Below is a titration curve of weak acid titrated with sodium hydroxide (NaOH). 8

Question

Below is a titration curve of weak acid titrated with sodium hydroxide (NaOH).

8. Below is a titration curve of a weak acid titrated with sodium hydroxide (NaOH) From the graph, estimate the pKa of the weak acid. Round to one decimal place pKa- a. b. When 0.5 mole equivalents of NaOH are added, what percent of the carboxyl groups will be deprotonated? In the "a" region of the graph, will most of the carboxyl groups be protonated or deprotonated? c. d. In the "b" region of the graph, will most of the carboxyl groups be protonated or deprotonated? 14 13 12 K-a-> 0.5 1.0 moles OH added/ mole weak acic

Explanation / Answer

Q8. (a.) From the graph , pKa = 7.1

(b.) When 0.50 mole equivalents of NaOH are added, then 50% of carboxyl groups will be deprotonated

(c.) In the "a" region of the graph, most of the carboxyl groups will be protonated

(d.) In the "b" region of the graph, most of the carboxyl groups will be deprotonated

Explanation

When a weak monoprotic acid is titrated with strong base like NaOH, titration is completed when moles of acid is equal to moles of base. This is known as equivalence point.

The half-equivalence point is defined as the amount of NaOH which is exactly half of what required at equivalence point is added to weak acid. Hence half the moles of acid will be neutralisesd (or deprotonated)

Also pKa is the pH at half equivalence point.

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