10.0 mL of 0.10 mol/L hydrochloric acid is made up to 1.00 L with water…

$K_w = [\text{H}_3\text{O}^+][\text{OH}^-] = 1.0 \times 10^{-14}$ at $298\ \text{K}$, and it grows with temperature. $\text{pH} = -\log_{10}[\text{H}_3\text{O}^+]$, $\text{pOH} = -\log_{10}[\text{OH}^-]$, and $\text{pH} + \text{pOH} = \text{p}K_w$, which is 14 at $298\ \text{K}$. A strong acid or base ionises fully. Neutral means $[\text{H}_3\text{O}^+] = [\text{OH}^-]$.