Weak Acid pH Calculator
pH and ionization for five common weak acids at any concentration.
What it does
Weak acids split partly: most molecules stay whole while a small share frees hydrogen ions. The equilibrium constant Ka with the starting concentration sets that share through a square root, then pH follows by logarithm.
The tool above takes one of five common acids with its molarity. It returns pH with hydrogen concentration and ionized percent beside them.
The default 0.1 M acetic acid (Ka 1.8e-5) gives root(1.8e-6) = 1.34e-3 M for pH 2.87 at 1.34% ionized. Formic at the same strength reads pH 2.37 with 4.24% ionized.
Equilibrium follows OpenStax Chemistry acid chapters with Khan Academy acid base units matching the approximation. Both restrict it to cases where ionization stays small.
How to use
- Pick the acid for its Ka value.
- Enter the starting molarity.
- Read pH first for the solution.
- Read ions and percent for the split.
- Distrust results past a few percent ionized.
Formula
- Ka
- Acid equilibrium constant of the pair.
- C
- Starting molarity before splitting.
- root
- Square root from the equilibrium algebra.
Valid while ionized share stays small (the 5% rule); past it, solve the quadratic exactly. Polyprotic acids use Ka1 here for the first step only.
Worked example
Example: 0.1 M acetic acid
Defaults: acetic Ka 1.8e-5, 0.1 M.
- Product: 1.8e-5 x 0.1 = 1.8e-6.
- Ions: root(1.8e-6) = 1.34e-3 M.
- pH: -log10(1.34e-3) = 2.87.
- Share: 1.34e-3 / 0.1 = 1.34%.
- Tool shows 2.87, 1.34e-3 M, 1.34%.
Reference table
| Acid | Ka | pH |
|---|---|---|
| Acetic | 1.8e-5 | 2.87 |
| Benzoic | 6.3e-5 | 2.60 |
| Formic | 1.8e-4 | 2.37 |
| Hydrofluoric | 6.8e-4 | 2.08 |
Limits
- Small ionization assumed (5% rule).
- First step only for polyprotic acids.
- Ideal dilute behavior assumed.
- Temperature shifts Ka slightly.
Common mistakes
- Using strong acid math (full split) instead.
- Entering percent instead of molarity.
- Trusting results past 5% ionized.
- Mixing Ka values between temperatures.
- Applying Ka1 to later buffer steps.
Try it yourself
Quick checks. Answers sit tucked under each question.
0.1 M formic?
root(1.8e-5) = 4.24e-3 M, pH 2.37.
0.01 M acetic?
root(1.8e-7) = 4.24e-4 M, pH 3.37.
Dilute 10x effect?
pH rises 0.5 (root of ten).
Questions
How do you find weak acid pH?
Root of Ka times concentration, then negative log: 0.1 M acetic is 2.87.
Why only partly ionized?
Equilibrium balances split molecules against whole ones at Ka.
What is the 5% rule?
Approximation holds while ionized share stays under 5%.
Strong vs weak acid?
Strong splits fully (pH from concentration); weak splits partly (pH from Ka).
Does dilution raise pH?
Yes by half a unit per tenfold for weak acids.
Which acid is strongest here?
Hydrofluoric at Ka 6.8e-4 among the five.