Question 1

Which of the following best explains why the pH at the equivalence point in a weak acid-strong base titration is greater than 7?

Accepted Answer

The conjugate base of the weak acid hydrolyzes in water, producing OH$$^{-}$$ ions.

Question 2

A 0.0250 M solution of benzoic acid (C$$_6H$_5$$COOH, $$K_a = 6.3 	imes 10$$^{-5}$$) is titrated with 0.050 M NaOH. What is the initial pH of the benzoic acid solution before any NaOH is added?

Accepted Answer

pH $$= 2.87$$

Question 3

During the titration of a weak acid with a strong base, what is the significance of the half-equivalence point?

Accepted Answer

At the half-equivalence point, $$[HA] = [A^-]$$ and pH $$= pK_a.$$

Question 4

Which indicator would be most suitable for detecting the equivalence point in the titration of a weak acid with a strong base?

Accepted Answer

Phenolphthalein (color change interval: pH 8.3–10.0)

Question 5

A 0.100 M NH$$_3$$ solution ($$K_b = 1.8 	imes 10$$^{-5}$$) is titrated with 0.100 M HCl. What is the initial pH of the NH_3$ solution?

Accepted Answer

pH $$= 11.13$$

Question 6

What happens to the pH at the equivalence point as the $$K_a of a $$weak acid decreases in a titration with a strong base?

Accepted Answer

The pH at the equivalence point increases.

Question 7

In a titration of 40.0 mL of 0.0250 M benzoic acid with 0.050 M NaOH, what is the pH at the equivalence point?

Accepted Answer

pH $$= 8.73$$

Question 8

Which of the following statements is TRUE regarding the buffer region in a weak acid-strong base titration?

Accepted Answer

The buffer region occurs before the equivalence point, where both acid and its conjugate base are present.

Question 9

A titration curve for a weak base with a strong acid shows a gradual decrease in pH until the equivalence point, followed by a sharp drop. What causes the sharp drop in pH?

Accepted Answer

The weak base is completely neutralized, and excess strong acid is added.

Question 10

During the titration of 20.0 mL of 0.100 M NH$$_3$$ with 0.100 M HCl, what is the pH at the equivalence point?

Accepted Answer

pH $$= 5.13$$

Question 11

Which of the following is a key difference between the titration curve of a strong acid-strong base and a weak acid-strong base?

Accepted Answer

The weak acid-strong base titration curve has a buffer region and a higher pH at the equivalence point.

Question 12

In the titration of a polyprotic acid such as H$$_3PO$_4$$ with NaOH, what is observed on the titration curve?

Accepted Answer

Multiple equivalence points and buffer regions corresponding to each proton lost.

Question 13

Which equation is used to calculate the pH in the buffer region of a weak acid-strong base titration?

Accepted Answer

pH $$= pK_a + \log \left( \frac{[A^-]}{[HA]} \right)$$

Question 14

Why is methyl red an unsuitable indicator for the titration of a weak acid with a strong base?

Accepted Answer

Its color change interval occurs before the equivalence point is reached.

Question 15

A student titrates 50.0 mL of 0.100 M H$$_3PO$_4$$ with 0.100 M NaOH. At which point on the titration curve does the concentration of H$$_2PO$_4^-$$ equal the concentration of HPO$$_4$$^{2-}$$?

Accepted Answer

At the second half-equivalence point.

Question 16

What is the primary role of an indicator in an acid-base titration?

Accepted Answer

To signal the equivalence point by changing color in the region where pH changes rapidly.

Question 17

In a titration of a weak base with a strong acid, what is the pH at the half-equivalence point?

Accepted Answer

pOH $$= pK_b$$

Question 18

A titration curve for a weak acid with a strong base shows a buffer region. What chemical species are present in significant amounts in this region?

Accepted Answer

Both the weak acid and its conjugate base.

Question 19

Which of the following best estimates the $$pK_a of a $$weak acid if the pH at the half-equivalence point is 4.75?

Accepted Answer

$$pK_a = 4.75$$

Weak Acid-Strong Base and Weak Base-Strong Acid Titrations: Principles, Curves, and Indicators

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