Free MCAT-style practice, 5A
MCAT Acid-Base, Buffer and Titration Practice Questions
Ten free MCAT-style practice questions on pH and pOH, weak acid Ka, conjugate pairs, Henderson-Hasselbalch buffers, blood bicarbonate, titration curves, indicators and polyprotic acids, with full explanations.
Quick concept primer
Most acid-base questions come down to deciding what is actually in the flask. A strong acid or base dissociates completely, so its pH follows straight from its concentration (at 25 C, pH + pOH = 14). A weak acid only partly dissociates; when it is much more concentrated than its Ka, [H+] is about the square root of Ka times the concentration. For a conjugate pair, Ka times Kb equals Kw, so a weaker acid has a stronger conjugate base.
A buffer is a weak acid plus its conjugate base in comparable amounts. The Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]), gives its pH, and a buffer works best within about one unit of its pKa. When strong base is added to a weak acid, do the neutralization first in moles, then apply the equation to what remains. On a titration curve the half-equivalence point has pH = pKa, and the equivalence pH depends on the salt formed: above 7 for a weak acid with strong base, below 7 for a weak base with strong acid. Choose an indicator whose color change range contains the equivalence pH.
Traps: dilution with water barely changes a buffer's pH because the ratio is unchanged, and the equivalence point is not the same as pH 7 unless both partners are strong.
10 practice questions
Question 1
What is the pH of a 0.020 M aqueous solution of sodium hydroxide at 25 C?
- 1.70
- 10.30
- 12.30
- 12.70
Show answer and explanation
Answer: C. 12.30
NaOH is a strong base, so [OH-] = 0.020 M and pOH = -log(0.020) = 1.70. At 25 C pH = 14.00 - 1.70 = 12.30. Choice A is the pOH reported as if it were the pH, and choices B and D do not equal 14.00 - 1.70; check the arithmetic by confirming that pH + pOH = 14.
Question 2
A 0.10 M solution of a monoprotic weak acid HX has Ka = 1.0 × 10-5. What is the approximate pH?
- 2.0
- 3.0
- 4.0
- 5.0
Show answer and explanation
Answer: B. 3.0
Because Ka is tiny compared with the concentration, x is small and [H+] is about the square root of (1.0 × 10-5 × 0.10) = 1.0 × 10-3 M, so pH is about 3.0. Choice A would require 10 percent dissociation, far more than this Ka allows, and 5.0 is simply the pKa, which applies only when acid and conjugate base are present in equal amounts.
Question 3
A student adds 0.050 mol of NaOH to 1.0 L of solution containing 0.20 mol of acetic acid (pKa about 4.76). Assuming no volume change, what is the pH?
- 4.28
- 4.76
- 5.24
- 3.76
Show answer and explanation
Answer: A. 4.28
Neutralize first: 0.050 mol OH- converts 0.050 mol acetic acid to acetate, leaving 0.15 mol acid and 0.050 mol acetate. Then pH = 4.76 + log(0.050/0.15) = 4.76 - 0.48 = 4.28. Choice C inverts the ratio, choice B ignores that the amounts are unequal, and choice D uses a tenfold ratio.
Question 4
A lab needs a buffer that holds a cell culture medium near pH 7.4. Which conjugate acid, with the approximate pKa listed, is the best choice for the acid component?
| Conjugate acid | Approximate pKa |
|---|---|
| Acetic acid | 4.8 |
| Ammonium ion | 9.3 |
| Hydrogen carbonate ion (HCO3-) | 10.3 |
| Dihydrogen phosphate ion (H2PO4-) | 7.2 |
- Acetic acid
- Ammonium ion
- Hydrogen carbonate ion
- Dihydrogen phosphate ion
Show answer and explanation
Answer: D. Dihydrogen phosphate ion
A buffer resists pH change best when the target pH lies within about one unit of the pKa, where both acid and base forms are present in useful amounts. Only dihydrogen phosphate (pKa about 7.2) is that close to 7.4. Acetic acid, ammonium and hydrogen carbonate all have pKa values more than 1.5 units away, so at pH 7.4 nearly all of each pair would sit in one form.
Question 5
During an anxiety attack, a person breathes rapidly and deeply for several minutes. Which change in arterial blood is expected?
