m2_prac_summer08

					Chemistry 14BL                                                                                      Summer 2008
                                  Study Guide for the Cumulative Exam #2


                          Exam #2 -       Thursday (July 31) at 9am (50 MINUTES)

        DON'T BE LATE - NO EXTRA TIME WILL BE GIVEN IF YOU ARE LATE TO THE EXAM.

                      Exam #2 is a CLOSED book and CLOSED notes exam.
                        There is NO REGRADE or MAKE UP for EXAM #2.
                 You cannot pass the course without taking cumulative exam #2.
         Make sure you bring a pen, a calculator (check the batteries!) and your student ID.


Exam #2 will include ALL the topics discussed in 14BL lectures. Pay special attention to the
following topics: acid-base equilibrium (including buffers); melting point theory, theory on
crystallization (or recrystallization) and chemical kinetics (i.e. the topics that discussed after exam
#1). Also review topics such as, Beer’s law, error analysis and solution concentrations for the final
exam.

When studying for the exam, FOCUS on CONCEPTUAL UNDERSTANDINGS OF THE MATERIALS.
DO NOT SIMPLY READ OR MEMORIZE LECTURE NOTES. You MUST understand the concepts
behind the experiments.

(I) Acid-Base Equilibrium & Buffers

Make sure you check any assumption that you made during the calculations (especially on the
exam)

(1) Derive the Henderson-Hasselbalch equation for buffers starting from a buffer solution that contains a weak
                                      -                                                      -5
acid (HA) and its conjugate base (A ). You may assume that the weak acid has a Ka of 10 or less. For simplicity,
                                                                                                               -
set the initial molar concentration (M) for the weak acid (HA) to be X and the initial molar concentration of A to be
Y. Based on the derivation, what are the assumptions when deriving the Henderson-Hasselbalch equation for
buffers.

(2) Base on your derivation in (1), explain why the Henderson-Hasselbalch equation will not work if the Ka for a
                                       -5
monoprotic weak acid is greater than 10 .

(3) Tris-hydroxymethyl-aminomethane (TRIS) is a weak base. It is widely used in biochemical research for the
preparation of buffers. Compute the pH of a solution that contains 0.050 moles of TRIS and 0.020 moles of HCl in a
total volume of 2.0L. Note: Use RNH2 to represent TRIS. pKb for TRIS = 5.92
                                                -5
(4) Aspirin is a weak acid with a Ka of 3.0X10 . Calculate the pH of a solution by dissolving 0.65g of aspirin in
water and diluting it to 50.0ml. You may use the symbol RCOOH to represent aspirin when writing chemical
reaction. Molecular weight of aspirin = 180.0g/mol

(5) Calculate the pH of a solution prepared by mixing 20.00ml of the aspirin solution in question #4 with 5.00ml of
0.200M NaOH. BEFORE using ICE box to solve for the pH of this mixture, think about whether the pH should
be less than 7, equal to 7 or greater than 7 simply by comparing the moles of aspirin and the NaOH.
(6) Calculate the pH of a solution prepared by mixing 20.00ml of the aspirin solution in question #4 with 10.00ml of
0.200M NaOH. BEFORE using ICE box to solve for the pH of this mixture, think about whether the pH should
be less than 7, equal to 7 or greater than 7 simply by comparing the moles of aspirin and the NaOH.

(7) Calculate the pH for the titration of 40.00ml of 0.1000M solution of ethylamine (C2H5NH2) with 0.1000M HCl
for the following volumes of added HCl:

                                        0.00ml; 20.00ml; 40.00ml and 50.00ml
                             -4
Kb for ethylamine = 6.4X10

(8) List three different ways of preparing a buffer solution.

(9) Consider the following solutions: (Note: There are no mathematics involve in this question)
(This question is a review from Exam #1)

(i) A solution mixture consists of 0.10 moles of a weak monoprotic acid and 0.20 moles of NaOH.
(ii) A solution mixture consists of 0.10 moles of a strong monoprotic acid and 0.20 moles of NaOH.
(iii) A solution mixture consists of 0.10 moles of a strong monoprotic acid and 0.10 moles of NaOH.
(iv) A solution mixture consists of 0.20 moles of a weak monoprotic acid and 0.20 moles of NaOH.
(v) A solution mixture consists of 0.20 moles of a weak base and 0.20 moles of NaOH.
(vi) A solution mixture consists of 0.20 moles of a weak base and 0.20 moles of HCl.
(vii) A solution mixture consists of 0.20 moles of a weak base and 0.10 moles of HCl.

(a) Which one of the above solutions has a pH of 7? Explain.
(b) Which one of the above solutions is a buffer? Explain.
(c) Which one of the above solutions is a weak base but NOT a buffer? Explain.
(d) Which one of the above solutions is a weak acid but NOT a buffer? Explain.

