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The pH scale is defined as pH = -log10[H+]. In pure water at 25°C, [H+][OH-] = Kw = 1.0 x 10-14, so pH + pOH = 14.
Strong acids (HCl, HNO3, H2SO4) dissociate completely in water: a 0.01 M HCl solution has [H+] = 0.01 M = 10-2 M, giving pH = -log10(10-2) = 2.0. Neutral solutions have pH = 7.0; acidic < 7.0; basic > 7.0.
Worked example
1.A 0.01 M solution of strong acid HCl has a pH of 2.0.
2.A solution with pH = 9.0 is acidic.
3.In water at 25°C, the sum of pH and pOH always equals 14.0.
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