The molality of a 10 %(v / v) solution of di-bromine solution in CCl₄ (carbon tetrachloride) is 'x'. x= ____________ × 10⁻² M. (Nearest integer)
[Given : molar mass of Br₂=160 g mol⁻¹
atomic mass of C=12 g mol⁻¹
atomic mass of Cl=35.5 g mol⁻¹
density of dibromine =3.2 g cm⁻³
density of CCl₄=1.6 g cm⁻³]

Solution

Chemistry ma’am · 4 steps

Take 100 mL of solution: 10 mL is bromine, and only the other 90 mL is the solvent.

1. Read the label. A 10% (v/v) solution has 10 mL of Br₂ in every 100 mL of solution. Taking the volumes as additive, the solvent CCl₄ is 100 - 10 = 90 mL.
2. Moles of solute. Mass of Br₂ = 10 mL × 3.2 g cm⁻³ = 32 g. Moles = 32/160 = 0.2 mol.
3. Mass of solvent. Mass of CCl₄ = 90 mL × 1.6 g cm⁻³ = 144 g = 0.144 kg. The molar mass of CCl₄ is not needed, because molality only uses the mass of the solvent.

   m = n_(Br₂)/(w_(CCl₄) (kg)) = 0.2/0.144 = 1.389 mol kg⁻¹

4. So. x = 1.389 = 138.9 × 10⁻², which rounds to 139.

The common slip: 125. Taking all 100 mL as CCl₄ makes the solvent 160 g and gives 0.2/0.16 = 1.25, that is 125, not 139.

The idea to keep. For any v/v or w/w percentage, assume 100 units of solution and split it into solute and solvent first. Molality always divides by the solvent's mass in kg, never the solution's.

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