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Exam02-solutions

# Exam02-solutions - Version 091 Exam02 Walker(53985 This...

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Version 091 – Exam02 – Walker – (53985) 1 This print-out should have 39 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Rainwater is naturally acidic, but in which area of the US is the rain often much more acidic? 1. Northeast correct 2. Central 3. Northwest 4. Southwest Explanation: Measurements taken by the Central Ana- lytical Laboratory from the mid 1970s onward indicate that the rainwater in the Northeast is consistently more acidic than in other parts of the US. 002 5.0 points If the oxidation state of H is +1 and O is - 2, what is the oxidation state of C in C 2 H 4 O? 1. - 1 correct 2. +1 3. - 2 4. +2 5. - 1 2 Explanation: To calculate oxidation numbers, remember that the the sum of the numbers must equal the total charge on the molecule or in the case of neutral species, zero. Let x be the oxidation number of C. 2 x + 4(+1) + ( - 2) = 0 C 2 H 4 O : 2 x + 2 = 0 2 x = - 2 x = - 1 003 5.0 points If the pH of an aqueous solution changes from 5.2 to 8.6, what has happened to the hydro- nium ion concentration? 1. It increased. 2. It decreased. correct 3. It did not change. 4. It became less than zero. 5. It became zero. Explanation: 004 5.0 points Which of the following is an acid? 1. HNO 3 correct 2. NH 3 3. CH 4 4. NaOH 5. NaCl Explanation: 005 5.0 points Bonus question. The juice from purple cabbage serves as a pH indicator. When we did the experiment in class, the indicator in solution was first col- ored , then turned when we breathed into it. The color change was due to the . 1. pink; colorless; H 2 CO 3 2. blue; pink; H 2 CO 3 correct 3. colorless; pink; H 2 CO 3 4. green; purple; H 2 CO 3 Explanation: Austin tap water is basic due to the lo- cal limestone; purple cabbage juice is blue

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Version 091 – Exam02 – Walker – (53985) 2 in basic solutions. When we breathed into the solution, CO 2 from our exhaled breath combined with water to form carbonic acid. More breaths created more of the acid un- til the acid molecules outnumbered the basic molecules. In this acidic environment the in- dicator turned to pink/purple. 006 5.0 points A solution with a higher pH is more acidic. 1. False correct 2. True 3. The answer cannot be determined with- out additional information. Explanation: pH = - log [H 3 O + ], so as [H 3 O] increases, pH decreases. 007 5.0 points In a fuel cell, the reaction at the anode is 1. H 2 2 H + + 2 e correct 2. H + + OH H 2 O 3. O 2 2 O 2 + 2 e 4. No reaction; Pt is inert. 5. H 2 O H + + OH Explanation: Oxidation occurs at the anode. In the hy- drogen fuel cells studied, the hydrogen loses electrons (is oxidized) to form H + . 008 5.0 points The reaction Ag + + Cu Ag + Cu + is an example of a(n) 1. precipitation reaction. 2. oxidation/reduction reaction. correct 3. acid/base reaction. 4. combustion reaction. 5. destructive reaction.
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