For question 2, the difference between HCl and HCN is that HCl is a strong acid, and HCN is a weak acid. This means that HCl dissociates (more or less) completely in water, which means that more hydronium ions are created - you would mention this equation:
HCl(aq) + H2O(l) -> Cl-(aq) + H3O+(aq)
Meanwhile, because HCN is a weak acid, it only partially dissociates in water, meaning that far less hydronium ions ions are created - you would do a similar equation except with a reversible arrow, mentioning that the equilibrium is far shifted to the left hand side, and because the hydronium concentration is much lower, the pH is much higher (pH=-log[H3O+])