What it does
Ka tells you how far an acid splits into H⁺ and its conjugate base in water. Because Ka values span many powers of ten, chemists usually quote pKa instead. This calculator converts between Ka and pKa, and also gives Kb and pKb for the conjugate base (or the conjugate acid, if you start from a base). Add a concentration to estimate the pH of a weak acid or weak base solution.
How it's calculated
pKb = −log₁₀(Kb) · Kb = 10−pKb
Ka × Kb = Kw · pKa + pKb = pKw = 14.00 at 25 °C
The last line links an acid to its own conjugate base, like acetic acid and acetate, or ammonium and ammonia. It doesn't connect two unrelated compounds. pKw is 14.00 at 25 °C (a careful evaluation by Bandura and Lvov gives 13.99) and changes with temperature, so you can type a different pKw if you're working at another temperature.
Significant figures: in a logarithm, only the digits after the decimal point count. So a Ka with 3 significant figures (1.75 × 10−5) gives a pKa with 3 decimals (4.757). The calculator follows that rule and also shows the unrounded values.
Worked example
Acetic acid has Ka = 1.75 × 10−5. pKa = −log₁₀(1.75 × 10−5) = 4.757. Its conjugate base, acetate, has pKb = 14 − 4.757 = 9.243, so Kb = 5.71 × 10−10.
For 0.1 M acetic acid, solving Ka = x2 ÷ (0.1 − x) gives [H⁺] = x = 0.00131 M, a pH of about 2.88, with only 1.3% of the acid ionized.
Some pKa values at 25 °C
| Acid | pKa | Ka |
|---|---|---|
| Phosphoric acid (first H⁺) | 2.148 | 7.11 × 10−3 |
| Hydrofluoric acid | 3.17 | 6.8 × 10−4 |
| Formic acid | 3.745 | 1.80 × 10−4 |
| Benzoic acid | 4.202 | 6.28 × 10−5 |
| Acetic acid | 4.757 | 1.75 × 10−5 |
| Carbonic acid (first H⁺) | 6.352 | 4.45 × 10−7 |
| Hypochlorous acid | 7.53 | 3.0 × 10−8 |
| Ammonium ion | 9.244 | 5.70 × 10−10 |
Values from Martell and Smith's Critical Stability Constants, as tabulated in Harvey's Analytical Chemistry 2.1.
Limitations
- The pH estimate treats activity as concentration and ignores water's own ions, so it works best for concentrations well above 10−6 M.
- For acids with more than one H⁺ (phosphoric, carbonic, citric), each step has its own Ka. The pH estimate uses only the one you enter.
- Strong acids such as HCl have Ka values too large to measure well; treat them as fully ionized.