CH335			      Oral Review Questions
Fall, 1998	
Dr. Ross

	The questions below will be used to initiate the Final Oral Exam for the 
course.  As has been the case all semester, we will concentrate on the CONCEPTS 
embodied in these questions.  At the time of the oral exam each member of 
your group will draw a number to determine which question he/she will answer.  
The questioning will then continue in order to examine all related concepts.  
Other members of the group will provide support and act as resources, if necessary.


	In preparing for the oral review, you may use any resources you wish (text,
library, other faculty members, other students, or me) to help your group 
understand the concepts highlighted by these questions.


	Best wishes, and if you have questions, please let me know.


* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

1. Below is a summary of the important analytical steps in the analysis of your unknown for experiment I.







	50 mls of 1 M NaOH was pipetted into a 500 ml volumetric flask, diluted to 
     the line and mixed.








	3.8 to 4.2 grams of dried KHP were transfered to a 250 ml volumetric flask,
	dissolved, diluted to the line and mixed.  50 ml aliquots were taken by 
    pipet and titrated against the titrant to standardize it.








	1.4 to 1.6 grams of the dried unknown was weighed carefully and transferred
	to a 250 ml Erlenmeyer flask.  It was dissolved in 50 mls of water and 
    titrated against the titrant.



From all the data, the % KHP in the unknown was calculated.



There are many potential sources of error in this experiment.



 



	a.  What do you feel are the two most likely potential sources of 
        determinant   errors? Justify your choices.




	b.What do you feel are the two largest sources of indeterminant   error? 
		Justify  your  choices.




	c.Discuss briefly how the errors in (a) and (b) above will affect the
		accuracy  and precision  of this experiment.














 2.  On page 67 of your text the author presents the following formula, which may 
    be used to  perform the t test for comparing the means of two different 
    analytical methods.



 












 



a.  What is the meaning of each parameter in the equation.










b.  Explain briefly how the t test is used and how the value of t is affected by 
    each parameter in the equation.





3.  The following set of results was obtained for "identical" determinations of 
    the percentage chloride in an unknown sample.





		     			

   

      
         
Weight of Sample
         
         
(in grams)

      		
      
         
% Cl in sample
         
         

      		
   

   

      
         
0.5678

      		
      
         
33.43

      		
   

   

      
         
0.2390

      		
      
         
33.14

      		
   

   

      
         
0.9469

      		
      
         
33.52

      		
   

   

      
         
0.4052

      		
      
         
33.32

      		
   

   

      
         
0.1519

      		
      
         
32.85

      		
   

   

      
         
1.615

      		
      
         
33.57

      		
   









             
							

1)   Analyze the results as completely as possible, indicating specifically the 
    errors involved and how to treat them.  Is there evidence for a determinate 
    error?



2)   Based on your analysis of the results, what is likely to be the best estimate 
     of the "true" percentage of chloride in the sample?  Be sure to justify your 
     answer.



 



NOTE: Caution!! This is not a trivial
question.













4.     Silver nitrate can be purchased as an analytical reagent in very high 
     purity(99.9 percent).  However, if care is not exercised in handling it, 
     some of it will be photoreduced to silver metal and nitrogen oxide gases.



			AgNO3 (s) + hv   ===>     Ag(s) + gases






Suppose for a particular sample of AgNO3 that two
percent of the moles had been reduced to silver metal. Without
knowing this, an analyst weighed out 5.000 grams of the mixture and
diluted it to 500.0 mls in a volumetric flask. (NOTE: for the
following, assume that silver metal does not dissolve.) (MW of
AgNO3 = 169.0, Ag = 107.9).





	1) What would be the molarity of the silver nitrate that the analyst would
	calculate (i.e., based on 99.9 percent purity?)

	2) What is the actual molarity?

	3) What is the absolute error in the molarity?

	4) What is the relative percent error in the molarity?












5.    Consider the precipitation of substances by slowly pouring the precipitating
	agent into the  sample while stirring vigorously.