- PCO2 rises and pH falls
- PCO2 falls and pH rises
- PCO2 falls and pH falls
- PCO2 and pH are both unchanged because the bicarbonate buffer absorbs the effect
Show answer and explanation
Answer: B. PCO2 falls and pH rises
Hyperventilation removes CO2 faster than tissues produce it. By Le Chatelier's principle the equilibrium CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- shifts left, consuming H+, so pH rises (respiratory alkalosis). Choice A describes hypoventilation. The bicarbonate system cannot cancel the effect because CO2 is itself one of its components.
Question 6
Lactic acid samples are analyzed by adding 0.15 M KOH to 20.0 mL portions of 0.15 M acid. In one run, the meter reads pH 3.80 when exactly 10.0 mL of KOH has been delivered. Which conclusion is correct?
- The acid's pKa is 3.80, and the pH at the equivalence point is above 7
- The acid's pKa is 3.80, and the pH at the equivalence point is exactly 7
- The acid's pKa is 10.20, and the pH at the equivalence point is below 7
- The acid's Ka is 3.80, and the equivalence point occurs at 10.0 mL
Show answer and explanation
Answer: A. The acid's pKa is 3.80, and the pH at the equivalence point is above 7
Equivalence needs 20.0 mL of the equally concentrated base, so 10.0 mL is the half-equivalence point, where acid and conjugate base are equal and pH = pKa = 3.80. At equivalence the flask holds only lactate, a weak base, so the pH is above 7. Choice D confuses Ka with pKa and misplaces the equivalence volume, and 10.20 is 14 minus the pKa.
Question 7
Aqueous ammonia is titrated with hydrochloric acid, and the equivalence point is calculated to be near pH 5.3. Using the indicator data below, which indicator is the best choice?
| Indicator | Color change range (pH) |
|---|---|
| Phenolphthalein | 8.2 to 10.0 |
| Thymol blue (second change) | 8.0 to 9.6 |
| Methyl red | 4.4 to 6.2 |
| Alizarin yellow R | 10.1 to 12.0 |
- Phenolphthalein
- Thymol blue
- Methyl red
- Alizarin yellow R
Show answer and explanation
Answer: C. Methyl red
An indicator should change color over a range that contains the equivalence pH, so the color change coincides with the steep part of the curve. A weak base titrated with a strong acid gives an acidic equivalence point because the product, NH4+, is a weak acid. Methyl red (4.4 to 6.2) brackets 5.3; the other three change color in basic solution, well before equivalence.
Question 8
A buffer contains 0.10 M HA and 0.10 M NaA. All of the following statements about it are true EXCEPT:
- Its pH equals the pKa of HA
- It resists pH change about equally well whether a small amount of strong acid or strong base is added
- Doubling both concentrations increases its buffer capacity
- Diluting it tenfold with pure water lowers its pH by about one unit
Show answer and explanation
Answer: D. Diluting it tenfold with pure water lowers its pH by about one unit
Henderson-Hasselbalch depends on the ratio [A-]/[HA], and dilution changes both concentrations by the same factor, so the ratio and the pH stay essentially the same. Dilution does reduce capacity, which is why choice C (more moles of both forms) is true. Choices A and B follow from the 1:1 ratio.
Question 9
Phosphoric acid has pKa values of about 2.1, 7.2 and 12.3. In a phosphate solution adjusted to pH 9.7, which species is present at the highest concentration?
- H3PO4
- H2PO4-
- HPO42-
- PO43-
Show answer and explanation
Answer: C. HPO42-
pH 9.7 lies between the second pKa (7.2) and the third (12.3). It is 2.5 units above pKa2, so HPO42- outnumbers H2PO4- by about 300 to 1, and 2.6 units below pKa3, so HPO42- outnumbers PO43- by about 400 to 1. Fully protonated H3PO4 is negligible.
Question 10
Hydrocyanic acid, HCN, has Ka of about 6.2 × 10-10 at 25 C. What is Kb for the cyanide ion, CN-?
- 1.6 × 10-5
- 6.2 × 10-10
- 6.2 × 10-24
- 1.6 × 109
Show answer and explanation
Answer: A. 1.6 × 10-5
For a conjugate acid-base pair, Ka × Kb = Kw = 1.0 × 10-14 at 25 C, so Kb = 1.0 × 10-14 / 6.2 × 10-10, about 1.6 × 10-5. Cyanide is therefore a reasonably strong weak base. Choice C multiplies instead of dividing, and choice D is 1/Ka, not Kw/Ka.