; the extra mass is the NaOH.

1. Fix the volume. Take 1000 cm³ of solution. Its mass is 1000 × 1.2 = 1200 g.
2. Water and NaOH. The density of water is given because the solid NaOH is taken to add no volume, so the water also fills 1000 cm³. That is 1000 g of water, which leaves 1200 - 1000 = 200 g of NaOH.
3. Moles of NaOH. The molar mass of NaOH is 23 + 16 + 1 = 40 g mol⁻¹, so there are 200/40 = 5 mol of NaOH.

   m = nₛₒₗᵤₜₑ/(mass of solvent in kg) = (5 mol)/(1 kg) = 5 mol kg⁻¹

4. So. The molality of the solution is 5 m.

The common slip: 4. Dividing the 5 mol by the whole 1.2 kg of solution instead of the 1 kg of water gives 4.17, which rounds to 4.

The idea to keep. Molality divides by the mass of the solvent only. When a question gives a solution density along with the water density, take a fixed volume and subtract the water's mass to find the solute's mass.

Answer from the NTA final answer key. Solution by PrepWiser.