Question

What is the pH of a 8.2× 10–8 M solution of HCl(aq) at 25 °C? Report your answer to the hundredths place.

Answer 1

Initial concentration of H⁺ = 8.2 x 10^-8 M

Considering the dissociation of water

              H₂O                      H⁺           +       OH^-

Initial ..................................................8.2 x 10^-8 ..............0

Equilibrium ....................................8.2 x 10^-8 + x..............x

Kw = [H⁺] x [OH^-] = (8.2 x 10^-8 + x)x = 1 x 10^-14

x² + 8.2 x 10^-8 x - 1 x 10^-14 = 0

On solving,

x = 6.7078 x 10^-8 M

Total H⁺ = 8.2 x 10^-8 + 6.7078 x 10^-8 = 1.49 x 10^-7 M

pH = -log[H⁺]= - log (1.49 x 10^-7 ) = 6.82