A commercially sold conc. HCl is 35% HCl by mass. If the density of this commercial acid is 1.46 g/mL, the molarity of this solution is:
(Atomic mass : Cl = 35.5 amu, H = 1 amu)

Solution

Chemistry ma’am · 3 steps

Take exactly 1 L of the acid; the density gives its mass, and the 35% gives the HCl inside it.

1. Mass of 1 L. Molarity is moles of solute per litre of solution, so work with 1000 mL. Its mass is 1000 × 1.46 = 1460 g.
2. Mass of HCl. 35% of that mass is HCl: 0.35 × 1460 = 511 g. The molar mass of HCl is 1 + 35.5 = 36.5 g mol⁻¹.

   M = (511 g)/(36.5 g mol⁻¹ × 1 L) = 14.0 mol L⁻¹

3. So. The molarity is 14.0 M, which is option (c). The shortcut M = (10 × d × w%)/(molar mass) = (10 × 1.46 × 35)/36.5 gives the same answer.

The idea to keep. Mass percent and density together give molarity. Fix the volume at 1 L, use density to get its mass, then take the percent of that mass as solute.

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