Properties of Buffers Practice

Sample questions

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1. Sample 1difficulty 2/5

   A buffer typically consists of

   • A

     Weak acid + its conjugate base (or weak base + conjugate acid)

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   • B

     Two strong acids

   • C

     Pure water

   • D

     Strong acid + strong base

   Why

   Both partners present in comparable amounts to neutralize added acid or base.

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2. Sample 2difficulty 3/5

   What is the pH when the conjugate base/acid ratio = 1?

   • A

     7.00

   • B

     4.74

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   • C

     9.26

   • D

     5.74

   Why

   log(1) = 0, so pH = pKa = 4.74.

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3. Sample 3difficulty 3/5

   For a pH-5 buffer, choose a weak acid with pKa

   • A

     ≈ 14

   • B

     ≈ 9

   • C

     ≈ 2

   • D

     ≈ 5

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   Why

   Best buffer near pH = pKa, ideally within ±1 unit.

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4. Sample 4difficulty 4/5

   A buffer is made from 0.40 M $NH_3$ and 0.30 M $NH_4Cl$. What is the pH? ($K_b(NH_3) = 1.8\times 10^{-5}$)

   • A

     $pH \approx 9.38$

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   • B

     $pH \approx 9.26$

   • C

     $pH \approx 4.62$

   • D

     $pH \approx 8.92$

   Why

   $pK_a(NH_4^+)$ = 14 - $pK_b(NH_3)$ = 14 - 4.74 = 9.26. pH = $pK_a + \log([NH_3]/[NH_4^+])$ = 9.26 + $\log(0.40/0.30)$ = 9.26 + 0.125 = 9.38.

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5. Sample 5difficulty 4/5

   A buffer contains 0.20 M HF and 0.30 M NaF. The pH is ($K_a$(HF) = $7.2\times10^{-4}$, $\log 7.2 = 0.86$)

   • A

     3.14

   • B

     3.44

   • C

     3.32 ($pK_a + \log([F^-]/[HF])$)

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   • D

     10.86

   Why

   pH = $pK_a + \log([F^-]/[HF])$ = $-\log(7.2\times10^{-4}) + \log(0.30/0.20)$ = 3.143 + 0.176 = 3.32.

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