ChemTeam Video: pH - XVI - Mixture of two strong acids

A 2.13 L acid solution contains 0.360 g total of HCl and HBr. The pH of the solution is 2.440. What are the masses of HCl and HBr in the solution?

Solution:

[H+] = 10-pH = 10-2.440 = 3.63 x 10-3 mol/L

Let the mass of HCl = x, mass of HBr = (0.360 - x)

HCl(aq) -----> H+(aq) + Cl-(aq)

mole of HCl = mole of H+ in HCl = mass / molecular mass = x/36.5 g/mol

HBr(aq) -----> H+(aq) + Br-(aq)

mole of HBr = mole of H+ in HBr = (0.360 - x) / 80.9 g/mol

Concentration of TOTAL H+ in the solution = 3.63 x 10-3 mol/L

Apply the formula for molarity. That is molarity = mole / volume

[(x/36.5) + (0.360 - x)/80.9] / 2.13 = 3.63 x 10-3

Solve for x.

x = 0.218

Mass of HCl = 0.218 g, mass of HBr = 0.142 g