The molarity of a 70 % (mass/mass) aqueous solution of a monobasic acid (X) is _________ × 10⁻¹ M (Nearest integer)
[Given: Density of aqueous solution of (X) is 1.25 g mL⁻¹
Molar mass of the acid is 70 g mol⁻¹ ]

Solution

Chemistry ma’am · 3 steps

Take 100 g of solution, and let the density turn that mass into a volume in litres.

1. Pick a sample. Take 100 g of solution. At 70% (mass/mass) it holds 70 g of acid X, which is 70/70 = 1 mol.
2. Find the volume. Molarity needs the volume of the solution, not of the water. Volume = (100 g)/(1.25 g mL⁻¹) = 80 mL = 0.080 L.

   M = (1 mol)/(0.080 L) = 12.5 M

3. So. 12.5 M = 125 × 10⁻¹ M, so the answer is 125.

The common slip: 100. Taking the solution volume as 100 mL and ignoring the density gives 10 M, which is 100, not 125.

The idea to keep. To go from mass percent to molarity, use M = (10 × w% × d)/Mₛₒₗᵤₜₑ with d in g mL⁻¹. Molarity always counts litres of solution.

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