Lecture #26 What’s on the Final?. Chemistry 142 B Autumn Quarter, 2004 J. B. Callis, Instructor. General. Date as Announced in Syllabus 44 Multiple-Choice Questions - Chapters 1-8 22 questions from chapters 1-5 22 questions from chapters 6-8
Chemistry 142 B
Autumn Quarter, 2004
J. B. Callis, Instructor
A chemist studies three oxides of iodine, and finds their % oxygen as follows: 20.14 %, 23.97% and 22.10 %. (a) Calculate the mass of oxygen per gram of iodine in each compound. Express the result as a ratio. Then form the ratio of ratios by dividing by the ratio of the first compound. (b) How do the numbers in part (a) support Dalton’s atomic theory?
Problem 26-2: Bromoform is 94.85% bromine, 0.40% hydrogen and 4.75% carbon by mass. Determine its empirical formula.
x NaCl + y SO2 + z H2O + w O2 => u Na2SO4 + v HCl
2 SO2 + O2 + 2 H2O => 2 H2SO4
Suppose 400. g SO2, 175. g O2 and 125. g H2O are mixed and the reaction proceeds until one of the reactants is used up. Identify the limiting reactant and determine what masses of the other reactants remain.
x H+ + y H2O2 + z Fe2+ => u Fe3+ + v H2O
Problem 26-6: A 20.6 L sample of air is collected in Greenland at –20. oC at a pressure of 1.01 atm and forced into a 1.05 L bottle for shipment to Europe for analysis. Compute the pressure inside the bottle as it is opened in the laboratory at 21. oC.
Problem 26-7: At 5000 K, even the nitrogen molecule (N2) breaks down (into 2 atoms of nitrogen). At this temperature, when the total pressure of nitrogen is 1.00 atm, N2(g) is a = 0.65% dissociated at equilibrium: N2(g) = 2 N(g). Compute the equilibrium constant at 5000K.
Problem 26-8: What is the pH of 0.15 M methylammonium bromide, CH3NH3Br? (Kb of CH3NH2 = 4.4 x 10-4
Problem 9: You have at your disposal an ample quantity of a solution of 0.0500 M NaOH and 500 mL of a solution of 0.100 M formic acid (HCOOH, Ka = 1.77 x 10-4). How much of the NaOH solution should be added to the acid solution to produce a buffer of pH 4.00?
Ans: Use the base to produce a sufficient amount of the formate ion to provide a buffer of the desired pH. Do the problem at the Henderson-Hasselbalch level, ignoring the ionization of water. Allow for dilution of the original acid solution with the base, just as in a titration.
The relevant reaction is:
Problem 10: A saturated solution of Mg(OH)2 at 25oC is prepared by equilibrating solid Mg(OH)2 with water. Concentrated NaOH is then added until the solubility of Mg(OH)2 is 0.001 times that in H2O alone. (Ignore the change in volume resulting from the addition of NaOH.) The solubility product Ksp of Mg(OH)2 is 1.2 x 10-11 at 25oC. Calculate the concentration of hydroxide ion in the solution after the addition of the NaOH.
Ans: First calculate the solubility of Mg(OH)2 in water. Then calculate the concentration of [Mg2+] after addition of OH- put this into the mass action expression and solve for [OH-].
Addition of OH- shifts the equilibrium to the left and the concentration of Mg2+ must diminish to maintain the mass-action expression at a constant value. After the addition of base, the new concentration of Mg2+ is