(10) (a) Explain why the equivalence point pH=7 for a strong acid and strong base titration?
(b) Explain why the equivalence point pH >7 for a weak acid and strong base titration?

Your 14A/B textbook will be an excellent resource for the concepts (NOT JUST THE
MATHEMATICS) of buffers and acid-base equilibrium chemistry. Also review your post-lab reports
for experiment # 3a & 3b.

(II) Beer's Law

(11) (a) Calculate the predicted absorbance of a solution if its concentration is 0.0278 M and its molar extinction
coefficient is 35.9 L/(mol cm). The depth of the cell is 9.0 mm.
(b) If the molar extinction coefficient is not given, how can one calculate the concentration of an unknown solution
by using Beer’s law?

(12) Calculate the absorbance of the solution if the transmitted light intensity is 71% of the initial light beam
intensity.

(13) In general, what would happen to the absorbance of a solution if a student forgot to calibrate the spectrometer
using a blank solution?

Review the concepts (i.e. post-lab report, lecture guide and lecture worksheets) that covered in experiment #2 on
Beer’s law.

The on-line tutorials (Beer’s law & solution concentrations) listed on VOH report guidelines for experiment #2
may also be useful when studying for the exam.
(III) Theory on Crystallization (or Recrystallization)
(14) What is the purpose of recrystallization? Outline the criteria for choosing a good crystallization solvent.
(15) What is the major difficulty when performing recrystallization?
(16) Outline the general procedures in recrystallization.
(17) Does polarity play a role when picking a solvent for any given product in recrystallization?
(18) Consider the following two solvents

                                     Solubility
Solvent A                52g of product in 100mL at 1000C
                         29g of product in 100mL at 250C

Solvent B                 8.1g of product in 100mL at 1000C
                                                          0
                          0.91g of product in 100mL at 25 C
(a) Which solvent is a good crystallization solvent for the product? Explain your reasoning.
(b) If a student started out with 2.5021g of crude product, what will be the % yield after one crystallization step by
using the minimum amount of solvent A that is required for the crystallization step?
(c) If a student started out with 2.5021g of crude product, what will be the % yield after one crystallization step by
using the minimum amount of solvent B that is required for the crystallization step?

Review also the concepts in mixed solvent recrystallization (similar to what you did in the aspirin experiment).
Think about the reason why mixed solvents are used in the aspirin experiment for crystallization. Refer to the lecture
guide and the Mohrig textbook (refer to the lab schedule for assigned readings) for more details on theory of
recrystallization.

(IV) Melting Point Theory
Know the effect of impurities on melting point. You should also understand how to read the binary mixture melting
point phase diagram (refer to lecture guide). Understand the concepts in mixed melting point determination etc.
Review the lecture guide and the Mohrig textbook on melting point theory. Understand the various types of
intermolecular forces that can affect melting point of a compound.

(V) Chemical Kinetics
Review the lecture guide & 14B textbook on chemical kinetics. You should know how to use kinetics rate data to
find out the rate law (like what we did in lecture). Know the relationship between the various integrated rate
equation and the order of the chemical species in the kinetics rate law (refer to lecture guide). There are plenty of
questions you can practice from your 14B general chemistry textbook on chemical kinetics.

(VI) Error Analysis and Concentration Units (Review from exam #1)
(19) Calculate the percent error for the following quantities.
    a. (20.54  0.02 X 0.254  0.003) / (3.21  0.05)
    b. 30.078  0.003 - 20.174  0.001 + 9.813  0.005
(20) Calculate the absolute errors for the quantities in question 19.

(21a) Mg(OH)2 is one of the active ingredients in Maalox (an antacid). Suppose you prepared a stock solution by
dissolving 0.0558g (weight obtained from digital balance) of Mg(OH) 2 in a 50-mL volumetric flask. Calculate the
molarity, ppm and w/v % of the Mg(OH)2 stock solution. Your final answers should have the correct units and
significant figures.
(21b) Calculate the % relative error (for the molarity) in the Mg(OH) 2 solution. (Refer to page 27 of the manual for
the absolute errors in lab equipment).
(21c) Calculate the absolute error (in molarity) for the Mg(OH)2 solution.
(21d) Suppose only one hydroxide dissociates completely in Mg(OH)2. Calculate the normality (N) for the Mg(OH)2
solution in part (a). Show all your reasoning including any conversion factors between molarity and normality.
Review concepts in solutions concentrations and serial dilution. Know the units of ppm, ppb,
w/v%, normality(N), molarity(M) and error analysis (refer to lecture guide #1 for general principle
on error analysis ). Your post-lab report #2 is an excellent reference for Beer’s law, concentration
units, serial dilution and error analysis.
                                             Solutions & Answers to Practice Problems

                                                                                                                  -
     1. Suppose that the amount of dissociation is z. X, Y are the INITIAL concentration of the HA and A respectively. Set up the
     ICE box in terms of X, Y and z.