 

   

      
         
Experiment

      		
      
         
type of precipitate

      		
      
         
ppting agent

      		
      
         
sample

      		
      
         
precipitate

      		
   

   

      
         
I

      		
      
         
Crystalline

      		
      
         
Na2PtCl6

      		
      
         
KCl

      		
      
         
K2PtCl6

      		
   

   

      
         
II

      		
      
         
Crystalline

      		
      
         
K2C2O4

      		
      
         
CaBr2

      		
      
         
CaC2O4

      		
   

   

      
         
III

      		
      
         
Colloidal

      		
      
         
Na2S

      		
      
         
NiNO3

      		
      
         
NiS

      		
   

   

      
         
IV

      		
      
         
Colloidal

      		
      
         
KI

      		
      
         
AgNO3

      		
      
         
AgI

      		
   









		                                                	


   For each of the four experiments in the table, indicate:

	a) What ion will most likely be primarily adsorbed during the 
        precipitation?

	b) What ion will most likely be secondarily adsorbed during the 
        precipitation?

	c) What ion will most likely be primarily adsorbed after precipitation is
		complete?

	d) What ion will most likely be secondarily adsorbed after precipitation is
		complete?

	e) What compound will most likely be coprecipitated?

	f) Will coprecipitation of the above compound lead to high, low, or 
        unchanged results in the percent of sample?  Explain.













 6  0.50 mls of 0.1 M KBr (FW = 119) and 50 mls of 0.1 M NaCl (FW = 58) are both
	added to 75 mls of 0.1 M AgNO3  (FW = 169) to make a total of 175 mls of 
    resulting solution.  If Ksp for AgBr is 8 x 10-13 and Ksp for AgCl is 
    1 x 10-10 what are the concentrations of all species in solution when the 
    system reaches equilibrium?














7.    Consider the following experimental information:  Given: Ammonia, vinegar,
	water and a phenolphthalein solution.

	a.	10 drops of vinegar are added to 50 mls of water.  When 5 drops of
		phenolphthalein  solution are added, the water remains colorless.  This
		solution is discarded.

	b.	10 drops of ammonia are added to 50 mls. of water.  When 5 drops of
		phenolphthalein  solution are added, the water turns deep red.

	c.	When 40 drops of vinegar are added to the solution resulting in  b 
		above, the solution remains deep red.

	d.	When 8 more drops of (total = 48) of vinegar are added to the 
        solution resulting in c  above, the solution turns very light pink.

	e.	When 2 more drops (total = 50) of vinegar are added to the solution
		 resulting in  d  above, the solution becomes colorless.

	f.	Answer the following questions about this experiment.

		1.  What are the independent  variables in the experiment?


		2.  What are the dependent  variables in the experiment?


		3.  What variables are controlled  in parts  b,  c,  d  and  e  of this
			experiment?


		4.  What can be inferred  from the results in part  e?


		5.  What determines  if a solution turns red when phenolphthalein
			is added?













 8.     An analyst has a 0.2500 gram sample that is 10.00 percent Ca.  In setting 
    up a gravimetric  method of analysis she is considering three possible weighing
	forms:  CaO (MW = 56), CaC2O4 (MW = 128) or Ca2P2O7  (MW = 254).

	a)Assuming the MW of Ca = 40, what weight (to the fourth decimal place) of
		precipitate  would she get for each weighing form?


	b)Assuming a weighing error of 0.0002 grams, what relative error (in parts
		per thousand would be associated with each weighing form?













 9.    A solution that is 0.2 F in Pb+2 is titrated with 0.4 F I- .  A second 
    portion of the original 0.2 F Pb+2 solution is titrated with 0.2 F CrO4-2.  
    Both I- and CrO4-2 form insoluble precipitates with Pb+2.

	Ksp for PbI2  is  2.5 x 10-13,   Ksp for  PbCrO4 is  1.8 x 10-14 .

	a)Sketch the approximate titration curves for the two titrations 
		(pPb [vertically] vs %   titrated).

	b)Which titration would be the most satisfactory?  Explain













10.   A sample of impure NaOH, which has been partially converted to Na2CO3 by
	exposure to CO2,  is analyzed by titrating a 188.5 mg sample with 
    0.1065 M HCl. 
	The volume required to reach the phenolphthalein endpoint is 39.19 mL, 
    while the volume required to reach the   bromocresol green endpoint 
    is 40.67 mL. 
	Calculate the percentages of NaOH and Na2CO3  in the sample.














11.  A mineral sample containing dolomite (CaCO3 : MgCO3) and weighing 865 mg is
	treated with 10.00 mL of 1.542 M HCl to dissolve the carbonate material.  
    The resulting mixture is  boiled to remove CO2, and the excess HCl is 
    titrated with 0.2163 M NaOH.  If 26.03 mL is  required to reach the 
    methyl red endpoint, calculate the percentage of dolomite in the  sample.




