Q: During the titration of 50.00 mL of 0.10 M acetic acid ($$K_a = 1.8 \times 10$$^{-5}$$) with 0.10 M NaOH, what is the pH after adding 25.00 mL of NaOH?

pH = pK_a = $$-\log(1.8 \times 10$$^{-5}$$) = 4.74$$

Question 1

Which of the following best explains why the pH at the equivalence point of a strong acid-strong base titration is always 7.00 at 25°C?

Accepted Answer

Because the strong acid and strong base completely neutralize each other, leaving only water and neutral salts, and the pH is determined by the auto-ionization of water.

Question 2

During the titration of 50.00 mL of 0.10 M acetic acid ($$K_a = 1.8 	imes 10$$^{-5}$$) with 0.10 M NaOH, what is the pH after adding 25.00 mL of NaOH?

Accepted Answer

pH = pK_a = $$-\log(1.8 	imes 10$$^{-5}$$) = 4.74$$

Question 3

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

Accepted Answer

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

Question 4

A 50.00 mL sample of 0.10 M acetic acid is titrated with 0.10 M NaOH. What is the pH at the equivalence point? ($$K_a = 1.8 	imes 10$$^{-5}$$, K_w = 1.0$$ 	imes $$10^{-14})$$

Accepted Answer

pH = 8.72

Question 5

Which indicator would be most appropriate for detecting the endpoint in the titration of a strong acid with a strong base?

Accepted Answer

Either phenolphthalein or methyl red, because the equivalence point is at pH 7 and both indicators change color near this pH.

Question 6

In a titration of 0.10 M NH$$_3$$ with 0.10 M HCl, the pH at the equivalence point is 5.28. What is the main reason for this acidic pH?

Accepted Answer

The conjugate acid NH$$_4^+$$ hydrolyzes to produce H$$_3O$^+$$, making the solution acidic.

Question 7

A weak diprotic acid H$$_2A is $$titrated with a strong base. Which of the following is true about the titration curve?

Accepted Answer

There are two buffer regions and two equivalence points, corresponding to the two ionizable protons.

Question 8

Which equation is used to calculate the pH of a buffer solution containing a weak acid (HA) and its conjugate base (A$$^-$$)?

Accepted Answer

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

Question 9

If 55.00 mL of 0.10 M NaOH is added to 50.00 mL of 0.10 M acetic acid, what is the pH of the solution? (Assume all reactions go to completion.)

Accepted Answer

pH = 11.68, because there is excess strong base present.

Question 10

Which of the following best describes the color change of an acid-base indicator?

Accepted Answer

The color change occurs over a range of about 2 pH units, centered around the indicator's pK$$_a.$$

Question 11

A student is titrating a weak acid with a strong base and observes a buffer region on the titration curve. What is happening chemically in this region?

Accepted Answer

Both the weak acid and its conjugate base are present, resisting changes in pH upon addition of small amounts of acid or base.

Question 12

Why is the pH at the equivalence point of a weak acid-strong base titration greater than 7?

Accepted Answer

The conjugate base of the weak acid hydrolyzes to produce OH$$^-$$, making the solution basic.

Question 13

Which of the following is the correct expression for the percent ionization of a weak acid?

Accepted Answer

$$	ext{Percent ionization} = \frac{[H_3O^+]_{eq}}{[HA]_i} 	imes 100$$

Question 14

A solution contains both HIn and In$$^-$$, the acid and base forms of an indicator. At what pH will the solution appear to have an intermediate color between the two forms?

Accepted Answer

At $$\mathrm{pH} = \mathrm{p}K_a of $$the indicator

Question 15

Which of the following best describes the use of a universal indicator?

Accepted Answer

A universal indicator is a mixture of several indicators that provides a continuous color change over the entire pH range.

Question 16

In the titration of a weak base with a strong acid, what is the main species present at the equivalence point?

Accepted Answer

The conjugate acid of the weak base (e.g., NH$$_4^+ in $$the case of NH$$_3)$$

Question 17

Which of the following equations correctly relates $$K_a$$, $$K_b$$, and $$K_w$$ for a conjugate acid-base pair?

Accepted Answer

$$K_a 	imes K_b = K_w$$

Acid-Base Titrations and Indicators: Aqueous Ionic Equilibria

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