Phenol has a pKa of 10.0. A solution is made by dissolving 0.050 mol of phenol in water to make 1.0…

Step 1. The constant. $K_a = 10^{-10.0} = 1.0 \times 10^{-10}$.

Step 2. The hydrogen ions. $[\text{H}^+] = \sqrt{1.0 \times 10^{-10} \times 0.050} = \sqrt{5.0 \times 10^{-12}} = 2.2 \times 10^{-6}\ \text{mol/L}$.

Step 3. The pH. $\text{pH} = -\log_{10}(2.2 \times 10^{-6}) = 5.65$.