If the [H+] of a solution is 0.001 mol/L what is the pH?
A. 12
B. -2
C. 1
D.2

pH = 3 (though the option is missing)

Explanation:

We have a solution with a concentration of the hydrogen ion, [H⁺] = 0.001 mol/L = 0.001 M. We can calculate the pH using its definition.

pH = -log [H⁺]

pH = -log 0.001

pH = 3

Since the pH < 7, the solution is acid. Other way to recognize that a solution is acid is when [H⁺] > 10⁻⁷ M at 25°C.