pH calculator

A pH calculator works out how acidic or alkaline a solution is from its concentration. pH = −log₁₀[H⁺], so a solution with [H⁺] = 0.01 mol/L has a pH of 2. Our pH Calculator handles strong and weak acids and bases, buffers, and finding Ka from a measured pH, and shows the answer on a pH scale with every step.

It opens on 0.1 mol/L ethanoic acid (acetic acid in the US), a weak acid, and common acids and bases with their Ka and Kb values are built in.

Try it

pH Calculator

Calculate pH, pOH and Ka for strong and weak acids

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How to use the pH calculator

1. Choose what you have: Strong acid, Strong base, Weak acid, Weak base, Find Ka from pH, Buffer, or Convert pH, pOH, [H⁺], [OH⁻].
2. For a strong acid or base, pick it from the list, such as HCl or Ca(OH)₂, so the calculator knows how many H⁺ or OH⁻ ions each one releases, and type the concentration.
3. For a weak acid or base, pick one from the list to fill in its Ka or Kb, or type your own value. You can enter K or pK.
4. Read the pH, pOH, [H⁺] and [OH⁻], and the pH scale showing where your solution sits.
5. Follow the working, which shows each step from the concentration to the pH.

For a weak acid, the calculator uses the usual approximation, then checks it against the exact quadratic and says whether it holds. It also shows what share of the acid is ionised, on a species graph. Concentrations are in mol/dm³, which is the same as mol/L. For practice, Practice and help sets random pH questions with worked solutions.

The pH formulas

pH 7 is neutral at 25 °C, below 7 is acidic and above 7 is alkaline (basic). Each step of 1 on the pH scale is a factor of 10 in [H⁺], so pH 3 is ten times more acidic than pH 4.

pH of a strong acid or base: the pH calculator from molarity

A strong acid or base ionises completely, so [H⁺] or [OH⁻] comes straight from its concentration.

Example 1: a strong acid

Find the pH of 0.025 mol/L hydrochloric acid. HCl releases one H⁺ each, so [H⁺] = 0.025:

Example 2: a strong base that releases two OH⁻

Find the pH of 0.010 mol/L calcium hydroxide, Ca(OH)₂. Each one releases two OH⁻, so [OH⁻] = 0.020:

Choose Ca(OH)₂ in the list and the calculator uses 2 OH⁻ for you. For very dilute solutions, below about 10⁻⁶ mol/L, it also counts the H⁺ from water itself: 1 × 10⁻⁸ mol/L HCl gives a pH of 6.98, not 8, which would wrongly make an acid alkaline.

pH of a weak acid or base

A weak acid only partly ionises, so you need its acid dissociation constant, Ka. For a weak acid HA at concentration c, assuming only a little ionises:

Example 3: ethanoic acid

Find the pH of 0.20 mol/L ethanoic acid, with Ka = 1.74 × 10⁻⁵.

Choose Weak acid, pick ethanoic acid and type 0.2. The calculator gives pH 2.73, confirms the approximation holds, and shows that only 0.93% of the acid has ionised.

Example 4: a weak base

For a weak base, use Kb to find [OH⁻] the same way, then the pOH, then the pH. For 0.15 mol/L ammonia, Kb = 1.8 × 10⁻⁵, so [OH⁻] = √(1.8 × 10⁻⁵ × 0.15) = 1.64 × 10⁻³, pOH = 2.78 and pH = 11.22. The calculator gives 11.21 from the exact quadratic, with 11.22 from the approximation alongside.

Buffer pH calculator: Henderson-Hasselbalch

A buffer is a weak acid mixed with its conjugate base, which keeps the pH steady when small amounts of acid or alkali are added. Its pH comes from the Henderson-Hasselbalch equation:

Example 5: an ethanoate buffer

A buffer contains 0.10 mol/L ethanoic acid and 0.20 mol/L sodium ethanoate. pKa = −log₁₀(1.74 × 10⁻⁵) = 4.76, so:

Choose Buffer and the calculator gives pH 5.06, the ratio 2, and the range it works best over, about pH 3.76 to 5.76, which is one unit either side of the pKa.

Finding Ka from a measured pH

Choose Find Ka from pH to work backwards. If a 0.10 mol/L weak acid has a measured pH of 2.38, then [H⁺] = 10⁻²·³⁸ = 4.17 × 10⁻³, and Ka = (4.17 × 10⁻³)² ÷ (0.10 − 0.00417) = 1.81 × 10⁻⁴. The calculator gives this and pKa = 3.74.

Common mistakes

• Forgetting the minus sign. pH = −log₁₀[H⁺]. Without the minus, a pH comes out negative for most acids.
• Using the base's concentration as [H⁺]. For a base, find [OH⁻] and pOH first, then pH = 14 − pOH.
• Missing the 2 for diprotic acids and bases. 0.01 mol/L Ca(OH)₂ gives 0.02 mol/L OH⁻.
• Treating a weak acid as strong. 0.2 mol/L ethanoic acid has a pH of 2.73, not 0.70, because only a small share ionises.
• Mixing Ka and pKa. pKa = −log₁₀ Ka. A smaller Ka, and a bigger pKa, means a weaker acid.
• Using 14 at other temperatures. pH + pOH = 14 only at 25 °C.

Practise acids and bases in a short timed paper

pH and Ka calculations come up in AP Chemistry and College General Chemistry practice, and in A-level Chemistry: Year 2 in the UK. Each daily paper is a short timed paper of 8 questions worth 25 to 35 marks, and the papers mix topics. For the concentration step on its own, use the Molarity Calculator.

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