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pH Calculations

Chemistry Quick Review of pH

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Litmus paper gives a rough estimate of pH.

Litmus paper gives a rough estimate of pH.

David Gould, Getty Images
Here's a quick review of how to calculate pH and what pH means with respect to hydrogen ion concentration, acids, and bases.

Acids, Bases, and pH

There are several ways to define acids and bases, but pH only refers to hydrogen ion concentration and is only meaningful when applied to aqueous (water-based) solutions. When water dissociates it yields a hydrogen ion and a hydroxide.

H2O ↔ H+ + OH-

When calculating pH, remember that [] refers to molarity, M.

Kw = [H+][OH-] = 1x10-14 at 25°C
for pure water [H+] = [OH-] = 1x10-7
Acidic Solution: [H+] > 1x10-7
Basic Solution: [H+] < 1x10-7

Calculate pH and [H+]

pH = -log10[H+]
[H+] = 10-pH

Example:

Calculate the pH for a specific [H+]. Calculate pH given [H+] = 1.4 x 10-5 M

pH = -log10[H+]
pH = -log10(1.4 x 10-5)
pH = 4.85

Example:

Calculate [H+] from a known pH. Find [H+] if pH = 8.5

[H+] = 10-pH
[H+] = 10-8.5
[H+] = 3.2 x 10-9 M

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