Answer
(a) $$ H_2O(l) + H_2O(l) \leftrightharpoons H_3O^+(aq) + OH^-(aq)$$
(b) $$K = [H_3O^+][OH^-]$$
(c) $K_w$ is the special symbol for this specific constant.
(d) At room temperature, $K_w = 1.0 \times 10^{-14}$, so: $$1.0 \times 10^{-14} = [H_3O^+][OH^-]$$
At pure water: $$[H_3O^+] = [OH^-] = 1.0 \times 10^{-7} M$$
(e) The definition of acidic and basic is directly related to the relation between the concentrations of $H_3O^+$ and $OH^-$, if $[H_3O^+] > [OH^-]$, the solution is acidic, if $[OH^-] > [H_3O^+]$, the solution is basic.
