1. (15 pts.) A 3.00 L sample of urban air was bubbled through a solution containing 50.0
mL of 0.0116 M Ba(OH)2, which caused the CO2 in the sample to precipitate as
BaCO3. The excess base was back-titrated to a phenolphthalein endpoint with 23.6
mL of 0.0108 M HCl. Calculate the parts-per-million of CO2 in the air (i.e. mL
CO2/106 mL air); use 1.98 g/L as the density of CO2.
2. (15 pts.) Your first assignment at El Smello Perfume Co. is to determine the proper
setting for filling the bottles. The company wishes to keep profits high by using as
little perfume as possible, but a government regulation states that no more than 2.5%
of the bottles may contain less than the stated volume on the bottle. If the bottles you
are filling are stated to contain 100 mL, what is the minimum volume you should use
for filling if the standard deviation in the fill level is 1.6 mL. (Hint: assume an
infinite number of samples where the CI is
for 95% confidence
for 99% confidence).
3. (20 pts.) Calculate the pZn of a solution prepared by mixing 25.0 mL of 0.0100 M
EDTA with 50.0 mL of 0.00500 M Zn2+. Assume that both the Zn2+ and EDTA
solutions are buffered with 0.100 M NH3 and 0.176 M NH4Cl.
4. (15 pts.) Calculate the pH resulting from mixing 15.00 mL of 0.800 M HIO3 (pKa =
0.77) with 45.00 mL of 0.0200 M NaOH.
5. (20 pts) Isotope dilution is a mass spectrometric technique in which a known amount
of an unusual isotope (called the spike) is added to an unknown as an internal
standard for quantitative analysis. The ratio of the isotopes is measured and from this
ratio the quantity of the element in the original unknown can be calculated. For
example, natural vanadium atom fractions 51V = 0.9975 and 50V = 0.0025. The
atomic fraction is defined as:
atom fraction of 51V = atoms 51V/(atoms 51V + atoms 50V)
A spike enriched in 50V has atom fractions 51V = 0.6391 and 50V = 0.3609.
a) Let A be 51V and B be 50V. Let Ax be the atom fraction of A in an unknown.
Let Bx be the atom fraction of B in the unknown. Let As and Bs be the
corresponding atom fractions in a spike. Let Cx be the total concentration of
all isotopes of vanadium ( mol/g) in the unknown, and let Cs be the
concentration in the spike. After mixing mx grams of unknown with ms
grams of the spike, show that the ratio of isotopes in the mixture (denoted R)
is given by
b) Solve the above equation for Cx to show that
c) A 0.40167 g sample of crude oil containing an unknown concentration of
natural vanadium was mixed with a 0.41946 g spike containing 2.2435
mol/g enriched with 50V (atom fractions: 51V = 0.6391 and 50V = 0.3609).
The measured isotope ratio by mass spectrometry was R = 51V /50V =
10.545. Determine the concentration of vanadium ( mol/g) in the crude oil.
6. (20 pts.) Consider the electrochemical cell described below.
The cell solution was made by mixing
25.0 mL of 4.00 mM
25.0 mL of 4.00 mM
25.0 mL of 0.400 M acid,
25.0 mL of
, with pKa = 9.50
The measured voltage was -0.440 V. Calculate the molarity of the KOH solution.
Assume that essentially all of the copper(I) is in the form
. For the right side
(the cathode), the half cell reaction is:
7. (15 pts.) From the standard potential
Tl+ + e- = Tl(s)
E = – 0.336 V
Determine the standard potential of.
Tl2S(s) + 2e- = 2Tl(s) + S2Given that the Ksp for Tl2S is 1.2 x 10-22.
8. (15 pts.) An ion selective electrode used to measure
obeys the equation
is also sensitive to
When the electrode was immersed in 100.0 mL of unknown containing
, the reading was
) was added to the unknown, the reading increased to
Find the concentration of
in the original unknown.
9. (20 pts.) The sulfur in a 5.00 g sample of steel was evolved as H2S and then collected
in a solution of CdCl2 to produce the precipitate CdS. The CdS was then titrated with
excess I2 and the remaining I2 was then back titrated with 4.82 ml of 0.0510 M
sodium thiosulfate. If the concentration of the I2 is 0.0600 M and a total of 10.0 ml is
added, calculate the %S in the steel. The relevant reactions are:
CdS + I2
I2 + 2S2O32-
S + Cd2+ + 2IS2O62- + 2I-
Titration (excess I2)
10. (15 pts.) Inorganic phosphorous is spectrophotochemically measured in blood serum
by formation of a reduced heteropoly acid (heteropoly blue) and comparison of its
absorbance with a standard treated by the same procedure. If a 1.00 mL blood serum
sample gives an absorbance of 0.217, while a 2.00 mL aliquot of a standard
containing 91.2 mg KH2PO4 per liter gives an absorbance of 0.285, calculate the
milligram percent of P (milligrams per 100.0 mL) in the blood sample. Note: both
aliquots are diluted to the same volume prior to absorbance measurements.
11. (15 pts.) Butanoic acid has a partition coefficient of 3.0 (favoring benzene) when
distributed between water and benzene. Find the formal concentration of butanoic
acid in each phase when 100.0 mL of 0.10 M aqueous butanoic acid is extracted with
25.0 mL of benzene at a pH of 4.00.
12. (15 pts.) A 480 mg sample of cat food is digested with HNO3 and HClO4 and diluted
to 25.0 mL in a volumetric flask. Three aliquots of 2.00 mL each are drawn, to which
are added 0.1 mL, 0.2 mL and 0.3 mL spikes of 100.0 ppm Fe standard solution. The
absorbance of an unspiked 2.00 mL aliquot is measured by AAS, and the absorbance
of the spiked samples is measured as well. The data is presented in the Table below.
Correct the absorbance of the spiked samples for dilution and plot the absorbance
versus ppm Fe added. Extrapolate the graph to the point where absorbance is equal to
zero to determine the amount of Fe in the cat food sample in ppm.
+ 0.1 mL
+ 0.2 mL
+ 0.3 mL