pH Calculator

1) Concentration mode (you already know molarity, M)

1. Choose the chemical from the dropdown (this loads its formula, molar mass, and pKₐ).
2. Enter the solution concentration (mol/L) in “Concentration of solution (mol/L)”.
3. Click CALCULATE to compute pH using the weak acid/base approximation (internally: Kₐ=10⁻ᵖᴷᵃ, [H⁺]≈√(Kₐ·C) for acids; a similar relation is used for bases).
4. Read the result under “pH” and verify the context: very dilute or very concentrated solutions, polyprotic acids, or strong acids/bases can deviate—treat this as a quick estimate.

Tip: For buffers near pH ≈ pKₐ, Henderson–Hasselbalch often gives a better estimate if you know the [base]/[acid] ratio.

2) Weight mode (you weighed a solid and know final volume)

1. Choose the chemical (loads formula, molar mass, pKₐ).
   Enter the solid weight (grams) you dissolved.
   Enter the total solution volume (liters) after dilution.
2. Click CALCULATE; the tool converts to molarity = mass / (MW × volume), then estimates pH from pKₐ as above.
3. Check plausibility: If you used a strong acid/base (e.g., HCl, NaOH), the simple weak-equilibrium formula may over/underestimate; rule-of-thumb baselines are pH ≈ −log₁₀ C for strong acids and pOH ≈ −log₁₀ C → pH = 14 − pOH for strong bases.

Tip: For polyprotic acids (citric, phosphoric), the first dissociation dominates at low pH, but full accuracy needs stepwise equilibria; use this as a first pass.

3) Volume mode (you dispensed a liquid reagent from a stock)

1. Choose the chemical (loads formula, molar mass, pKₐ).
2. Enter the stock concentration (mol/L) of the liquid reagent.
   **Enter the volume added (liters) of that reagent to your mix.
   **Enter the total final volume (liters) after mixing/dilution.
3. Click CALCULATE; the tool computes final molarity = (C_stock × V_added) / V_total, then estimates pH using pKₐ.

Interpretation: If you are mixing acid with base (or vice versa), this simple mode does not do stoichiometric neutralization—use it when a single acid/base defines the solution. For acid–base mixtures or buffers, switch to buffer math (Henderson–Hasselbalch, charge balance) for accuracy.

General tips for all modes

1. Keep units consistent (L, mol/L, g).
2. Results assume 25 °C and modest ionic strength; temperature or high ionic strength can shift pKₐ and activities.
3. For buffers near pH ≈ pKₐ, knowing the [A⁻]/[HA] ratio yields better predictions; for capacity questions use β ≈ 2.303·C·(Kₐ[H⁺])/(Kₐ+[H⁺])².
4. Treat outputs as screening estimates; for critical work, solve the full equilibrium (mass/charge balance) or validate with a calibrated pH meter.