NAME:_____KEY__________________ Chem 51, fall, 2004, first midterm exam
A periodic table is included as an insert. You may use the back for scratch work but
enter ALL work to be graded in the space provided with each question. Show your work
in all questions involving computation in order to receive credit. All reactions are
conducted in water at 25.00C. Useful data: Ka(HOCl) = 3.2 x 10-8; Kw = 1.00 x 10-14.
1) (40 points) Write a balanced, net-ionic equation for the reactions that occur when
aqueous solutions or slurries of the following sets of compounds are mixed. The second
reagent, given in bold face, is present in large excess.
a) lithium hydroxide + hydrofluoric acid
HF is a weak acid.
OH-(aq) + HF(aq) H2O(l) + F-(aq)
b) copper metal + hot, 6 M nitric acid
Nitric acid at elevated temperature is a very strong oxidizing agent.
i) reduction half reaction
either e- + 2 H+(aq) + NO3-(aq) NO2(g) + H2O(l)
or 3 e- + 4 H+(aq) + NO3-(aq) NO(g) + 2 H2O(l)
ii) oxidation half reaction
Cu(s) Cu+2(aq) + 2 e-
either Cu(s) + 4 H+(aq) + 2 NO3-(aq) 2 NO2(g) + 2 H2O(l) + Cu+2(aq)
or 3 Cu(s) + 8 H+(aq) + 2 NO3-(aq) 2 NO(g) + 4 H2O(l) + 3 Cu+2(aq)
c) limonite [iron(III) hydroxide] + potassium cyanide
complexation and dissolution reactions
Cyanide is a very strong ligand.
Fe(OH)3(s) + 6 CN-(aq) Fe(CN)6-3(aq) + 3 OH-(aq)
2) (20 points) Most students conduct an analysis in triplicate. Simplissimus, a student
with superb technique who gets things right on the first try, sees no point is running a
procedure more than once. Prudentia tries to convince him that he is wrong but he won't
listen. Your task is to draft a succinct argument that Prudentia could use to convince
Simplissimus that he is wrong.
A good scientist validates results and replicate measurements are necessary to determine
the magnitude of random error which is always present. Simplissimus is overconfident
and is also unaware of the consequences of systematic error which can invalidate any
experiment, however precise. Checking of his technique via analysis of a standard and
calibration of his equipment are imperative.
3) (35 points) Household bleach is a 0.76 M aqueous solution of sodium hypochlorite,
NaOCl. The hypochlorite anion is the conjugate base of hypochlorous acid, HOCl.
a) Calculate the pH of bleach.
This is a weak base problem with the reaction
OCl-(aq) + H2O(l) HOCl + OH-(aq)
so Kb = [OH-][HOCl]/[OCl-]
where Kb = Kw/Ka = (1 x 10-14)/(3.2 x 10-8) = 3.1 x 10-7
Hypochlorite is a weak base; a small fraction is hydrolyzed and [OCl-] 0.76 M.
Neglecting the contribution of hydroxide from water, one concludes from the
above hydrolysis reaction that [OH-] = [HOCl]
Therefore, 3.1 x 10-7 = [OH-]2/(0.76 M), yielding [OH-] = 0.00049 M
and pOH = 3.31 and pH = 14.00 - pOH = 10.69
b) Bleach is used when clothing is soiled by colored substances. What is the
chemical basis for the decolorizing potency of bleach?
Some but not all dyes are indicators. Hence hypochlorite's functioning as a
bleaching agent is normally not due to its basic properties. The oxidation number
of chlorine, +1, reveals the source of its bleaching power. It is a weak oxidizing
agent and attacks electron-rich colored material.
4) (20 points) Limestone is predominantly calcium carbonate. Small amounts of other
minerals are found in limestone deposits. Identify these minerals by way of their
chemical formulae and provide the chemical reasoning behind your inference.
We are looking for water-insoluble ionic substances. The key is the consideration of the
calcium and carbonate ions produced when some limestone dissolves. Natural waters
would be expected to have appreciable concentrations of the +2 cations of the other
members of the alkaline earth family. Hence, we expect to find magnesium, strontium,
and barium carbonate. Actually, a common associated mineral is a mixed magnesium,
calcium carbonate, MgCa(CO3)2, known as dolomite. Natural waters might have
appreciable concentrations of other anions which would precipitate the calcium. Hence,
calcium sulfate and calcium fluoride are expected.
5) (35 points) Blood is a buffer whose pH, 7.4, is determined by the equilibrium between
carbonic acid and monohydrogen carbonate.
a) What does the pH of the blood, a result of evolution, suggest about the pKa of
In the design of buffers we match the pKa of the acid with the target pH so
carbonic acid would be expected to have a pKa close to 7.4.
b) Codeine, C, is a base with zero net charge and is prescribed as an analgesic.
The corresponding conjugate acid, CH+, is a cation with a pKa of 8.12. Calculate
the ratio of the concentration of free base to that of the conjugate acid, [C]/CH+],
in the blood of a patient who is using the drug. Also calculate the fraction of the
total codeine that is in the free base form, C.
A ratio of concentrations is requested. This is a buffer problem.
The relevant equation is CH+(aq) C(aq) + H+(aq)
[H+] is obtained from the pH; [H+] = 10-7.4 = 4.0 x 10-8
Ka = 10-8.12 = 7.6 x 10-9 = [H+][C]/[CH+]
Hence, [C]/[CH+] = Ka/[H+] (7.6 x 10-9)/(4.0 x 10-8) = 0.19
N.B. no knowledge of the absolute concentrations is required!
fC = [C]/([CH+]+[C]) = ([C]/[CH+])/(1 + [C]/[CH+]) = 0.19/(1 + 0.19) = 0.16.