How is the pH of a weak acid calculated?

• Use the Henderson-Hasselbalch equation

• Use the Ka of the weak acid to solve for [H⁺]

• Use the Kb of the weak base to solve for [OH⁻]

• Use the pKa to find the pH directly

In the buffer zone before the equivalence point, which equation is used to find the pH?

• pH = -log[H⁺]

• pH = pKa + log [A⁻]/[HA]

• pH = 14 - pOH

• pH = -log[OH⁻]

At the half-equivalence point of a titration, what is true about the pH?

• pH = 7

• pH = pKa

• pH = 14

• pH = 0

Answer explanation

The half-equivalence points occurs when half of the moles of acid have been neutralized. This means the moles of acid are equal to the moles of conjugate base, and pH = pKa.

What happens at the equivalence point in a weak acid and strong base titration?

• Only conjugate acid is present

• Moles of weak acid are equal to the moles of strong base

• Only strong base is present

• The solution becomes acidic

Answer explanation

While this is the simplest calculation in a strong acid + strong base titration (it's always 7.0), the equivalence point is the most difficult pH calculation in all other types of titrations.

Check out this video for an example of this type of calculation.

After the equivalence point, what is used to determine the pH?

• The concentration of weak acid

• The excess [OH^-] in solution from the strong base

• The initial concentration of the acid

• The concentration of the conjugate acid

The ​ (a)   of a titration occurs when the moles of acid are equal to the moles of base. The​ ​ ​ (b)   is the point at which the indicator changes color, signifying the end of the titration.

• equivalence point
• endpoint
• half-equivalence
• pH
• buffer zone

Arrange the following acids from weakest to strongest.

• Hydrochloric acid (HCl)

  pKa = -5.9

• Hydrofluoric acid (HF)

  pKa = 3.20

• Acetic acid (CH₃COOH)

  pKa = 4.76

• Formic acid (HCOOH)

  pKa = 3.75

What is the pH at the equivalence point if 30.0 mL of 2.0 M HCl is titrated with 6.0 M Ca(OH)₂?

Answer explanation

In a strong acid + strong base titration, the pH is always 7.0 at the equivalence point. Additionally, this type of titration will not have a half-equivalence point as there is no buffer zone in a strong acid + strong base titration.

Calculating the pH at the equivalence point of a weak acid + strong base or weak base + strong acid titration will not be as simple!

If the equivalence point of a titration occurs at a pH of 4.2, choose the best indicator.

• Methyl orange

  pH = 3.1 - 4.4

• Phenolphthalein

  pH = 8.3 - 10

• Methyl red

  pH = 4.4 - 6.2

Answer explanation

Choosing an indicator won't be an exact science, but the goal is to have the endpoint as close to the equivalence point as possible. Choose the indicator that has a pH range closest to the pH at the equivalence point.

If the Ka of oxalic acid is 5.4 x 10^-2 at 25°C, what is the Kb of its conjugate base?

• 1.85 x 10^-13

• 1.0 x 10^-14

• 1.27

• Not enough information is given to determine this

Answer explanation

To solve for K_b, remember that Kw = Ka x Kb.

At standard conditions (25°C), Kw, or the dissociation constant of water, is 1.0 x 10^-14.

Kb = Kw/ Ka = (1.0 x 10^-14)/( 5.4 x 10^-2)