pH calculator
Strong and weak acids and bases: choose the species, type the concentration and get pH, pOH, [H⁺], [OH⁻] and how much of it ionizes. Free in Periodic Table and Chemistry for iPhone, iPad and Mac.
How to calculate pH
- Strong acid: it ionizes completely, so [H⁺] equals its concentration times the H⁺ it releases. pH = −log[H⁺]. For 0.01 M HCl, pH = 2.00.
- Weak acid: only part of it ionizes. With Ka = 10^(−pKa), solve x² / (C − x) = Ka for x = [H⁺].
- Quick estimate for a weak acid: pH ≈ ½ (pKa − log C).
- pOH = 14 − pH at 25 °C, because Kw = 1.0 × 10⁻¹⁴.
Worked example: 0.1 M acetic acid (pKa 4.74)
| Quantity | Value |
|---|---|
| Quick estimate | pH ≈ ½ (4.74 + 1) = 2.87 |
| [H⁺] | 1.33 × 10⁻³ mol/L |
| [OH⁻] | 7.50 × 10⁻¹² mol/L |
| pOH | 11.12 |
| Ionized | 1.33 % |
| pH | 2.88 |
The quick estimate gives 2.87. The app solves the full charge balance, including the ionization of water, and gets 2.88: that is why a very dilute acid never goes past pH 7.
What the app does for you
- Strong and weak acids and bases, with the number of H⁺ or OH⁻ per formula (HCl 1, Ba(OH)₂ 2).
- Pick a weak acid or base and its pKa comes from the app’s acids and bases table, or type your own pKa or pKb.
- pH, pOH, [H⁺], [OH⁻] and percent ionization at once, and a history of your calculations.
- With Pro: buffers, with the acid and base forms of the pair (Henderson–Hasselbalch).
Questions.
- What is pH?
- pH = −log[H⁺], the negative logarithm of the hydrogen ion concentration in mol/L. At 25 °C, 7 is neutral, below 7 acidic, above 7 basic.
- Is the pH calculator free?
- Yes. Strong and weak acids and bases are free in the app. Buffers are part of Pro.
- Why can’t a very dilute acid reach pH 7?
- Because water itself ionizes and gives 1.0 × 10⁻⁷ mol/L of H⁺. The app includes it in the balance, so an acid, however dilute, stays below 7.
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