                                                                    +              -
                         HA      +          HOH =           H3 O        +        A

                 I       X                  -----           -----                Y
                 C       -z                 -----            +z                  +z
                 E       X-z                -----           +z                   Y+z


     X,Y & z have units of M.

             -       +
     Ka = [A ] [H3O ] / [HA] = (Y+z)(z) / (X-z)

                                       -5
     If Ka is less than or equal to 10 , we can then assume that z<<Y & z<<X; therefore,

     Ka  (Y)(z) / X

     Take the –log on BOTH SIDE OF THE EQUATION:                    -log Ka = -log ( (Y)(z) / X) = -log Y –log z + log X

   This equation can now be simply to (using the definitions of pKa and pH):

     pKa = -log Y +pH + log X

                                                                        -
     pH = pKa + log Y - log X = pKa + log (Y/X) = pKa + log { [A ] / [HA]}

     2. If the Ka becomes too high, the dissociation cannot be neglected. During the process of deriving the buffer equation (refer to
     the derivation shown above), our assumption is that the dissociation of the weak acid can be ignored. If the dissociation cannot be
     neglected, the assumption that we made during the derivation will fail and so does the buffer equation.

     3. pH= 8.26 (Use your 14A/B textbook as a reference for the titration of a weak base with a strong acid)


     4. pH= 2.83;        5. pH=4.88;        6. pH =12.31

     (Refer to the acid-base equilibrium examples on lecture guide for the titration of a weak acid HA with NaOH for questions 4,5 &
     6. Concepts for questions 4,5 & 6 are identical to the example we covered in lectures.)


     7. pH= 11.89 (0mL of HCl); 10.81 (20.00mL of HCl); 6.05 (40.00mL of HCl); 1.95 (50.00mL of HCl)

     Note: Question #7 is similar to question #3.


     8. (i) directly mix a weak acid with its conjugate base; (ii) add a limited amount of strong acid to a weak base; (iii) add a limited
     amount of strong base to a weak acid.
                                       Solutions & Answers to Practice Problems

9. (a) solution (iii)       (b) solution (vii)            (c) solution (iv)           (d) solution (vi)

For question #9, simply compare the starting moles of the two reagents (ONLY if one is an acid and the other is a base) and see
whether the resulting mixture will give the desired product(s). There is no need for you to set up an equilibrium table for each
solution mixture in order to correctly answer this question. Review the concepts of buffer from your 14B textbook.

For part (a), the final product between a reaction of a strong acid and strong base is NEUTRAL.

For part (b), a buffer solution must consist of a weak acid and its conjugate base at equilibrium. For solution (vii), the moles of
the weak base is greater than the moles of the strong acid. Therefore, the resulting mixture will be a buffer.
Refer also to the answer for question #8.

For part (c), at the equivalence point between the reaction of a weak acid and a strong base will yield a solution that contains
only the weak base. Solution (iv), is the only one that has equal number of moles of weak acid and a strong base.

For part (d), at the equivalence point between the reaction of a weak base and a strong acid will yield a solution that contains
only the weak acid. Solution (vi), is the only one that has equal number of moles of weak base and a strong acid.

10. (a) Final products are H2O and a neutral (i.e. pH = 7) salt;
(b) Final products are H2O and a weak base. Hydrolysis of the weak base gives out OH-.

11. (a) 0.90; (b) use the Beer’s law calibration graph by preparing a series of standard solutions (refer to the lecture materials
and the online tutorial for more information)

12. 0.15

13. The absorbance will goes up due to the interference of absorption from other chemical species and the (0,0) point will not be
satisfied on the Beer’s law calibration graph.

14, 15 ,16 Refer to the “crystallization” lecture guide and the Mohrig textbook for the theory on crystallization.

17. Yes, since "like dissolves like". Polar compound will dissolve in polar solvent. Using the idea of “like dissolves like”, one can
then pick the proper solvent for crystallization according to the criteria discussed in lecture.

18. (a) Solvent B (refer to the lecture notes on the criteria for selecting a good crystallization solvent).
(b) 44%
(c) 89%

19. (a) 3% ; (b) 0.05% (Report to only 1 significant figure)                20.       (a)  0.05 ; (b)  0.009

21a. 0.0191 M; 1.12X103 ppm; 0.112% 21b. 0.4% (you will get 0.5% if you round off number during each step of the
                                                calculation!!)

21c.      0.0001M (actual value is  0.00008M);                  21d.      0.0191 N Mg(OH)2

				
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