From a strong acid strong base titration curve how is the equivilance point used to work out the concentration of the acids involved ?

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mlsiasebs eNotes educator| Certified Educator

At the equivalence point of a titration, the moles of H+ will be equal to the moles of OH-.  If we know the concentration and volume of one of the solutions (let's say the base), then we can find the moles of base (this would be moles of the compound, i.e. Ca(OH)2) and use the stoichiometry from our balanced chemical equation to find the concentation of the acid.  The mole ratio from the equation will take into account the fact that Ca(OH)2 has 2 moles of OH- for every one mole of the compound.

For example,

Ca(OH)2 + 2 HCl --> CaCl2 + 2H2O

Let's assume we know the volume of HCl and we are titrating with a known concentration of Ca(OH)2

For every 1 mole of Ca(OH)2 that reacts, I need 2 moles of HCl to neutralize it.  At the equivalence point, we will have addd enough base to react with all the acid and be left only with salt and water.  The result of this is that on the titration curve, we see a rapid increase in the pH value.  Before the equivalence point, we would have excess acid present so the pH would be low.  As we get closer to the equivalence point, we see an increase in pH but only small increments.  At the equivalence point, we have neither excess acid nor base.  After the equivalence point, we will have excess base and will see a sharp rise in pH until it plateaus.

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