How do you calculate the pH of a weak acid?

For a weak acid HA with acid dissociation constant Ka and concentration C, set up the equilibrium HA = H+ + A- together with water autoionization. This tool solves the full charge-balance equation numerically rather than using the simplified square-root-of-Ka-times-C approximation, so the pH is correct even for very dilute or very weak acids where the approximation fails.

What is the Henderson-Hasselbalch equation?

The Henderson-Hasselbalch equation gives buffer pH as pH = pKa + log([A-]/[HA]), where [A-] is the conjugate base concentration and [HA] the weak acid concentration. It is an approximation valid when both concentrations are reasonably large. This tool shows the Henderson-Hasselbalch value alongside the rigorous equilibrium solution so you can see when they agree.

How do you find molar solubility from Ksp?

For a salt that dissolves into a cations and b anions, the solubility product is Ksp = (a·s)^a · (b·s)^b, where s is the molar solubility. For example AgCl gives Ksp = s^2 so s = sqrt(Ksp); CaF2 gives Ksp = 4s^3 so s = cube root of Ksp/4. Adding a common ion shifts the equilibrium and lowers solubility — enter a common-ion concentration to see the effect.

What is the common ion effect?

The common ion effect is the decrease in solubility of a salt when one of its ions is already present in solution. By Le Chatelier's principle, extra common ion pushes the dissolution equilibrium back toward the solid, so less of the salt dissolves. The Solubility mode lets you add a common-ion concentration and recomputes the reduced molar solubility.

What does the species distribution diagram show?

A species-distribution (or alpha) diagram plots the fraction of each acid-base species as a function of pH. For a monoprotic acid it shows the fraction present as HA versus A-: the two curves cross at pH = pKa, where the acid and its conjugate base are equal. It makes buffer regions and titration endpoints easy to see.

Is this pH and equilibrium calculator free?

Yes. It is completely free, needs no signup, and the chemistry is solved on the server with the open-source chempy and SciPy libraries for rigorous, textbook-accurate results.

Equilibrium & pH Calculator

Acid name (label only)

pKa

Concentration (M)

Rigorous solve (full charge balance), so it stays correct for dilute, very weak, or strong acids. Click a row in the reference for a pKa. Polyprotic acids: the (1st)/(2nd) entries solve a single step — use Custom for the full system.

Base name (label only)

pKb

Concentration (M)

pKa of the conjugate acid is taken as 14 − pKb.

pKa (weak acid)

[acid] (M)

[conjugate base] (M)

Shows the rigorous pH, the Henderson–Hasselbalch value, and buffer capacity.

Salt name (label only)

Ksp

cations (a)

anions (b)

[common ion] (M)

common ion is

For C_aA_b: Ksp = (a·s)^a(b·s)^b. Add a common-ion concentration to see solubility drop.

Equilibria — one per line, reactants = products; K

Initial concentrations (M) — species: value

Solved with chempy.equilibria. Handles polyprotic systems — add a line per dissociation step.

pKa reference

Acid–base equilibria, solved properly

Most online pH calculators use the shortcut [H⁺] ≈ √(Ka·C), which quietly breaks for dilute or very weak acids. This tool builds the actual equilibrium — including water autoionization — and solves the full charge-balance equation numerically, so the pH is right across the whole range. Behind the scenes it runs chempy and SciPy on the server.

What each mode does

Weak acid / base — pH, pOH, [H⁺], [OH⁻], conjugate concentrations and % ionization, plus a titration curve and species-distribution diagram.
Buffer — rigorous pH next to the Henderson–Hasselbalch estimate, with buffer capacity.
Solubility (Ksp) — molar solubility and ion concentrations, with the common-ion effect.
Custom — write any set of equilibria (polyprotic, complexation…) and solve the whole system at once.

Frequently asked

Set up HA = H⁺ + A⁻ with its Ka plus water autoionization, then solve the charge-balance equation numerically. This avoids the √(Ka·C) approximation, so it's accurate even for dilute or very weak acids.

pH = pKa + log([A⁻]/[HA]) for a buffer. It's an approximation valid when both concentrations are sizeable; the tool shows it next to the exact solution.

For a salt giving a cations and b anions, Ksp = (a·s)ᵃ(b·s)ᵇ. AgCl → s = √Ksp; CaF₂ → s = ∛(Ksp/4). A common ion lowers s — enter its concentration to see by how much.

The fraction of each species versus pH. For a monoprotic acid the HA and A⁻ curves cross at pH = pKa, which is the centre of the buffer region.

Yes — free, no signup, solved with the open-source chempy and SciPy libraries.

Menu

⭐ Popular Tools

🕒 Recently Used

📁 All Categories

Quick Links

Support

Equilibrium & pH Calculator

Result

Acid–base equilibria, solved properly

What each mode does

Frequently asked

Support This Free Tool