How this instrument works
Acid-base titration is a technique for finding an unknown concentration, or checking a known one, by slowly adding a base of known concentration to a measured volume of acid (or vice versa) until the reaction is exactly complete — the equivalence point, where moles of base added exactly match the moles of acid present, adjusted for the reaction's stoichiometry. This instrument computes the volume of base required to reach that point, given both molarities, the acid's volume, and the reaction's stoichiometric ratio.
The underlying relationship, Macid x Vacid = Mbase x Vbase x stoichiometric ratio, comes directly from moles being conserved at the equivalence point: moles of acid consumed equal moles of base added times whatever mole ratio the balanced equation specifies. For a simple 1:1 acid-base pair like HCl and NaOH, that ratio is 1, and equal molarities of acid and base require equal volumes to reach equivalence. For a diprotic acid like sulfuric acid reacting with a monoprotic base, the ratio is 2, since each mole of acid needs two moles of base to fully neutralize.
In practice, the equivalence point is located experimentally with an indicator that changes color near that point, or with a pH meter tracking the titration curve, and the volume actually measured that way should match what this calculation predicts if the acid's true concentration matches what was assumed. That comparison is exactly how titration is used to determine an unknown concentration: measure the actual volume needed, then solve the same relationship for the unknown molarity instead of the volume.
- Enter the acid's known molarity into Acid molarity (mol/L).
- Enter the volume of acid being titrated into Acid volume.
- Enter the titrant base's known molarity into Base molarity (mol/L).
- Enter the base-to-acid mole ratio from the balanced equation into Stoichiometric ratio (base : acid) — 1 for HCl + NaOH, 2 for a diprotic acid needing two moles of base per mole of acid.
- Read Base volume needed directly beneath the fields, in the same volume unit you entered for Acid volume — it recalculates the instant any input changes.
- Base molarity (mol/L) and Stoichiometric ratio (base : acid) must both be greater than zero — the instrument divides by both.
Worked example — 25 mL of 0.1 M acid, 1:1 reaction
Enter 0.1 into Acid molarity (mol/L), 25 into Acid volume, 0.1 into Base molarity (mol/L), and 1 into Stoichiometric ratio (base : acid) — a simple 1:1 acid-base pair such as HCl reacting with NaOH, both at the same 0.1 M concentration. Base volume needed reads 25.0.
That comes from (0.1 x 25) / (0.1 x 1) = 2.5 / 0.1 = 25.0 mL: because acid and base share the identical molarity and a 1:1 stoichiometric ratio, it takes exactly as much base volume as acid volume to reach the equivalence point. Raising the base's molarity while keeping everything else fixed would proportionally shrink this volume, since a more concentrated base needs less of it to supply the same moles.
Questions
Why does a 1:1 reaction with equal molarities need equal volumes?
Because moles of acid must equal moles of base at the equivalence point for a 1:1 stoichiometric ratio, and moles equal molarity times volume. If both solutions have the same molarity, the only way for Macid x Vacid to equal Mbase x Vbase is for Vacid and Vbase to also be equal — the concentrations cancel out, leaving the volumes matched directly.
What does the stoichiometric ratio actually change?
It scales how many moles of base are needed per mole of acid, taken directly from the balanced chemical equation's coefficients. A ratio of 2, as with sulfuric acid (H2SO4) reacting with sodium hydroxide (2 NaOH), means twice as many moles of base are required per mole of acid compared to a 1:1 reaction, which increases the required base volume by that same factor, all else equal.
Can I use this to find an unknown acid concentration instead?
Yes, conceptually — this instrument solves for base volume given a known acid molarity, but the same equation, Macid x Vacid = Mbase x Vbase x ratio, can be rearranged to solve for Macid instead if you measure the actual base volume needed to reach equivalence experimentally and already know Mbase, Vacid and the ratio. That rearrangement is the whole basis of using titration to determine unknown concentrations in the first place.
Does the equivalence point always occur at pH 7?
Not necessarily — pH 7 only occurs at equivalence for a strong acid reacting with a strong base, where the resulting salt doesn't affect the solution's pH. Titrating a weak acid with a strong base, for example, produces a conjugate base that makes the solution mildly basic at equivalence, so the equivalence point sits above pH 7 even though the mole-based volume calculation performed here is unaffected by that distinction.
What volume unit does the answer come out in?
Whatever unit you entered for Acid volume — the calculation is a pure ratio and doesn't care whether that unit is millilitres, litres, or any other consistent volume unit, as long as it's used consistently. Entering Acid volume in millilitres returns Base volume needed in millilitres; switching to litres for the input switches the output to litres as well.