Question

A 1.80 L acid solution contains 0.45 g total of HCl and HBr. The pH of the solution is 2.40.what are the masses of HCl and HBr in the solution?

Answer 1

Since pH = 2.4

So, -log [H⁺] = 2.4

So, [H⁺] = 10^-2.4 = 3.981 x 10^-3 M

= 3.981 x 10^-3 moles in 1 liter.

So, total [H⁺] present in 1.8 L of solution = 1.8 x 3.981 x 10^-3 moles

= 7.166 x 10^-3 moles

now let y gram of HCl and (0.45-y) gram of HBr are taken. then we can say that moles of HCl and HBr add up to the total moles of H⁺ present in solution i.e. 7.166 x 10^-3 moles.

molar mass of HCl = 36.5 g

molar mass of HBr = 81 g

So,

(y/36.5) + (0.45-y)/81 = 7.166 x 10^-3

81y + 16.425 - 36.5y = 36.5 x 81 x 7.166 x 10^-3

81y - 36.5y = 21.186279 - 16.425

44.5y = 4.761279

y = 0.107g HCl present.

(0.45-y) = 0.45-0.107 = 0.343g HBr present.