Smith-Palmer Chemistry 265 – Analytical and Environmental
2010
Part A Short Snappers
[2]
1. Calculate the pH of a solution prepared by mixing 2.0 mL of a strong acid
solution of pH 3.00 with 3.0 mL of a strong base solution of pH 10.00.
H+ : 2.0 mL x 10-3 M = 2.0 x 10-3 mmol H+
OH-: 3.0 mL x 10-4 M = 3.0 x 10-4 mmol OHDifference = 1.7 x 10-3 mmol H+/ 5.0 mL = 3,4 x 10-4 M
[1]
pH = 3.47
2. State Beer’s law.
A = εbc
[1]
3. What is the general principle behind all chromatography (2 words)?
Differential holdup
[2]
4. What is the full name for EDTA?
Ethylenediaminetetraacetic acid
How many metal ions can each EDTA ion bind with?
one
[3]
5. Write the charge balance equation for an aqueous solution of Al2(SO4)3.
3[Al] + [H+] = 2[SO42-] + [H SO4-] + [OH-]
[4]
6. The pH of blood is 7.40. What is the ratio of bicarbonate to carbonic acid in
the blood.
For carbonic acid, pKa1 = 6.351 and pKa2 = 10.329
7.4 = 6.35 + log ([HCO3-]/[H2CO3])
[HCO3-]/[H2CO3] = 11
Is this a good buffer system? No – ratio is larger than 10
What will be the pH if the blood is diluted by a factor of 5?
7.4
[1]
7. Define the cathode in a galvanic cell.
Where reduction occurs
[5]
8. Calculate the theoretical potential for the following cell.
Indicate whether it is galvanic or electrolytic.
Pt, H2 (0.740 bar) ⅼH+ (5.67 x 10-7) ⅼⅼPd2+ (0.0120 M) ⅼPd
Pd2+ + 2e- ⇋ Pd
Ec =0.915 -
Ea – 0 -
Eo = 0.915 V
0.0559
1
log
= 0.858 V
2
0.0120
0.740
0.0592
log
= -0.366 V
2
(5.67 x10 7 ) 2
Ecell = 0.858 V – (-0.366 V) = 1.22 V
galvanic
[2] 9. What are some properties that are necessary for any reaction that is going to be
used as the basis for a titration.
Fast
Quantitative
Available indicator
Known stoiciometry
[3]
10.
Name a reference electrode that is commonly used in the lab.
Write the Nernst equation for the half-reaction involved in the electrode
and explain why the potential stays constant.
Standard calomel electrode
Hg2Cl2 + 2e- ⇋ 2 Hg (s) + 2 Cl-
0.0592
log [Cl-]2
2
Use a saturated solution of chloride – so the concentration will always be
constant = nothing else affects the equation
E = Eo -
Also could use silver/silver chloride electrode
NOT the hydrogen electrode
[2]
11.
One of your friends said they were going to titrate Na Cl with silver nitrate
and determine the endpoint potentiometrically. Your other friend did not
understand (they have been skipping class) and asked you to explain what
that meant. Please write out how you would explain this.
Use silver wire and reference electrode in solution. Measure potential as silver
nitrate is added
Jump occurs at endpoint
[3]
12.
Later you talked again with the friends who were going to titrate Na Cl
with silver nitrate and they said they were just going to use an indicator.
Suggest an indicator they could use for that particular titration. Explain
why it changes colour at the endpoint.
What is the colour change which occurs.
Chromate – it will ppt with silver just past equivalence point – will see red ppt
Dichlorofluorescein – will adsorb on AgCl ppt once there is an excess of silver ions
making it positively charged
Will create a pink ppt
[2]
13. Write the reactions whereby carbon dioxide dissolves in water to form
carbonic acid, then show its two step dissociation.
CO2 + H2O ⇋ H2CO3
H2CO3 ⇋ H+ + HCO3HCO3- ⇋ H+ + CO32-
[4]
Answer Four of the following:
What percentage of the air is carbon dioxide?
0.04%
Would carbon dioxide dissolve more in base or acid?
base
What happens to the amount of dissolved carbon dioxide when water is heated?
decreases
What is happening when we say marble buildings dissolve as a result of acid
rain. Write the equation.
