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Exam 2-solutions

# Exam 2-solutions - Version 456 Exam 2 laude(51635 This...

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Version 456 – Exam 2 – laude – (51635) 1 This print-out should have 20 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 6.0points 100 mL of 0.50 M BaCl 2 is added to 100 mL of 0.50 M HCl. Which of the following equations is the correct charge balance for this system? 1. 2[Ba 2+ ] + [H + ] = 2[OH - ] + 2[Cl - ] 2. [Ba 2+ ] = 2[OH - ] 3. 2[Ba 2+ ] + [H + ] = [OH - ] + [Cl - ] correct 4. [Ba 2+ ] + [H + ] = 3[Cl - ] 5. [Ba 2+ ] + [H + ] = [OH - ] + [Cl - ] 6. 2[Ba 2+ ] = [Cl - ] 7. 2[Ba 2+ ] + [H + ] = 2[OH - ] + [Cl - ] Explanation: 002 6.0points What is the pH at the half-stoichiometric point for the titration of 0.22 M HNO 2 (aq) with 0.01 M KOH(aq)? For HNO 2 , K a = 4 . 3 × 10 - 4 . 1. 2.01 2. 7.00 3. 3.37 correct 4. 2.31 5. 2.16 Explanation: 003 6.0points For the titration of 50.0 mL of 0.020 M aque- ous salicylic acid with 0.020 M KOH(aq), cal- culate the pH after the addition of 55.0 mL of KOH(aq). For salycylic acid, p K a = 2.97. 1. 10.02 2. 12.30 3. 12.02 4. 10.98 correct 5. 7.00 Explanation: 004 6.0points WITHDRAWN 005 6.0points What is the final pH of a solution containing 100 mL of 0.2 M HX and 300 mL of 0.1 M NaX after 0.01 mol of NaOH is added? The p K a is 3.00. 1. 2.40 2. 2.70 3. 3.00 4. 12.40 5. 3.60 correct Explanation: Initially (100 mL) (0 . 2 M) = 20 mmol HA (300 mL) (0 . 1 M) = 30 mmol A - Now add the impurity: 0.01 mol of NaOH = 10 mmol OH - : HA H + + A - ini 20 0 30 Δ - 10 +10 fin 10 40 Thus A - HA = 40 10 = 4 and pH = 3 . 0 + log(4) = 3 . 60206 . 006 6.0points

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Version 456 – Exam 2 – laude – (51635) 2 Which of the following statements concerning polyprotic acids is false? 1. Of the p K a values for a polyprotic acid, p K a 1 is always the smallest. 2. Polyprotic acids have one more buffer region than amphiprotic region. 3. A diprotic acid exists in three forms in solution. 4. The ability to resolve the equivalence points in titration curves becomes easier as the number of acidic protons increases. cor- rect Explanation: Additional acidic protons smooth the titra- tion curve and make it more difficult to resolve individual endpoints, or any other feature of the titration curve for that matter. Since p K a 1 necessarily describes the most acidic proton of a polyprotic acid, it will be the smallest of the acid’s p K a 1 s. Diprotic acids can exist as H 2 A, HA - or A 2 - . When deal- ing with polyprotic acids, several simplifying assumptions are possible if the K a values are several orders of magnitude apart 007 6.0points
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Exam 2-solutions - Version 456 Exam 2 laude(51635 This...

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