solution8_pdf - dang(rtd356 – H08 Buffers – Mccord...

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dang (rtd356) – H08: Buffers – Mccord – (52450) 1 This print-out should have 18 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Which of the five solutions below is most ba- sic? 1. 0.10 M CH 3 COOH and 0.20 M NaCH 3 COO 2. 0.10 M aq NH 3 and 0.20 M NH 4 Cl 3. 0.10 M CH 3 COOH and 0.10 M NaCH 3 COO 4. 0.10 M aq NH 3 and 0.10 M NH 4 Cl cor- rect 5. 0.10 M HClO and 0.10 M NaClO Explanation: All are buffer where pH = p K a + log parenleftbigg [base] [acid] parenrightbigg 002 10.0 points Explain why the salt of a weak acid, as well as the acid itself, must be present to form a buffer solution. 1. The anion from the salt is needed to partially neutralize added base. 2. The anion from the salt is needed to partially neutralize added acid. correct 3. The cation from the salt is needed to partially neutralize added acid. 4. The cation from the salt is needed to partially neutralize added base. 5. Actually, a weak acid by itself is a buffer; no salt is needed. Explanation: The salt of the acid provides the anion which is the conjugate base of the buffer sys- tem: HA + H 2 O A - + H 3 O + This anion A - reacts with any added acid (H 3 O + ) to prevent any appreciable change in pH. 003 10.0 points What is the pH of an aqueous solution that is 0.018 M C 6 H 5 NH 2 ( K b = 4 . 3 × 10 - 10 ) and 0.12 M C 6 H 5 NH 3 Cl? 1. 9.37 2. 8.54 3. 2.87 4. 4.02 5. 10.19 6. 5.46 7. 3.81 correct 8. 4.63 Explanation: 004 10.0 points A buffer solution is made by dissolving 0.45 moles of a weak acid (HA) and 0 . 23 moles of KOH into 750 mL of solution. What is the pH of this buffer? K a = 4 . 5 × 10 - 6 for HA. Correct answer: 5 . 36609 pH. Explanation: n HA = 0 . 45 mol n KOH = 0 . 23 mol K a = 4 . 5 × 10 - 6 for HA You must substract the 0 . 23 moles of KOH from the 0.45 moles of HA because the strong base will neutralize the weak acid. You there- fore would make 0 . 23 moles of A - and be left with 0 . 22 moles of HA. You can now plug this ratio into the equilibrium equation or in the Henderson-Hasselbalch equation to get pH. 005 10.0 points
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dang (rtd356) – H08: Buffers – Mccord – (52450) 2 Which one of the following combinations is NOT a buffer solution? 1. NH 3 and NH 4 Br 2. NH 3 and (NH 4 ) 2 SO 4 3. HCN and NaCN 4. HBr and KBr correct 5. CH 3 COOH and NaCH 3 COO Explanation: A buffer must contain a weak acid/base conjugate pair. HBr/Br - is a strong acid conjugate pair. CH 3 COOH / CH 3 COO - , HCN / CN - and NH + 4 / NH 3 are weak acid/base conjugate pairs. 006 10.0 points Which of the following mixtures will be a buffer when dissolved in a liter of water? 1. 0.2 mol HF and 0.1 mol NaOH correct 2. 0.2 mol HBr and 0.1 mol NaOH 3. 0.3 mol NaCl and 0.3 mol HCl 4. 0.4 mol NH 3 and 0.4 mol HCl 5. 0.1 mol Ca(OH) 2 and 0.3 mol HI Explanation: Eliminate answers that are obviously incor- rect. The choice with “0.2 mol HBr” and “0.1 mol Ca(OH) 2 ” are strong acids and strong bases respectively; therefore, NOT buffers.
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