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Unformatted text preview:  r: I ' 5mm 0 M s u 254 Aquatic Chemistry
Practice for ﬁnal exam Problem 1. ,5 Let’s see how the pH of rainwater changes at it passes through the sulfuric acid plume
associated with a smokestack. The rainwater starts out pure, at equilibrium with Pam) = 10'35 atm. Please draw the
carbonate system and show the pH (this is the same as What we did in class). What is the
alkalinity of this water? Now working with the same graph, let’s assume that (112804)T =‘ 10‘1 M diSSOIVes in the 1 water as it passes over the smokestack. The H2504 dissociates to form 2H+ and 3042’, and because the second pKa is approximately 2, we knowthere is no protonation at pH >
4 2. What is the alkalinity of this water? Please draw a line for 8042‘ on the graph, and
solve for the pH of the rainwater. You can make a Tableau using components dominant
at pH 4.5 to get an Alk equation, and use this to solve the graph, or you can do it by
considering all constraints on the system. nilllllllmlla
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t Problem 2. a) A freshwater is at equilibrium W%C3C03m. glut all of the carbonate species as a
function of pH if [Ca2+] = 10‘3 M. (*1 ' poiLIESL/
caC03(s) = Ca2+ + €03.2 K = 1043
H2C03* = H" + HCOS' K = 10'63 '
HCO ' = H" + CO 2' K = 1040'3 *
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pH Space for working Problem 2. l=__.‘..»...~.c H: _. ._ t. hm t: .. IA . b) To the water from part a, you add 10“5 M of a Pb salt, which forms sz” upon
dissociation. Consider just one hydroxide complexation reaction: Pb2+ + OH‘: PbOH+ K = 1064 Please sketch the concentration of Pb2+ free ion that would exist in yen: solution if no precipitation takes place (you can just plug in values at a few points to get the general
shape of the curve). (*10 points) [moss Hot“ C%“1C<>s*1 ‘1 I O G
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0) Does this complex priecipitate as PbC03(s,? If so, in what pH range does solid form, and in what pH range is all of the lead dissolved ‘2 Show graphically and mathematically
solve for the critical pH Where solid dissolves or precipitates. You can assume that the system remains at equilibrium with CaCOsm with [Ca2+] buffered at 10'3M. 2' Pb2+ + C032. I<so 7* 1013'2 Problem 3. In this problem, you will see how with a simple pH measurement, you can determine the
KSP of a solid! Imagine you are studying a hazardous waste containing cadmium and you need to know
the solubility product for the form of CdCOm appearing in your system (which may be
different from that for CdCOxsiomim), which appears in your text). You add some
CdCOm) to pure water at equilibrium with the atmosphere (PCO2 = 10’” atm, just the
carbonate system present) and measure the pH as pH 7.1. The solid does not dissolve
completely (it’s present at saturation). What is the K50 for CdC03(s)? Part A: In order to solve this problem, ﬁrst calculate the concentration of the carbonate species in your system at this pH using mass laws, and sketch the open system diagram (graph on
next page): COME) : H2C03* K = 1015 111 2111114
H2C03* = H+ + HCO; K = 10—63
HC03” = fl” + C032" 7 K = 1010'3 .3, s [HrCoeui ‘1 IO lowS : xo'J/Yl Q a“ pH
PM as 7m [H(03~3 r. to to —, [0 m
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sufﬁcient to show that CdOH+ and Cd(OH)2 are negligible. At what pH would these complexes become important?
(Y ou’ll need to look at Appendix D.) C. as T— im“? + [Cam—P] +CCc‘ (om—Ll i . as =ksws(l [lﬁfow] Hangman
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E (Dag.5675 cum (9 LHﬂos‘iF 10""5 PCoL @ had. 93%“ ~— 1 Q {sumac3.3 r [0403 ®i\‘\LOBl L saw ® [H'QCOHZJ =16“ @< 2 l 03 C03 .— 40 3 ‘ @ [Gummy] “4L5? meow P @ 65kt. Lug? _ ﬂ
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PaItD 1 99°31] /7_ : LOFW1/L 2* to 2’/\/7 Finally, calculate the solubility product you are looking for. . ...
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This note was uploaded on 02/02/2012 for the course CEE 250 taught by Professor Terrihogue during the Fall '11 term at UCLA.
 Fall '11
 TerriHogue

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