Buffers: Quiz

Buffers: Quiz

7 questions

1. Question 1EasyWhich pair of substances forms a buffer?

   AHCl and NaClBNaOH and NaClCAcetic acid and sodium acetateDHNO₃ and KOH
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   Answer: Acetic acid and sodium acetate

   A buffer needs a weak acid and its conjugate base. Acetic acid is weak and acetate is its conjugate base. HCl/NaCl does not buffer because Cl⁻ is too weak a base.

2. Question 2EasyA buffer contains equal concentrations of acetic acid and acetate (pKa = 4.74). What is its pH?

   A4.74B7.00C9.26D2.37
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   Answer: 4.74

   With [A⁻] = [HA], log(1) = 0, so pH = pKa = 4.74.

3. Question 3MediumWhat is the pH of a buffer that is 0.10 M acetic acid and 0.20 M sodium acetate? (pKa = 4.74)

   A4.44B4.74C5.04D5.44
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   Answer: 5.04

   pH = 4.74 + log(0.20/0.10) = 4.74 + 0.30 = 5.04. The answer 4.44 puts the ratio upside down.

4. Question 4MediumWhat is the pH of a buffer that is 0.20 M acetic acid and 0.10 M sodium acetate? (pKa = 4.74)

   A4.44B4.74C5.04D4.14
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   Answer: 4.44

   pH = 4.74 + log(0.10/0.20) = 4.74 − 0.30 = 4.44. More acid than base gives a pH below the pKa.

5. Question 5HardA buffer contains 0.10 mol of acetic acid and 0.10 mol of acetate. 0.010 mol of NaOH is added. What is the new pH? (pKa = 4.74)

   A4.65B4.74C4.83D12.00
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   Answer: 4.83

   OH⁻ reacts with acetic acid: HA = 0.09 mol, A⁻ = 0.11 mol. pH = 4.74 + log(0.11/0.09) = 4.83. The pH rises only slightly.

6. Question 6MediumYou need a buffer at pH 9.5. Which conjugate pair is the best choice?

   AAcetic acid/acetate (pKa 4.74)BAmmonium/ammonia (pKa 9.25)CFormic acid/formate (pKa 3.74)DHCl/Cl⁻
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   Answer: Ammonium/ammonia (pKa 9.25)

   Choose a pair whose pKa is within about 1 unit of the target pH. 9.25 is close to 9.5.

7. Question 7EasyIn a buffer, which species removes added $\ce{H3O+}$?

   AThe weak acid, HABThe conjugate base, A⁻CWaterDThe spectator ion (e.g. Na⁺)
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   Answer: The conjugate base, A⁻

   $\ce{A- + H3O+ -> HA + H2O}$. The base part of the buffer neutralizes added acid; the acid part neutralizes added base.