Chem1BExam3SampleTestKey

Chem1BExam3SampleTestKey - Chem 1B Exam 3 Sample Questions...

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Chem 1B Exam 3 Sample Questions Instructions: For all mathematical problems, show your work. Give the basic formula (if there is one), rearrange it if necessary, plug in data with units, round properly to the correct number of significant figures, and place a box around the answer. For essay questions, be complete, but concise. Complete sentences with proper spelling and punctuation are expected. Useful information: K w = 1.0 × 10 -14 Spectrochemical series: CO > CN 1- > NO 2 1- > en > NH 3 > H 2 O > OH 1- > F 1- > Cl 1- > Br 1- > I 1- 1. (10 pts.) How many moles of sodium benzoate, NaC 7 H 5 O 2 , must be added to 1.00 L of 0.150 M benzoic acid, HC 7 H 5 O 2 , to form a buffer solution of pH 4.60? K a for benzoic acid is 6.4 × 10 -5 . HC 7 H 5 O 2 (aq) + H 2 O( l ) H 3 O + (aq) + C 7 H 5 O 2 1- (aq) pH = pK a + log [C 7 H 5 O 2 1- ] [ HC 7 H 5 O 2 ] pH = pK a + log[C 7 H 5 O 2 1- ] - log[HC 7 H 5 O 2 ] pH + log[HC 7 H 5 O 2 ] - pK a = log[C 7 H 5 O 2 1- ] log[C 7 H 5 O 2 1 ] = 4.60 + log(0.150) - [-log(6.4 × 10 -5 )] = -0.42 [C 7 H 5 O 2 1 ] = 0.38 M 0.38 mol HCNO must be added. 2. (10 pts.) To 50.0 mL of 0.120 M hydrazoic acid, HN 3 , is added 25.0 mL of 0.175 M sodium hydroxide solution. Calculate the pH of the resulting solution. K a for hydrazoic acid is 1.9 × 10 -5 . HN 3 (aq) + H 2 O( l ) H 3 O + (aq) + N 3 1- (aq) Initial moles HN 3 = 0.0500 L × 0.120 M = 0.00600 mol Added moles OH 1- = moles HN 3 consumed = moles N 3 1- generated = 0.0250 L × 0.175 M = 0.00438 mol pH = pK a + log [N 3 1- ] [HN 3 ] pH = -log(1.9 × 10 -5 ) + log 0.00438/V (0.00600 - 0.00438)/V pH = 4.72 + log(2.70) = 5.15
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3. (10 pts.) Calculate the molar solubility of La(OH) 3 . K sp = 2.0 × 10 -21 La(OH) 3 (s) La 3+ (aq) + 3OH 1- (aq) K sp = [La 3+ ][OH 1- ] 3 Let x = molar solubility of La(OH)
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Chem1BExam3SampleTestKey - Chem 1B Exam 3 Sample Questions...

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