pH Formula and pH Calculator
The pH Formula and pH Calculator
pH and pOH formulas for strong and weak acids and bases, with worked examples and a strong-acid pH calculator.
What is pH?
pH expresses acidity as the negative logarithm of the H+ concentration. At 25 °C pH 7 is neutral, below 7 is acidic and above 7 is basic. Each unit drop in pH means ten times more H+.
pH and pOH formulas
| Solution type | Concentration formula |
|---|---|
| Strong acid (HCl, HNO3, H2SO4) | [H+] = a × M |
| Weak acid (CH3COOH, HF) | [H+] = √(Ka × M) |
| Strong base (NaOH, KOH, Ca(OH)2) | [OH−] = b × M |
| Weak base (NH3) | [OH−] = √(Kb × M) |
| Buffer | pH = pKa + log([salt] ÷ [acid]); see buffer solutions |
Worked examples
Example 1: strong acid
What is the pH of 0.01 M HCl?
- [H+] = 1 × 0.01 = 10−2 M
- pH = 2
Answer: pH 2.
Example 2: diprotic strong acid
What is the pH of 0.005 M H2SO4 (assuming full ionisation)?
- [H+] = 2 × 0.005 = 0.01 M
- pH = 2
Answer: pH 2.
Example 3: strong base
What is the pH of 0.001 M NaOH?
- [OH−] = 0.001 M, so pOH = 3
- pH = 14 − 3 = 11
Answer: pH 11.
Example 4: weak acid
What is the pH of 0.1 M acetic acid (Ka = 1.8 × 10−5)?
- [H+] = √(1.8 × 10−6) = 1.342 × 10−3 M
- pH = 2.87
Answer: pH 2.87.
Frequently asked questions
How do I calculate pH without a scientific calculator?
For 10−n, pH = n. For x × 10−n, pH = n − log x (log 2 = 0.30, log 5 = 0.70).
Can pH be negative?
Yes, for very concentrated strong acids; 2 M HCl has a theoretical pH of −0.3. The calculator above is limited to pH above 0.
How is pH measured?
Universal indicator paper for a quick estimate and a pH meter for results to 0.01.
Why is H2SO4 not exactly twice?
Its second ionisation is incomplete (Ka2 ≈ 1.2 × 10−2), so in more concentrated solutions [H+] is slightly below 2 × M.
Prepare solutions with analytical-grade reagents
AMI Scientific is one of the official TCI distributors in Indonesia. Reagents with lot CoA & SDS, formal quotations and tax invoices for universities and laboratories.
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