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Chemistry 31A Autumn 2004
Professors Chidsey & Zare
Exam 1
Name:
SUNetID:
@stanford.edu
Honor Code Observed:
(Signature)
9:00am
10:00am
2:15pm
3:15pm
S02 OC103
S04 OC103
S06 OC110
S08 200-201 S12 Ricker
S15 Ricker
S09 50-51P
S13 Lagunita
S16 Lagunita
S14 Stern
S18 Stern
S17 Wilbur
S19 Wilbur
Circle
S03 OC110
your
section
S05 OC110
7:00pm
8:00pm
Instructions:
• This is a closed book and closed notes exam.
• You may use a calculator.
• For more space, use the back of a page and so indicate below the problem and in the boxes below.
• Box your answers! Show your work for full credit. Crossed out or erased material will not be graded.
• Be careful of units and significant figures.
• This exam contains 4 pages, none of them blank. COUNT YOUR PAGES.
Useful information:
Avogadro's Number = 6.022137x1023 = 1 mol
1 u = 1 g mol-1 = 1.660540x10-24 g = 1.660540x10-27 kg
∆E = c02 ∆m
where c0 = 2.99792458x108 m s-1
Some Normal Average Atomic Masses
H
1.0079 u
C
12.011 u
N
14.007 u
O
15.999 u
Cl
35.453 u
Ca
40.078 u
Cu
63.546 u
STUDENT FILL THIS OUT!
I
have
used the back of the
grade
initials
following pages (check box):
1-3
33
1.
4
30
2.
5-6
37
3.
Total
100
4.
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1. (6 pts) Fill in the blank: a megawatt is to a milliwatt as a kilowatt is to a _______________watt.
The abbreviation for mega is M, the abbreviation for milli is m, the abbreviation for kilo is k, and
the abbreviation for what you put in the blank is: ________.
2. (12 pts) In 1896 the French scientist Henri Becquerel (1852-1908) was studying the mineral called
pitchblende that contained uranium, when he discovered that it spontaneously emits high-energy
radiation. This spontaneous emission of radiation is called radioactivity. At Becquerel's suggestion
Marie Curie and her husband, Pierre, began their famous experiments to isolate the radioactive
components of the mineral. Further study of the nature of radioactivity, principally by the British
scientist Ernest Rutherford (1871-1937), revealed three types of radiation: alpha ( ), beta ( ), and
gamma ( ) radiation. Each type differs in its response to an electric field, as shown in the figure
below. Identify on the figure below which type of radiation is alpha, which is beta, and which is
gamma radiation. Also indicate the sign of charge of the particles in each beam.
3. (15 pts) Carbon-14 is the radioactive isotope most often used to date the age of once living
materials such as wood and bone. The decay rate of carbon-14 is such that half its nuclei
disappear in 5,730 years. Dating is possible because the fractional abundance of carbon-14
available to living organisms is constantly replenished, so one knows the fraction of carbon-14 in
the organism at its death. The replenishment occurs by neutron-capture reactions high in the
earth’s atmosphere where cosmic rays are absorbed by the gases in the upper atmosphere,
generating the required high-energy neutrons plus other nuclear reaction products and, as a side
benefit, protecting us on the earth’s surface from most cosmic rays. Fill in the blanks in the
neutron-capture equation below that replenishes the supply of radioactive carbon-14.
+ 10n →
14
page 2 of 4
C +
1
1
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name:_
4. (30 pts) Carbon-14 is made artificially by irradiation of a calcium compound with neutrons from a
nuclear reactor to make radioactive calcium bicarbonate (CaH2C2O6), which is then purified away
from the residue of the original calcium compound. Assuming all elements have their normal
abundances in the purified calcium bicarbonate except carbon which is entirely carbon-14 with an
atomic mass of 14.0032 u, what is the formula mass of this compound?
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5. (31 pts) In order to determine how a drug is metabolized in the liver of mammals, the drug is
synthesized with carbon derived entirely from the radioactive calcium bicarbonate from problem 4.
The intention is to administer the drug to rats, remove the rats’ livers at predetermined times after
administering the drug, separate the many compounds in each rat’s liver and measure their
radioactivity to determine what compounds are derived from the drug as the degradation proceeds.
To confirm that the desired drug has been synthesized, a sample of the drug is burned in a
combustion train. 13.801 mg of CO2 and 4.504 mg of H2O are obtained. Assuming the drug
contains only carbon-14 and hydrogen of normal abundance, what is the empirical formula of the
drug?
6. (6 pts) What experimental evidence indicated that the volume of the atom contains for the most
part little mass and that most of the mass of the atom is concentrated in a very small volume called
the nucleus? Please answer this question in two or three sentences.
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