titrations of weak acids and weak bases chapter 17

Titrations of Weak Acids and Weak Bases

Chapter 17

Titration of a Weak Acid with a Strong Base

• The initial pH (initial acid: use Ka of the acid and the initial concentration of the acid to calculate this pH (ICE Table)

• Major substance present is the acid not dissociated


• Between the initial pH and the equivalence point (buffer mixture): prior to equivalence point, the acid is neutralized, and its conjugate base is being formed– Thus mixture contains a

mixture of the acid and conjugate base

– To calculate pH:• Henderson-Hasselbach

equation


• Half way between start and equivalence point is the half-equivalence point– At this point, the concentration of acid =

concentration of the conjugate base• pH = pKa for the acid

• Major substances present:– H3O+, anion of the acid, cation of the base, and undissociated

acid


• At each point below the equivalence point, the pH of the solution during titration is determined from the amounts of the acid and its conjugate base present at that particular time.– If [A-]>[HA], pH > pKa– If [A-]<[HA], pH < pKa


• Unlike in the previous case, the conjugate base of the acid affects the pH when it is formed.

• The pH at the equivalence point will be >7 because the conjugate base will be a major substance present since the acid and base neutralize each other

• Phenolphthalein is commonly used as an indicator in these titrations.


• After the equivalence point (excess base): excess base is present and only thing that determines the pH


With weaker acids, the initial pH is higher and pH changes near the equivalence point are more subtle.

3 differences between strong acid-strong base & weak acid-strong

base titrations• The solution of the weak acid has a higher

initial pH than a solution of a strong acid of the same concentration

• The pH change at the rapid-rise portion of the curve near the equivalence point is smaller for the weak acid than it is for the strong acid

• The pH at the equivalence point is above 7.00 for the weak acid-strong base titration

Titration of a Weak Base with a Strong Acid

• The pH at the equivalence point in these titrations is < 7.– The conjugate acid is

still present enough to decrease the pH

• Methyl red is the indicator of choice.

Titrations of Polyprotic Acids• In these cases there is

an equivalence point for each dissociation of a proton.

• The number of titration curves in the graph tells how many labile protons there are that can be removed from the acid

Choosing Indicators• Choose an indicator solution whose pKa is

closest to the pH at the equivalence point• Because the pH changes so sharply with little

change in the concentration of the acid or base added near equivalence point as long as the indicator changes in that range it will work

• Choose an indicator that changes color on the steep part of the curve

titrations of weak acids and weak bases chapter 17

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