AP® Chemistry · Unit 8 · Topic 8.4
Acid-Base Neutralization
Neutralization is a proton transfer between an acid and a base that reacts moles of H+ with moles of OH-; whatever is left over, and what that leftover species is, sets the final pH.
Mix a strong acid and a strong base, and H+ and OH- combine to form water. If you mix equal moles, neither survives, and you're left with a solution of a neutral salt dissolved in water, so the pH sits at 7.00. The moment the moles aren't equal, whichever species is in excess decides the pH: leftover H+ makes it acidic, leftover OH- makes it basic.
To track this, build a stoichiometry table. Find initial moles of H+ and OH- (molarity times volume for each), let them react 1:1, and see what's left. Whatever remains in excess gets divided by the new total volume, since mixing two solutions dilutes everything, and that concentration goes into -log for pH or pOH. This is just limiting reactant logic borrowed from stoichiometry, now applied to protons instead of grams.
The strong acid plus strong base case is the simple one. Once a weak acid is involved, neutralization no longer lands you at pH 7 even with exactly matched moles, because the product isn't neutral. That sets up the next idea: titration curves, where the equivalence point's pH depends on what species is sitting in the beaker once the original acid or base is used up, and buffers, which exploit the fact that a weak acid and its conjugate base can coexist in solution and resist pH change.
Key terms
- Neutralization
- A reaction in which an acid and a base transfer protons until one or both are used up.
- Equivalence point
- The point in a titration where moles of acid added exactly equal moles of base originally present, or vice versa.
- Stoichiometry table
- A table tracking initial, change, and final moles of reacting species to find what remains after a reaction.
- Hydrolysis
- A reaction of an ion with water that produces H+ or OH-, shifting the pH away from 7.
- Conjugate base
- The species left after an acid donates a proton, which can itself act as a weak base in water.
Watch it explained
- How it worksHow do you track moles of acid and base through a neutralization with a stoichiometry table?
- Worked exampleFind the pH after mixing 25.0 mL of 0.100 M HCl with 15.0 mL of 0.100 M NaOH.
- Common mistakeWhy isn't the equivalence point of a weak acid titration at pH 7?
- Going furtherWhat is left in the beaker when a weak acid is exactly neutralized by a strong base?
Check yourself
End of topic.