pH Calculator

pH Calculator Guide

A pH calculator computes pH, pOH, [H+] and [OH-] for strong acids/bases, weak acids/bases, and buffers. It applies equilibrium constants and the Henderson-Hasselbalch equation where valid.

What is pH Calculator?

The pH calculator solves common lab problems: strong acid/base pH from molarity, weak acid dissociation using Ka/Kb, buffer pH from pKa and ratio, and conversions between pH and concentration.

How to Use the pH Calculator

1. Choose system: strong acid/base, weak acid/base, or buffer.
2. Enter concentration(s), Ka/Kb or pKa/pKb, and volume if dilution matters.
3. Calculate pH, pOH, [H+], and [OH-]; see intermediate steps.
4. (Optional) include temperature if your lab requires non-25 C corrections.
5. Check assumptions (e.g., x << C for weak acids) or switch to the exact quadratic.

Formulas & Methods

• Definitions: pH = -log10[H+], pOH = -log10[OH-], pH + pOH = 14.00 at 25 C.
• Strong acid: [H+] approx C_acid (if no significant water or autoprotolysis).
• Strong base: [OH-] approx C_base; pH = 14 - pOH.
• Weak acid (HA): Ka = [H+][A-]/[HA]. Solve x^2 + Ka*x - Ka*C = 0 where x = [H+].
• Henderson-Hasselbalch (buffer): pH = pKa + log10([A-]/[HA]).
• Dilution: use moles and total volume to update concentrations before pH math.

Assumptions & limitations

• Default assumes 25 C and ideal dilute solutions (activity ~ concentration).
• For very dilute strong acids/bases, include water autoionization (Kw).
• High ionic strength requires activity corrections; advanced models are beyond scope.

Examples

Example A — Strong acid
0.010 M HCl: [H+] ~ 0.010, pH = 2.00.

Example B — Weak acid
C = 0.050 M, pKa = 4.76 (acetic acid).
Ka = 10^-4.76 = 1.74e-5.
Solve x^2 + Ka*x - Ka*C = 0 → x ~ 0.0023 M, pH ~ 2.64 (exact quadratic; HH gives ~2.62 near).

Example C — Buffer
0.10 M acetic acid and 0.10 M acetate: pH = pKa + log10(1) = 4.76.

| System | Inputs | Output | |---|---|---| | Strong acid | 0.010 M HCl | pH 2.00 | | Weak acid | 0.050 M, pKa 4.76 | pH ~ 2.64 | | Buffer | pKa 4.76, ratio 1.0 | pH 4.76 |

Pro Tips & Best Practices

• Use the quadratic when C/Ka is small or the x << C assumption fails.
• Check that buffers stay within pH = pKa +/- 1 for good capacity.
• Combine moles first, then compute concentrations after mixing.
• Temperature shifts Kw; recalc 14.00 term if not at 25 C.
• Report sig figs consistent with input molarities and pKa precision.

Related Calculators

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• Stoichiometry Calculator
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FAQ

Q: How do you calculate pH from acid concentration?

A: For strong acids, pH ~ -log10[H+]. For weak acids, use Ka and solve the equilibrium or use the Henderson-Hasselbalch equation for buffers.

Q: What is the relation between pH and pOH?

A: pH + pOH = 14.00 at 25 C because Kw = 1.0e-14 in pure water at that temperature.

Q: How do buffers work?

A: Buffers contain a weak acid and its conjugate base; pH ~ pKa + log10([base]/[acid]) within the buffer range.

Q: Do temperature and ionic strength matter?

A: Yes—Kw and activity coefficients change with temperature and ionic strength, shifting pH slightly.

Q: Can I compute pH of mixtures?

A: Simple mixtures can be approximated; exact results need full equilibrium and charge balance.

Engineering note: Standard assumptions: 25 C, aqueous solutions, activities ~ concentrations. For rigorous work, apply activity corrections.

Call to Action

Enter concentration and pKa/Ka to compute pH, pOH, and ions—switch modes for strong, weak, or buffer systems and see the steps.