pH Calculator: Strong and Weak Acids and Bases, Buffers, pOH and Ion Concentration

Find the pH of an acid, a base or a buffer, with the working.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose strong acid, strong base, weak acid, weak base, buffer, or a pH to concentration conversion.

Enter the concentration, and Ka or Kb for a weak one, or the pKa and the salt and acid concentrations for a buffer.

The calculator gives the ion concentration, the pH, the pOH and the hydroxide concentration, and for weak acids the percent ionised and a check of the square root approximation.

Formulas used

  • \mathrm{pH} = -\log_{10}[\mathrm{H^+}]
  • \mathrm{pH} + \mathrm{pOH} = 14
  • x = \frac{-K + \sqrt{K^2 + 4Kc}}{2}
  • \mathrm{pH} = \mathrm{p}K_a + \log_{10}\frac{[\mathrm{A^-}]}{[\mathrm{HA}]}

Worked example

With the default numbers in the calculator above:

  1. Hydrogen ion concentration (a strong acid ionises completely): 0.1 mol/L
  2. pH: 1.00 (acidic)
  3. pOH, and the hydroxide ion concentration (at 25 C, pH + pOH = 14): 13.00; [OH-] = 1.0e-13 mol/L

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: What do you want? Weak acid (from Ka).

  1. Concentration of the ions formed, from the exact quadratic x^2 + Kx - Kc = 0: 0.0013327 mol/L
  2. Compare the approximation x = sqrt(Kc), which is good when the acid is less than 5 percent ionised: 0.0013416 mol/L (the exact answer differs by 0.67 percent)
  3. Percent ionised: 1.33 %
  4. pH: 2.88 (acidic)
  5. pOH, and the hydroxide ion concentration (at 25 C, pH + pOH = 14): 11.12; [OH-] = 7.5037e-12 mol/L

Common questions

How do I calculate pH?

pH is minus the log to base 10 of the hydrogen ion concentration in mol/L. For 0.1 M HCl it is -log(0.1) = 1.

What is the difference between a strong and a weak acid?

A strong acid ionises completely, so the ion concentration equals the acid concentration. A weak acid only partly ionises, so you need its Ka.

What is a buffer?

A solution of a weak acid and its salt that resists a change in pH. Its pH is given by the Henderson-Hasselbalch equation.

Why is pH + pOH equal to 14?

At 25 C the product of the hydrogen and hydroxide ion concentrations is 1 x 10 to the minus 14.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any first-year chemistry text (acids, bases and buffers).

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