Exercise 1 solution

Formula (PH article): pH = -log10[H+], with the concentration in mol/L.

(a) [H+] = 3.2 x 10^-4 mol/L
  pH = -log10(3.2 x 10^-4) = 3.49 (3.4949 before rounding)
  Check against a rough guess: 10^-4 would give pH 4 and 10^-3 would give pH 3; 3.2 x 10^-4 is
  between them, so a pH between 3 and 4 is right. It is below 7, so the solution is acidic.

(b) [H+] = 1.0 x 10^-9 mol/L
  pH = -log10(1.0 x 10^-9) = 9.00
  Above 7, so basic (alkaline) at 25 C.

Why (b) has fewer hydrogen ions than pure water: pure water at 25 C is pH 7, which is
[H+] = 1 x 10^-7 mol/L. Here the concentration is 100 times smaller (two pH units up).

Note: the arithmetic was run in Python (check, my own calculation).