During an acid–base titration at

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

25\,^{\circ}\text{C}

, a student adds

0.120\,\text{M}

NaOH from a buret to

30.0\,\text{mL}

of an unknown monoprotic weak acid, HA. The pH of the solution is carefully recorded throughout. The following observations are made: • At

15.0\,\text{mL}

of NaOH added, the pH is 4.40. • A sharp, nearly vertical rise in the titration curve begins near

28\,\text{mL}

of NaOH added and levels off near

32\,\text{mL}

. • The pH well past the equivalence point (at

35.0\,\text{mL}

of NaOH added) is measured to be 12.20. (a) Calculate the initial molarity of the weak acid solution. (b) Determine the acid-dissociation constant

K_a

for HA. (c) Using appropriate chemical equations, calculate the pH at the equivalence point. (d) Verify whether the measured pH of 12.20 at

35.0\,\text{mL}

is consistent with the presence of excess NaOH in the beaker. Support your answer with a quantitative calculation of the hydroxide ion concentration at this stage of the titration.

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