12.  A solution contains one of the following:

		a) H3PO4 				  d) KH2PO4 - K2HPO4
		b) H3PO4 - KH2PO4 		 e) K2HPO4
		c) KH2PO4 				 f) K2HPO4 - K3PO4

	One 50.00 mL aliquot of the solution was titrated with 0.1000 M NaOH and
	another separate  50.00 mL aliquot was titrated with 0.1000 M HCl.  The
	resulting titration curves are shown
	below.

	a) Which of the above was it?  Explain your answer.
		
	b) Calculate the actual molar composition of the solution (in terms of moles 
       per liter to three significant figures).  
		K1 = 7.1 x 10-3 , K2 = 6.3 x 10-8, K3 = 4.2 x 10-13



















13.   Sketch the approximate titration curve you would observe if 100 mls of a 
    mixture containing HCl (2 mmoles), HA (3 mmoles),  and HB (4 mmoles) is 
    titrated with 0.2 M NaOH.  Assume that Ka for HA is 10-4  and Ka for 
    HB is 10-8.  Be sure to label the volume axis carefully and observe the 
    pH axis as labeled.
















14.   Several relationships for approximating the [H+] (or [OH-] in cases of basic
	solutions) are   listed below.  For each question below, choose the one
	relationship that would be most  useful for approximating the [H+] (or [OH-]) 
    to within about 5 percent.


	A.  What relationship would you use to calculate the [H+] (or [OH-]) for the
		following   solutions:

	1)  is 0.01 F in HA (Ka = 1 x 10-6 )?

	2)  is 0.01 F in NaA (Ka for HA = 1 x 10-6 )

	3)  is 0.001 F in HA and 0.01 F in NaA (Ka for HA = 1 x 10-3 )

	4)  results when 10 mls of 0.2 F NaOH is added to 40 mls of 0.1 F HA 
		(Ka for HA = 1 x 10-6 )

	5) results when 10 mls of 0.001 F HCl is added to 100 mls of 0.01 F HA
	    (Ka for HA = 1 x 10-3 )
	
	















15.  How many grams of dipotassium phthalate must be added to 750 mL of 0.0500 F
	phthalic acid to give a buffer of pH 5.75?     
	For phthalic acid, pK1 = 2.95,   pK2 = 5.41



16. In the cell drawn below, the silver electrode weighs 20.2503 mg and is 
    immersed in 500.0 mL of 2.00 M AgNO3.  The copper electrode weighs 
    27.2953 mg and is in 500.0 mL of 1.00 x 10-2 M CuSO4 .

	For Cu+2 + 2e- = Cu,
	  Eo  = + 0.34

	For Ag+ + e- = Ag
	  Eo  = + 0.799









 




	a) What is the potential of the cell?

	b) Write the overall spontaneous reaction.

	c) How would you make the reaction proceed in reverse?

	d) In the spontaneous cell, which electrode is the cathode?

	e) In the spontaneous cell, which electrode is negative?

	f) In the spontaneous cell, which way are the electrons moving?

	g) What is the weight of the copper in the completely discharged cell?

	h) What is the potential for the silver electrode when discharged?

	i) What is the Keq of the reaction at a pH of 3.5?



17.   Calculate   Y-4  for EDTA in a solution of pH 10.  Assuming the formation 
    constant for the Ca2+  EDTA complex is 5 x 1010, calculate the pCa at 50, 100 
    and 150 percent titrated in the titration of 100 mL of 0.1000 M Ca2+ 
    with 0.1000 M EDTA.

18.  Solutions of HCl in water are essentially colorless, as are solutions of NaOH
    and  NaCl.   However, the acid-base indicator HIn and its anion In-  are both 
    colored. 
	Using the spectra below for HIn and In- , sketch the approximate
	spectrophotometric titration curves you would get at both  l 1  and  l 2 for the
	titration of a mixture of HCl (strong acid) and HIn (weak acid) with NaOH.




















19.  Cu+2 and Zn+2 both form complexes with ethylenediamine (en):

	For Cu(en)2+2 [b1  = 106, b2 = 1031 ] and for Zn(en2+2 [b1  = 1011, b2 = 1016]


	a)Calculate the conditional formation constants for the EDTA complexes of 
        both Cu+2 and Zn+2 under the conditions listed in the table below.  
        Show some sample calculations, but there is no need to show all 
        similar calculations.


