Fall_2009_105b_Practice_Exam_2_key

Fall_2009_105b_Practice_Exam_2_key - Practice exam#2 with...

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1. (22 pt) Answer the following multiple choice or short answer questions. a. Identify the following statements as true or false and correct the false statements. i. Strong acid can be used to change acetic acid molecules into acetate ions. ___________ ii. When hydroxide ions are added to a solution buffered with ammonia molecules and ammonium ions, the ammonia concentration will increase and the ammonium concentration will decrease. ______________ iii. The solubility of AgCl will be different in pure water than in a solution with a pH of 4.5. ______________ iv. If the enthalpy of a reaction is positive and the entropy of the reaction is negative, the reaction will be spontaneous at all temperatures. _______________ b. If you want to create a solution buffered at pH 3.15, which of the following acids would make the best buffer? i. HF (K a = 7.3 x 10 -4 ) ii. HNO 2 (K a = 6.6 x 10 -4 ) iii. C 2 H 6 NH (K b = 5.9 x 10 -4 ) iv. HCN (K a = 6.2 x 10 -10 ) c. Will solutions of the following salts be acidic, basic or neutral? If necessary, refer to the table of K a and K b values on the last page. NaOBr __________________ NH 4 NO 3 __________________ CaF 2 __________________ KI __________________ (CH 3 NH 3 ) 2 HPO 4 __________________ d. Which of the following molecules is the least soluble in pure water? i. PbF 2 (K sp = 4.0 x 10 -8 ) ii. PbCl 2 (K sp = 1.6 x 10 -5 ) iii. PbS (K sp = 7.0 x 10 -29 ) iv. Pb 3 (PO 4 ) 2 (K sp = 1.0 x 10 -54 )
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2. (10 pt) A solution contains 0.40 M ethylamine (C 2 H 5 NH 2 ) a weak base with K b of 4.3 x 10 -4 and 0.80 M pyridine (C 5 H 5 N) a weak base with K b of 1.5 x 10 -9 .
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