CHM2046Test3

# CHM2046Test3 - CHM2046 Test 3 Version A Answers underlined...

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CHM2046 Test 3 Version A Answers underlined 1a) What mass of ammonium chloride, NH 4 Cl, should be added to 1.50 L of a 0.155 M NH 3 to obtain a buffer with a pH of 9.55? pK a NH 4 + = 9.25; molar mass NH 4 Cl = 53.49 g/mol A) 3.39 kg B) 4.00 kg C) 6.24 g D) 20.8 g E) 9.3 g Use pH = pK a + log[base]/[acid] 9.55 = 9.25 + log([0.155]/[NH 4 Cl]) 9.55 – 9.25 = 0.30 = log(0.155/[NH 4 Cl]) (0.155/[NH 4 Cl]) = 10 0.30 = 1.995 [ N H 4 Cl] = 0.155/(1.995) = 0.0777 mol (53.49 g/mol) = 4.16 × (1.50) = 6.24 g 2a) Calculate the hydroxide ion concentration in an aqueous solution with a pH of 4.33 at 25°C. A) 10 2.1 10 M × B) 10 9.7 10 M × C ) 5 4.7 10 M × D) 5 3.8 10 M × E) 6 6.3 10 M × [H + ] = 10 -4.33 = 4.677 × 10 -5 ; [OH - ] = 1.0 × 10 -14 /4.677 × 10 -5 = 2.1 × 10 -10 M 3a) You wish to prepare an CH 3 CO 2 H buffer with a pH of 4.24. If the pK a of CH 3 CO 2 H is 4.74, what ratio of CH 3 CO 2 /CH 3 CO 2 H must you use? A) 0.10 B) 0.50 C) 0.32 D) 2.0 E) 2.8 Use pH = pK a + log[base]/[acid] 4.24 = 4.74 + log [CH 3 CO 2 ]/[ CH 3 CO 2 H ] -0.50 = log [CH 3 CO 2 ]/[ CH 3 CO 2 H ] [ C H 3 CO 2 ]/[ CH 3 CO 2 H ] = 10 -0.50 = 0.32 4a) A 1.00 L buffer solution is 0.150 M in HC 7 H 5 O 2 and 0.250 M in LiC 7 H 5 O 2 . Calculate the pH of the solution after the addition of 100.0 mL of 1.00 M HCl. The K a for HC 7 H 5 O 2 is 5 6.5 10 × . A) 4.19 B) 5.03 C) 4.41 D) 3.97 E) 5.91 originally [acid] = 0.150 mol [base] = 0.250 mol [HCl] added 0.100 mol after rxn 0.250 mol 0.150 mol ~0 pH = -log(6.5 × 10 -5 ) + log(0.15/0.25) = 4.19 + -0.22 = 3.97

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5a) Calculate the pH of a solution formed by mixing 250.0 mL of 0.15 M NH 4 Cl with 100.0 mL of 0.20 M NH 3 . The K b for NH 3 is 5 1.8 10 × . The mixture is a buffer! [acid] = 0.250 × 0.15 = 0.038 mol [base] = 0.100 × 0.20 = 0.020 mol pOH = pK b + log(acid/base) = 4.74 + log(0.038/0.020) = 4.74 + 0.28 = 5.02 pH = 14 - 5.02 = 8.98 A) 9.13 B) 9.25 C) 9.53 D) 4.74 E) 8.98 6a) The molar solubility of Ag 2 S is 16 1.26 10 M × in pure water. Calculate the K sp for Ag 2 S.
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## This note was uploaded on 09/21/2009 for the course CHM 2046 taught by Professor Dixon during the Spring '08 term at University of Central Florida.

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CHM2046Test3 - CHM2046 Test 3 Version A Answers underlined...

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