How many ml of concentrated HCL (36% HCL by mass, density= 1.18 g/my) are required to produce 10L of a solution that has a pH of 2.05?

Answer 1

HCl is a strong acid, so moles HCl equals moles of H⁺ (HCl ==> H⁺ + Cl^-)

From the pH, and the volume we can determine the moles of H+.

** pH = -log[H⁺], thus [H⁺] = 1x10^-2.05 = 8.91x10^-3 M

** 8.91x10^-3 mols/L x 10 L = 8.91x10^-2 moles of H⁺ required

Now, turning out attention to the concentrated HCl solution. 36% HCl by mass means there are 36 g HCl in 100 g of the HCl solution. We need to find the VOLUME of this solution that will provide 8.91x10^-2 (0.0891) moles of HCl

** 0.0891 mols HCl x 36.5 g / mol = 3.25 g HCl needed

** 3.25 g HCl x 100 g soln / 36 g HCl = 9.028 g of solution

** 9.028 g soln x 1 ml soln / 1.18 g = 7.65 mls solution = 7.7 mls (2 sig. figs.) or 8 mls (1 sig. fig.)

(36% HCl has 2 sig. figs. but 10 L has only 1 sig. fig., so take your pick)