3 Test Chemistry 253, Fall 2007 Name:_________________________________ Lab section/TA: __________________ You have one class period to complete this test. The test is closed book, closed note (only a calculator is allowed). Express all answers to the correct number of significant figures. You must show all work to get credit for each problem. Useful equations:
HA
[H ] [H ] K a
A
Ka [H ] K a
K'f Kf
Kf
[MY n 4 ] [M n ][Y 4- ]
Y 4-
[Y 4 ] [ EDTA ]
Ecell Ered
Y4
[MY n 4 ] [M n ][EDTA]
q n F
For aA + nebB; E
Eox
E
0.05916 [B]b log n [ A] a
F= 9.648 x 104 C/mol
E
constant
0.05916 log [A] k A,X [X] n
log
k A,X
0.51z 2
response to X response to A
Ac
[ C]
C
c
1
305
b2 2a 4ac
1 2
Y4
cz
i
2 i i
x
b
for EDTA:
Y4
pH 7 8 9 10 11
3.8 x 10-4 4.2 x 10-3 0.041 0.30 0.81
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Name:_________________________________ Lab section/TA: __________________ 1) (10 points) State whether the following statements are true or false: a) A metal ion indicator used in an EDTA complexometric titration must have a formation constant with the analyte that is less than that of the EDTA-analyte complex. ___________ b) A mass balance equation states that the sum of all positive charges in solution must equal the sum of all negative charges. ___________ c) One Ampere is equal to 1 coulomb/second of current flow. d) In a galvanic cell, electrons flow from the cathode to the anode. ___________ ___________
e) The more positive the standard reduction potential is for a chemical species, the easier it is to reduce that species. ___________
2) (10 points) Fill in each blank with the correct answer. a) A tenfold increase in [F-] induces a change of _____ mV in the potential of a Fselective electrode. b) Lewis Acids are electron pair _________. c) The unit of electric potential is the ________. d) The standard reduction potential for the Standard Hydrogen Electrode is ________ V. e) The __________________ formation constant describes the formation of a metal complex at any particular pH.
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Name:_________________________________ Lab section/TA: __________________ 3) a) (7 points) Using the standard reduction potentials Cu2+ + 2e- Cu(s) Co(OH)2 (s) + 2e- Co(s) + 2OHcalculate the cell potential for the reaction Cu2+ + Co(s) + 2OH- Cu(s) + Co(OH)2(s) when [Cu2+] = 0.010 M and [OH-] = 1 x 10-4. E = +0.339 V E = -0.746 V
b) (3 points) Is this reaction spontaneous the in direction written?
c) (5 points) What is Ecell if [OH-] in the above equilibrium is adjusted to 1 x 10-7 M?
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Name:_________________________________ Lab section/TA: __________________
4) (5 points) Use this graph to answer the following questions:
a) At what pH does [H2PO4-] = [HPO42-]? b) What is the predominant species at pH 8.5? c) Over what approximate pH range is H2PO4- the predominant species? 5) a) (5 points) Write a complete mass balance for an aqueous solution of Mg3(PO4)2 if the aqueous species present are Mg2+, PO43-, HPO42-, H2PO4-, and H3PO4.
b) (5 points) Write the charge balance equation for this solution, including ions produced from the dissociation of water.
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Name:_________________________________ Lab section/TA: __________________ 6) (10 points) In a titration of Cl- with AgNO3, what is the potential of a Ag indicator electrode at the titration endpoint, as measured against a Ag/AgCl reference electrode? (Ksp for AgCl = 1.8 x 10-10, E for Ag+ + e- Ag(s) = 0.799 V, E for AgCl(s) + e- Ag(s) + Cl- = 0.222 V)
7) (10 points) Gold can be deposited electrolytically with the net reaction Au + e- Au (s) What mass of gold is deposited using 0.100 A of current flowing for 120 seconds? (atomic weight of Au = 196.7 g/mol)
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Name:_________________________________ Lab section/TA: __________________
8) (10 points) At one time, the hydroquinone redox couple was used in a reference electrode, with a standard reduction potential of E = 0.700 V. Convert a potential of 0.315 V vs. the hydroquinone electrode to the Ag/AgCl reference electrode potential scale (E for the Ag/AgCl electrode = 0.222 V).
9) (5 points) A Na+-selective electrode has a potential of +0.258 V when immersed in a 0.010 M Na+ solution. When 0.050 M K+ is added to this solution, the electrode potential becomes +0.310 V. What is the selectivity factor for K+ relative to Na+?
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Name:_________________________________ Lab section/TA: __________________ 10) 30.00 ml of 0.0600 M Ca2+ was titrated with 0.0500 M EDTA at pH 9.0 (logKf for CaY2- = 10.65). a) (4 points) What is the conditional formation constant, K f', for CaY4- at this pH?
b) (2 points) What is the equivalence volume (Veq) for this titration?
c) (5 points) What is [Ca2+] at Veq?
d) (4 points) What is [Ca2+] after a further 4.00 ml of the EDTA titrant was added beyond Veq?
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