Question

Determine the pH of a solution containing 1.00 mol/L of HCN (Ka = 5.00×10–10) and 5.00×10–2 mol/L of HCl.

Answer 1

HCl is a strong acid and the H^+ will be 5.00E-2 for the HCl.

HCN is a weak acid. Calculate the H^+ contribution from that(decreased by the HCl as a common ion), then add the two together.
Total H^+ = H^+ from HCl + H^+ from HCN. Convert to pH.