2 H+ + CaCO3 ⇋ 2 HCO3- + Ca2+
How will the pH of seawater change (lower or higher) as CO2 levels in the
atmosphere increase?
lower
[2]
14.
Draw a block diagram of a UV-visible absorption spectrometer. Label the
boxes.
source
monochromator
sample cell
detector
[4]
15.
What are the two oxidation states for iron?
2+, 3+
What one is most stable in an aqueous acidic solution?
3+
What chemical can cause ferric ions to be reduced to ferrous ions?
Sn2+
Give the balanced equation for the reaction.
2 Fe3+ + Sn2+ ⇌ 2Fe2+ + Sn4+
[1]
16.
What is the voltage for a cell where the reaction has reached equilibrium?
0
Part B: Contaminated Site
You are an environmental chemist and have been called to examine a site where a
new industry wants to build a plant.
They are concerned about the contamination that may have been left in the soil by
previous owners, and also wonder whether the water in the stream will be suitable
for them to use in any of their applications.
[6]
1.
Choose three of the following water tests. Explain what it tests for and what
is done to carry out the test.
Alkalinity
Hardness
Biological oxygen demand
Acidity
Dissolved salts
2.
There was a big pile of soil/dirt at one corner of the site. There seemed to be a
mix of pale and dark particles. A sample weighing 3.50 g was collected. After
examining the sample in the microscope, it was determined that all the
particles were roughly the same size and almost a mm in diameter.
About a third of the particles were light coloured. These particle are thought to
contain silica. The dark particles will be analysed for their chromium content
by a method that gives a standard deviation of 4%.
If about 104 particles are collected in the sample, what will be the % error in
the sampling of the dark particles?
[2]
𝜎 = √𝑛 ∙ 𝑝 ∙ 𝑞 = 4 %
𝜎
𝑛∙𝑝
=
4%
2
104 ∙ 3
× 100%
= 0.7 %
[1]
What will be the total combined percentage error for the sampling and
analysis?
sT2 = (4 %)2 + (0.7 %)2
sT2 ≈ 4%
[1]
[3]
If the white particles were silica particles, what could be used to remove or
dissolve them?
Hot HF
3.
What is the basis for the hold-up in ion-exchange chromatography?
Explain how you could separate a neutral species from a charged species
using ion exchange chromatography.
Differential hold-up because of attraction of ions to oppositely charged functional
groups on the stationary resin.
[1]
4. One of the companies on the site previously had used a lot of lead in their
factory. You are trying to decide where it might have ended up on the site.
Why would the presence of lead be a problem to the environment?
Toxic to living things
[2]
The Ksp for PbCl2 is 1.70 x 10-5.
What is the solubility of PbCl2 in water?
Let solubility = x
Ksp = [Pb2+][Cl–]2
= x (2x)2
1.70 10-5 = 4x3
Solubility = x = 0.0162 M
[4]
What is the solubility of PbCl2 in 0.040 M NiCl2?
1.70 × 10-5
S
(s + 0.08)
s(s + 0.08)2 assume 0.08 > s
s(0.08)2 =
1.70 ×10−5
0.08 × 0.08
s(0.077 + 0.08)2
0.00265 M
Note: Necessary to solve cubic- but not expected here
[2]
If the PbCl2 was being dissolved in a solution containing 0.8 M Na2SO4,
would it be more or less soluble than when it was put in water? (No numerical
answer required) Why?
PbSO4 is insoluble
⇒ Less soluble
[5]
5. In one area of the site there is a container labeled chloroaniline. The purity
of the contents needs to be determined. Chloroaniline reacts with picric
acid to form amine picrates which absorb strongly at 359 nm (absorptivity
= a = 125 L. g-1.cm-1).
A 0.0265 g sample from the container is reacted with picric acid and
diluted to 1.00 L. The absorbance of the solution was measured in a cuvet
(test-tube) with a diameter (pathlength) of 1.00 cm. The absorbance was
0.547.
What is the percentage of choroaniline in the substance in the container?
A = a·b·c
0.547 = (125 Lg-1cm-1) · (1 cm) · c
0.00432 g/L = c
Assume 1 L sample so total 0.00433 g
%=
0.00433 𝑔
0.0265 𝑔
= 16.3 %
× 100%
Part C Titrations
[4] 1. In general, at the endpoint of a titration, one wants a steep jump. Why?
