Calculate the hydronium ion and hydroxide ion concentrations in human blood at 25oc

Using the equation, pH = - log [H+], we can solve for [H+] as,

- pH = log [H+],

[H+] = 10-pH.

Exponentiate both sides with base 10 to "undo" the common logarithm. The hydrogen ion concentration of blood with pH 7.4 is,

[H+] = 10-7.4 ≈ 0.0000040 = 4.0 * 10-8 M.