chem 173 exam 2 - CHEMISTRY 173 EXAMINATION II Dr Sebastian...

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CHEMISTRY 173 EXAMINATION II Dr. Sebastian Canagaratna May 1, 2008 NAME: Section: Lecture time Lab Instructor Section 9:00 Kaple 173.01 9:00 Gilligan 173.02 19:00 Gilligan 173.09 12:00 Kaple 173.12 You will lose credit if you do not show your work where appropriate
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Part I. (40 points) Answer the following appropriately ( filling in the blank, circling the correct answer, etc. ) 1. Write the net ionic equation , including phases and charges , which describe (a) the reaction occurring when a small amount of HCl(aq) is added to an aqueous solution containing both CH 3 NH 2 (aq) and CH 3 NH 3 Cl(aq) . ans : CH 3 NH 2 (aq) + H + (aq) -→ CH 3 NH + 3 (aq) (b) The dissolution of Ag 2 SO 4 (s) in water. ans : Ag 2 SO 4 (s) * ) 2Ag + (aq) + SO 2 - 4 (aq) (c) the base ionization of H 2 PO - 4 (aq) . ans : H 2 PO - 4 (aq) + H 2 O(l) * ) H 3 PO 4 (aq) + OH - (aq) (d) why an aqueous solution of NH 4 Cl is acidic. ans : NH 4 Cl -→ NH + 4 (aq) + Cl - (aq); NH + 4 (aq) + H 2 O * ) NH 3 (aq) + H 3 O + (aq) 2. The indicator methyl red is a weak acid with a K a of about 10 - 5 and may be represented as HIn. HIn is red, while In - is yellow. What is the color of a solution of 1.0 M HCl to which a few drops of methyl red have been added? . What is the color of a solution of 1.0 M NaOH(aq) to which a few drops of methyl red have been added? ans : red; yellow. 3. The pH at 25°C of a solution which is 0.100 M in HCN and 0.100 M in KCN is 9.21. The value of K a for HCN is therefore . ans : Buffer solution with [A - ] = [HA], pH = p K a = 9.21, K a = 10 - 9 . 21 = 6 . 17 × 10 - 10 4. If 5.00 mL of 0.1 M NaOH is added to 100 mL of the solution described in the above question, the pH after the addition is . ans : Do the table as a complete reaction, [CN - ]/[HCN] = (100+5)/100-5. pH = 9.25 5. True (T) or false (F) ? (a) Ammonia ( p K b =4.74) forms a buffer with NH + 4 whose best buffer capacity is in the range pH=3.74 to 5.74: ans : False; p K a is 14 - 4 . 74 . (b) AgCl(s) can be dissolved by the addition of HNO 3 (aq): ans : F (c) For spontaneous processes at constant temperature and pressure G of the system decreases: ans : T (d) In a titration of HF(aq) with NaOH, the pH at end point will be less than 7: ans : F. At the end-point we have a solution of F - . 6. Consider the slightly soluble salt PbBr
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chem 173 exam 2 - CHEMISTRY 173 EXAMINATION II Dr Sebastian...

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