Defines end point better
Sudden change of indicator
There are a number of variables that influence how big and/or steep the jump at
the endpoint is. Tell me about them. You can write in note form.
Concentration of analyte
Concentration of titrant
Speed of reaction
Ka or Kf or Ksp
[3]
[3]
2. A 10 mL aliquot of a solution of sodium glycinate, the sodium salt of the
amino acid glycine, was titrated with 0.0550 M hydrochloric acid. The
reaction was followed using a pH electrode, and the titration was
continued until the second endpoint had been passed. The jump for the
second endpoint occurred when 24.70 mLs of the acid had been added.
For glycine, pKa1 = 2.35
pKa2 = 9.778
Sketch the titration curve.
Mark in pHs at important points in the titration
What was the concentration of the sodium glycinate?
Can use the 1st endpoint
Then 1 : 1
0.0550 mmol/mL × 12.35 ml = 0.67925 mmol
[sodium gylc] = 0.0679 M
[2]
What is the pH at the first equivalence point?
𝑝𝐾𝑎1 +𝑝𝐾𝑎2
2
= 6.07
[4]
What is the pH at the second equivalence point?
[2]
What is the concentration of glycinate (L-) in the solution at the
equivalence point?
[L-] = Ka2
[2]
It was hard to decide exactly where the endpoint was.
Describe a way to massage the data to allow the endpoint to be determined
very exactly.
Derivative
[1]
If we had stopped collecting data a little way before the endpoint, what
could you do with your data to be able to find the endpoint?
Gran plot
Part D Calculation Fun- Choose any two.
[5]
1. Calculate the formation constant, Kf, for Ag(CN)2- if the following cell
develops a potential of 0.625 V.
Ag ⅼ Ag(CN) 2- (7.50 x 10-3 M), CN- (0.0250 M) ⅼⅼSCE
The potential for the SCE = 0.244 V
Ag+ + 2CN- ⇋ Ag(CN)2Ag+ + e- ⇋ Ag(s) Eo = 0.799 V
0.244 V- 0.625 = -.381 V
-.381 V = 0.799 𝑉 −
Kf =
0.059
1
1
𝑙𝑜𝑔 𝐴𝑔+
[𝐴𝑔(𝐶𝑁)−
2 ]
{𝐴𝑔+][𝐶𝑁−]2
Rearrange to get [Ag+] and substitute in Nernst above
Kf = 1.4 x 1019
[5]
2.
Calculate pNi2+ for the solution that results when 50.0 mL of 0.0500 M
EDTA is titrated into 50.00 mL of 0.0300 M Ni2+ at pH = 3.0.
Kf for NiY2- is 4.2 x 1018
At pH 3.0, α4 for EDTA =2.5 x 10-11
Also calculate pNi2+ at the equivalence point in the titration.
XS EDTA = 1.0 mmol
pNi = 7.85
[5]
Ka =
What is the pH of a solution of 0.010 M H2O2? Ka = 2.2 x 10-12
3.
[𝐻+][𝑃−]
0.1
[H+]total = [H+]water + [H+]peroxide
[H+]total [OH-] = Kw
[H+]water =[OH-]
[H+]total = [OH-] + [P-]
𝐾𝑤
= [𝐻 +]
[H+]total
= 4.8
pH = 6.32
𝑡𝑜𝑡𝑎𝑙
𝐾𝑎∗0.10
+ [𝐻 +]
x 10-7
𝑡𝑜𝑡𝑎𝑙
[5]
4.
What is the ionic strength of a solution that is 0.033 M in Mg(IO3)2?
Ionic
strength
Ba2+
IO3-
Activity Coefficients(ɤ ) at Indicated Ionic Strengths
0.001
0.005
0.01
0.05
0.1
0.868
0.964
0.744
0.928
0.67
0.902
0.46
0.82
0.38
0.78
The solubility of Ba(IO3)2 in a solution that is 0.033 M in Mg(IO3)2
is 1.56 x 10-6 M.
Using activities, calculate the Ksp for Ba(IO3)2.