[H⁺] of a solution with a pH of 7.9

In this problem we are given pH and asked to solve for the hydrogen ion concentration. Using the equation, pH = − log [H+] , we can solve for [H+] as,

− pH = log [H+] ,

[H+] = 10−pH,

by exponentiating both sides with base 10 to "undo" the common logarithm. The hydrogen ion concentration of "Solution A" is,

[H+] = 10−5.6 ≈ 0.0000025 = 2.51 × 10−6 M.

We can always check our solution by computing −log (2.5 × 10−6) ≈ 5.6