pH Calculator
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pH measures how acidic a solution is: pH = −log[H+]. At 25 °C, pH + pOH = 14, so a 0.01 M solution of hydrochloric acid has pH 2.00 and pOH 12.00. Choose what you know and the calculator does the rest.
Type 1e-5 for 1 × 10⁻⁵.
1 for HCl or NaOH, 2 for Ca(OH)₂.
Acetic acid Ka = 1.8 × 10⁻⁵; ammonia Kb = 1.8 × 10⁻⁵.
| Result | |
| pH | 2.00 |
| pOH | 12.00 |
| [H⁺] | 0.01 mol/L |
| [OH⁻] | 1 × 10⁻¹² mol/L |
| The solution is | acidic |
Show steps
- A strong acid ionizes completely, so [H⁺] = 1 × 0.01 = 0.01 M.
- pH = −log[H⁺] = −log(0.01) = 2.00.
- pOH = 14 − pH = 12.00.
How to use the pH calculator
- Pick what you know: the concentration of a strong acid or base, a weak acid or base with its Ka or Kb, an ion concentration, or a pH or pOH.
- Enter the numbers. For very small numbers type 1e-5 for 1 × 10−5.
- Press Calculate to get pH, pOH, [H+] and [OH−]. Open Show steps to see the work.
pH formulas
- pH = −log[H+] and [H+] = 10−pH
- pOH = −log[OH−] and [OH−] = 10−pOH
- pH + pOH = 14 and [H+][OH−] = 1.0 × 10−14 at 25 °C
Each step of 1 on the pH scale is a factor of 10 in [H+]. A solution with pH 3 has 10 times more hydrogen ions than one with pH 4.
Strong and weak acids
A strong acid such as HCl splits completely into ions, so [H+] equals the acid concentration. A weak acid such as acetic acid only partly ionizes. Its equilibrium is Ka = x² ÷ (C − x), where x is [H+]. The calculator solves this quadratic exactly instead of using the shortcut x ≈ √(Ka·C), which is only close when less than about 5% of the acid ionizes.
| Solution | Work | pH |
|---|---|---|
| 0.01 M HCl | −log(0.01) | 2.00 |
| 0.001 M NaOH | pOH = 3, pH = 14 − 3 | 11.00 |
| 0.10 M acetic acid, Ka = 1.8 × 10−5 | x = 1.33 × 10−3 | 2.88 |
| 0.05 M Ca(OH)2 | [OH−] = 0.10, pOH = 1 | 13.00 |
The pH scale
| pH | Example | Type |
|---|---|---|
| about 1 to 2 | Stomach acid | Strongly acidic |
| about 2.5 | Lemon juice | Acidic |
| 7 | Pure water at 25 °C | Neutral |
| about 7.4 | Blood | Slightly basic |
| about 11 | Household ammonia | Basic |
| about 13.5 | Lye solution | Strongly basic |
Below 7 is acidic, 7 is neutral and above 7 is basic. The neutral point of 7 holds at 25 °C; it shifts slightly at other temperatures.
pH practice questions
Test yourself with 6 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.
Acetic acid is weak, so only about 1% of it gives up H+. Fewer hydrogen ions means a higher pH (2.88 against 1.00).
Quiz complete
Frequently asked questions
How do you calculate pH?
Take the negative base-10 logarithm of the hydrogen ion concentration: pH = −log[H⁺]. For 0.0001 M H⁺, pH = −log(10⁻⁴) = 4.
How do you find pH from pOH?
At 25 °C, pH = 14 − pOH. A pOH of 3 gives a pH of 11.
How do you find the pH of a weak acid?
Solve Ka = x² ÷ (C − x) for x, the hydrogen ion concentration, then take −log x. For 0.10 M acetic acid with Ka 1.8 × 10⁻⁵, x = 1.33 × 10⁻³ M and pH = 2.88.
Can pH be negative?
Yes. A strong acid more concentrated than 1 M has [H⁺] above 1, so −log[H⁺] is below 0. For example 2 M HCl has pH about −0.3 by the simple formula.
Sources
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