HW7-ex3 - dsj294 hw07 Holcombe(53570 This print-out should...

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dsj294 – hw07 – Holcombe – (53570) 1 This print-out should have 25 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points When sodium formate is added to HCOOH(aq), 1. the pH of the solution increases. correct 2. the equilibrium concentration of HCOOH(aq) decreases. 3. the pH of the solution decreases. 4. the pH of the solution does not change. 5. the K a increases. Explanation: 002 10.0 points Some solid NH 4 Cl is added to a water solution of NH 3 . Which of the following statements is correct? 1. [OH - ] of the solution increases. 2. [H 3 O + ] of the solution remains the same. 3. The pH of the solution decreases. cor- rect 4. The pH of the solution increases. Explanation: The salt NH 4 Cl dissociates in solution, giv- ing the spectator ion Cl - and NH + 4 which hydrolyzes to NH 3 and H 3 O + . This decreases the pH. 003 10.0 points Which of the following mixtures can produce a buffer? I) NH 3 + NaOH II) NH 3 + NH 4 Cl III) H + + OH - IV) HClO 2 + NaClO 2 V) (CH 3 ) 3 NHCl + (CH 3 ) 3 N 1. II, IV, and V only correct 2. I, III, and V only 3. II, III, and IV only 4. IV only 5. II only Explanation: A buffer contains a weak acid or weak base, plus the salt of that weak acid or base; or, a mixture which will have this composition after any acid-base reactions occur. 004 10.0 points Which of the following is NOT true for a solution buffered at a selected pH? 1. The solution pH will not change with the addition of an acid. correct 2. The buffered solution contains an acid and its conjugate base. 3. The solution pH will increase a lesser amount than a non-buffered solution, when base is added. 4. The buffered solution could be made to be either acidic or basic. Explanation: Buffers are solutions which contain compar- ative amounts of a conjugate weak acid/base part and allow for only small changes in the pH of a chemical system, compared to non- buffered solutions, when an acid or a base is introduced. 005 10.0 points Which of the following would cause the pH of a 0.10 M solution of HF to decrease? 1. adding more water 2. adding some Ag + which would precipitate part of the F - from the solution as solid AgF correct
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dsj294 – hw07 – Holcombe – (53570) 2 3. dissolving gaseous NH 3 in the solution 4. adding solid NaF 5. adding solid KNO 3 Explanation: M HF = 0.10 M HF + H 2 O H 3 O + + F - HF is a weak acid. Adding NaF would create a buffer by supplying the conjugate base F - , consuming H 3 O + and making the solution less acidic (higher pH). Similarly, the base NH 3 would react with H 3 O + and pH would increase. Adding more water would dilute the solution, reducing the concentration. Solid KNO 3 , the salt of a strong acid and a strong base, would have no effect on pH. AgF is an insoluble salt and would precipitate out if Ag + ions are added and in turn equilibrium would shift to the right, producing more H 3 O + and lowering the pH.
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