pH
Acidity from hydrogen-ion concentration.
Formula
Variables
Example
pH of 0.01 M acid = 2
How it works
pH is a logarithmic way to express how much hydrogen ion, H+, is present in a solution. Because each 1-unit change in pH means a tenfold change in H+ concentration, the formula compresses very large concentration ranges into an easy-to-read scale. The negative sign makes acidic solutions, which have higher H+ concentrations, come out with lower pH values.
When to use it
Use this formula when you know the hydrogen-ion concentration of a solution and want to find its acidity on the pH scale. It is commonly used in chemistry labs, environmental testing, biology, and anything involving acids, bases, or aqueous solutions.
Worked example, step by step
Given: [H+] = 3.2 × 10^-5 mol/L Step 1: Substitute into the formula: pH = -log10[H+] Step 2: pH = -log10(3.2 × 10^-5) Step 3: log10(3.2 × 10^-5) = log10(3.2) + log10(10^-5) ≈ 0.505 - 5 = -4.495 Step 4: pH = -(-4.495) = 4.495 Final answer: pH ≈ 4.50
Common mistakes to avoid
- Forgetting the negative sign in front of the logarithm
- Using natural log (ln) instead of base-10 log
- Confusing hydrogen-ion concentration with pH itself
Did You Know?
Each pH unit is a tenfold change — stomach acid (pH 2) is about 100,000 times more acidic than blood (pH 7.4).
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