   

      
         
ION

      		
      
         
pH = 3 [en] =0

      		
      
         
pH = 3 [en] = 1

      		
      
         
pH = 10 [en] = 0

      		
      
         
pH = 10 [en] = 1

      		
   

   

      
         
Cu+2

      		
      
         

      		
      
         

      		
      
         

      		
      
         

      		
   

   

      
         
Zn+2

      		
      
         

      		
      
         

      		
      
         

      		
      
         

      		
   







	b)  What would be the most favorable conditions for titrating the sum of 
        Cu+2 and Zn+2  with EDTA?  Justify your answer.

	c)  What would be the most favorable conditions to titrate just one of the 
        two ions (Cu+2  or Zn+2) with EDTA?  Justify your answer.

20.   A chemist was once faced with the problem of calibrating her bathtub.  Her
	ingenious plan was to place a known amount of iron in the bathtub and fill it
	to capacity.  Then, by determining the concentration of iron in the bathtub
	water, she could calculate the  capacity.

	The method she used was to complex all the iron with bathophenanthroline     
	(bathophenanthroline is a particularly good reagent for determining the
	amount of iron in   bathtub water) and measure it spectrophotometrically.

			Fe+2 + 3BpH = Fe(BpH)3+2


	She dissolved 84 milligrams of pure iron in HNO3  and transferred it
	quantitatively to the  bathtub.  After adding a large excess of
	bathophenanthroline, reducing agent and buffer,  she filled the bathtub to
	capacity and mixed it well.

	A sample of the resulting solution was then placed in a 2.5 cm cell and the
	absorbance of  the solution was found to be 0.420 at a wavelength of 500.  
    What is the capacity of the  bathtub as expressed in liters? 
	(MW Fe = 56, MW BpH = 330 and e = 23,000)


21.  What is the solubility of CaF2 in a solution that is buffered at a [H+] of 
    3 x 10-4  M and has an initial [Ca+2 ] of 5 x 10-4 M?   (HINT: Your answer 
    should be good to about 5% relative error.)

	Ksp  for CaF2 = 3.9 x 10-11 and Ka  for HF is 6.8 x 10-4


22.  In this problem we will calculate the voltage of a cell in two ways.  The cell is




Pt(s)/H2(g,0.100 atm) H+(aq,pH=2.54) // Cl-(aq, 0.200 M) Hg2Cl2(s),Hg(l)/Pt (s)



	a)  Write a reduction half-reaction for each electrode.

	b) Write the Nernst equation for each half-reaction and evaluate E for each
		half-reaction  under the given conditions.  (For the Hg2Cl2 half-reaction,
		Eo = 0.268 V.)
  
	c)  Find the cell voltage from the relation E(cell) = E(+) - E(-).

	d)  Write the net cell reaction, assuming that the left electrode is the anode.

	e)  Write the Nernst equation for the net cell reaction in the form	









 




	f)   Evaluate the cell voltage in part e and compare it with the voltage in 
         part c.














23.   The spectrophotometric titration curves shown below are for the separate
	titrations of Ca2+ and Mg2+ with EDTA at 605 nm using calmagite as the 
    indicator. 
	Conditions for the two titrations were identical, and buffered at pH = 10.



















	Which of the following statements are true?

	  a.  The conditional formation constant for CaIn- is greater than that 
          for MgIn-.

	  b.  At 605 nm, MgIn- absorbs more strongly than H2In-.

	  c.  At 605 nm, CaIn- absorbs more strongly than H2In-.

	  d.  At 605 nm, EDTA absorbs more strongly than H2In-.

	  e.  At 605 nm, EDTA absorbs very little light.


24.    Which of the five indicator acids listed would be most suitable for the 
       titration of 0.2 M formic acid with 0.2 M NaOH?





   

      
         

      		
      
         
Ionization
 Constants

      		
   

   

      
         
Formic Acid

      		
      
         
2 x 10-4

      		
   

   

      
         
Indicator A -

      		
      
         
5 x 10-4

      		
   

   

      
         
B -

      		
      
         
5 x 10-11

      		
   

   

      
         
C -

      		
      
         
5 x 10-9

      		
   

   

      
         
D -

      		
      
         
2 x 10-6

      		
   

   

      
         
E -

      		
      
         
25 x 10-4

      		
   









	                	
				
			
			
			
		
			


25.   The weak acid H3X has the following pKa values:  pK1 = 3.00,  pK2 = 5.00,  
      pK3 = 7.00.   At pH 6.00 what are the concentrations of all forms of the 
      weak acid?