pH Calculator
Calculate pH, pOH, [H+], and [OH-] for acid-base problems.
Solve acid-base problems in five ways: strong acids, strong bases, weak acids from Ka, weak bases from Kb, or a straight conversion between pH, pOH, [H+], and [OH-]. Weak acid and base modes solve the equilibrium quadratic exactly.
All relationships assume 25 °C, where Kw = 1.0 × 10⁻¹⁴ and a neutral solution sits at pH 7.00.
How to Use the pH Calculator
1
Choose the mode. Pick strong acid, strong base, weak acid with Ka, weak base with Kb, or a straight conversion from a known pH, pOH, [H+], or [OH-].
2
Enter the values. Type the concentration with its unit and, for weak acids and bases, the Ka or Kb value — or pick one from the constant library.
3
Read all four quantities. The result reports pH, pOH, [H+], and [OH-] together with the worked steps and, for weak species, the percent ionisation.
What is the pH Calculator?
The pH calculator converts between pH, pOH, hydrogen ion concentration [H+], and hydroxide concentration [OH-] using the standard relationships pH = -log[H+] and pH + pOH = 14 at 25 °C. Enter any one value and get the other three instantly.
pH problems appear in every general chemistry course, from strong acid dilutions to titration setups. Moving comfortably between concentration and the log scale is the skill this tool reinforces: each result shows the conversion so you can reproduce it by hand in an exam.
Common Uses
- Convert [H+] concentration to pH for acid solutions
- Find pOH and [OH-] from a known pH
- Check acid-base homework answers
- Classify solutions as acidic, neutral, or basic
How to Solve It by Hand
Manual calculation is still important: identify the known variables, convert units before substitution, apply the equation, and check whether the result is chemically reasonable. The most common mistakes are inconsistent units, constants rounded too early, and skipping the interpretation step.
Practice Prompt
Try changing the default values and ask the lower-right chemistry chat why the result increased or decreased. That turns the calculator from a number machine into a study loop.
For a strong acid that fully dissociates, pH = -log[H+], where [H+] is the molar concentration multiplied by the number of ionisable hydrogens. A 0.01 M HCl solution gives pH = -log(0.01) = 2.00.
pH measures hydrogen ion concentration and pOH measures hydroxide ion concentration. At 25 °C they always add to 14, so pOH = 14 - pH. A pH of 3 corresponds to a pOH of 11.
A weak acid only partly dissociates, so you need its Ka. Set up an ICE table, substitute into Ka = x²/(C - x), and solve for x, which is [H+]. This calculator solves that quadratic exactly rather than using the x much less than C shortcut, so the answer stays correct even when ionisation exceeds 5 percent.
Enter the number of hydrogens the acid releases under the conditions you are modelling. H2SO4 is treated as 2 for its first two dissociations in dilute solution; for weak polyprotic acids such as H3PO4 use the stepwise Ka values instead, one at a time.
Only at 25 °C. Kw rises with temperature, so pure water at 50 °C is neutral at about pH 6.63 even though [H+] still equals [OH-]. Every calculation here assumes 25 °C, where Kw = 1.0 × 10⁻¹⁴.