Acid Base Calculator
This acid base calculator works out the pH and pOH of an acid or base solution from its concentration and how strongly it dissociates. Choose whether you are dealing with a strong acid, a weak acid, a strong base or a weak base, enter the molar concentration, and for a weak substance enter its dissociation constant, Ka for an acid or Kb for a base. For a strong acid or base the calculator treats dissociation as complete, so the hydrogen or hydroxide ion concentration equals the concentration you entered. For a weak acid or base it uses the standard approximation that the ion concentration is the square root of the dissociation constant times the concentration, valid when the amount that dissociates is small compared with the starting concentration. From there it reports the pH, the pOH, and both the hydrogen ion and hydroxide ion concentrations, plus whether the solution is acidic, neutral or basic. Chemistry students meet this calculation constantly when predicting the pH of a prepared solution, and it saves a lot of log work during problem sets and lab prep. The approximation is the one taught in first year courses; for very dilute or very weak systems, or for polyprotic acids, a fuller treatment is needed. Pair it with our pH and pOH tools when you already know the ion concentration and just want the conversions.
Strong acids and bases are assumed to dissociate fully. Weak acids and bases use the small dissociation approximation, ion concentration equals the square root of the constant times the concentration. The dissociation constant is ignored for strong types. Values are for 25 degrees Celsius. Estimate only.
How it works
For a strong acid the hydrogen ion concentration equals the concentration you enter, because dissociation is complete, and the pH is the negative base ten logarithm of that value. For a weak acid the hydrogen ion concentration is the square root of Ka times the concentration, a good approximation while only a small fraction dissociates, and again pH is the negative log. Bases are handled through the hydroxide ion the same way: strong bases give a hydroxide concentration equal to the concentration, weak bases use the square root of Kb times the concentration, and the pOH is the negative log of that. The pH and pOH always add to 14 at 25 degrees, and the two ion concentrations multiply to the water constant of one times ten to the minus fourteen, so the calculator fills in whichever pair you did not start from.
Worked example
Take a 0.1 mol per litre solution of acetic acid, a weak acid with a Ka of 1.8 times ten to the minus five. The hydrogen ion concentration is the square root of 1.8 times ten to the minus five multiplied by 0.1, which is the square root of 1.8 times ten to the minus six, about 1.34 times ten to the minus three mol per litre. The pH is the negative log of that, about 2.87, so the solution is clearly acidic. The pOH is 14 minus 2.87, which is 11.13, and the hydroxide concentration is one times ten to the minus fourteen divided by the hydrogen concentration, about 7.45 times ten to the minus twelve mol per litre. A strong acid at the same 0.1 mol per litre would instead give a pH of exactly 1.
Related calculators
- pH Calculator: convert between pH and hydrogen ion concentration.
- pH and pOH Calculator: pH, pOH and both ion levels.
- pOH Calculator: pOH from hydroxide concentration.
- Buffer pH Calculator: pH of a buffer with Henderson Hasselbalch.