Answer
$pH=12$
Work Step by Step
The formula for the pH of a substance, as a function of hydrogen ion concentration ($H^{+}$) is (pg. 465):
$pH=-\log_{10}[H^{+}]$
Applying this to our case ($H^{+}=10^{-12}$), we get:
$pH=-\log_{10}[H^{+}]=-\log_{10}{10^{-12}}=-(-12)\log_{10}{10}=12$
Here we used the power property of logarithms (pg. 462):
$\log_b{m^n}=n\log_b{m}$
We also used the fact that $\log_{10}{10}=1$ (because $10^1=10$).
