NAME: (print) __________________________
UIN #: ________________________
CHEMISTRY 107
Section 501
Exam #1–Version A
February 13, 2015
Dr. Larry Brown
This is a 50-minute exam, and contains 8 problems. There should be 10 numbered pages,
including this one; page 10 is blank. There is also a periodic table attached to the back of the
exam; you may tear the periodic table page off, and you do not need to turn that page in. Point
values for the different questions are as indicated. Some constants and conversion factors you
might need are listed below.
Please show ALL of your work as clearly as possible. This will help us award partial credit.
Answers without supporting work may not receive credit. You may use a calculator for this
exam, but you may NOT retrieve or use any alphanumeric information or algorithms that might
be stored in your calculator’s memory.
Please PRINT your name and UIN number above, and SIGN the honor code statement below.
Also, please put your name on every page of the exam, in case a page gets detached from the
exam.
Potentially Useful Information
PHYSICAL CONSTANTS
Avogadro’s Number NA = 6.022 × 1023 mol–1
CONVERSION FACTORS
1 amu = 1.660566 × 10–27 kg
1 kg = 1,000 g
1 g = 106 µg
1 L = 1,000 mL
1 fl oz. = 29.6 mL
1 cm = 10–2 m
1 µm = 10–6 m
1 nm = 10–9 m
"On my honor, as an Aggie, I have neither given nor
received unauthorized aid on this exam."
SIGNATURE: ___________________________________________
Exam © 2015 L.S. Brown
A1
(24 pts) 1. Answer each of the following questions, and write your answer neatly on the line provided.
These should not require any elaborate calculations, and will be graded as either right or wrong.
You do not need to show your work here. (3 pts each)
No more than 2 or 3 significant figures for the numerical ones here, please!
(For Grading)
Problem
(a). How many protons are present in a single sulfide ion, S2–?
______________________
(b). How many electrons are present in a single sulfide ion, S2–?
Score
1 (24)
2 (10)
______________________
(c). A student dissolves 14 g of NaF in enough water to make 0.5 L of
solution. What is the molarity of NaF in this solution?
3 (10)
4 (8)
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(d). A bottle contains 1.0 L of 0.50 M MgF2. What is the concentration of
fluoride ions (F–) in this solution? (MgF2 is a strong electrolyte.)
5 (12)
6 (12)
______________________
(e). Suppose 100 mL of the solution from (d) is diluted to a final volume
of 250 mL. What is the concentration of MgF2 in the diluted solution?
______________________
7 (12)
8 (12)
TOTAL
(f). What is the chemical term for burning something in oxygen?
______________________
(g). What is the chemical term for a substance that dissolves in water to produce a solution
containing hydronium ions (H3O+)?
______________________
(h). Two stable isotopes of copper (Z = 29) exist:
63
Cu and
65
Cu. Which isotope is more
abundant? (Remember that you have a periodic table available.)
______________________
A2
© 2015 L.S. Brown
NAME:_________________________________
2. The structure below is a compound known as xanthohumol. It is found in hops, and therefore
also in beer. Xanthohumol has a variety of effects on the brain, and recent research suggests that
it may potentially slow the development of both Alzheimer’s and Parkinson’s diseases.
HO
O
O
OH
OH
(5 pts) (a). What is the correct molecular formula for this compound?
(5 pts) (b). A standard 12-ounce bottle of a typical American lager beer contains about 11 µg of
xanthohumol. Given that the molar mass of xanthohumol is roughly 350 g/mol, approximately
how many molecules of xanthohumol would be present in such a bottle of beer? (That molar
mass is only approximate; you should not use it to check your formula from above. But please
use the 350 g/mol value for this calculation.)
© 2015 L.S. Brown
A3
3. Write properly balanced chemical equations for each of the processes described below.
Please write your final balanced equations neatly in the boxes provided!
(5 pts)
(a). Aqueous solutions of potassium phosphate (K3PO4) and calcium chloride (CaCl2) react to
produce solid calcium phosphate (Ca3(PO4)2) and aqueous potassium chloride (KCl).
(5 pts)
(b). Sulfuric acid (H2SO4) and sodium hydroxide (NaOH) solutions are mixed and undergo a
neutralization reaction.
A4
© 2015 L.S. Brown
NAME:_________________________________
(8 pts) 4. As computer processor speeds increase, it is necessary for engineers to increase the number of
circuit elements packed into a given area. Individual circuit elements are often connected using
very small copper “wires” deposited directly onto the surface of the chip. In a particular
processor, these copper interconnects are 20 nm wide. How many copper atoms would be in a
1.0-µm length of such an interconnect? (Assume a square cross-section, so that you are
considering a piece of copper that is 1.0 µm × 20 nm × 20 nm. The density of copper is 8.96
g/cm3. Show ALL of your work; do not use any unit conversion functions that may be available
on your calculator.)
© 2015 L.S. Brown
A5
(12 pts) 5. The characteristic odor of garlic is due to the presence of various sulfur-containing
compounds. One such compound was analyzed and found to contain 49.26% carbon, 6.89%
hydrogen, and 43.85% sulfur by mass. Another test shows that the molar mass of this compound
is between 90 and 160 g/mol. Find the molecular formula for the compound.
A6
© 2015 L.S. Brown
NAME:_________________________________
(12 pts) 6. Hydrogen cyanide is produced industrially by the high temperature reaction of gaseous
ammonia, oxygen, and methane.
2 NH3(g) + 3 O2(g) + 2 CH4(g) → 2 HCN(g) + 6 H2O(g)
If 500.0 kg of NH3, 1,000.0 kg of O2, and 500.0 kg of CH4 are combined, what is the maximum
mass of HCN that could be obtained? Please express your answer in kilograms.
Molar masses of the substances involved are as follows:
NH3: 17.034 g/mol
O2 :
32.00 g/mol
HCN:
H2O:
18.016 g/mol
27.03 g/mol
© 2015 L.S. Brown
CH4:
16.042 g/mol
A7
(12 pts) 7. The formula of an unidentified acid can be written as H3A, where ‘A’ represents the anion.
The acid reacts with potassium hydroxide according to the following equation.
H3A(aq) + 3 KOH(aq) → K3A(aq) + 3 H2O(l)
A lab technician takes 0.9466 g of this acid and dissolves it in 50.00 mL of water. If 29.04 mL of
0.5090 M KOH is required to neutralize the resulting solution, what is the molar mass of H3A?
A8
© 2015 L.S. Brown
NAME:_________________________________
(12 pts) 8. Magnesium carbonate (MgCO3) and calcium carbonate (CaCO3) both decompose when
heated, forming CO2 gas and the corresponding oxides.
MgCO3(s) → CO2(g) + MgO(s)
CaCO3(s) → CO2(g) + CaO(s)
When a particular mixture of MgCO3 and CaCO3 is heated, it releases 47% of its mass as CO2, so
that the oxide products have 53%of the mass of the original sample. What mass percentage of
MgCO3 was present in the original mixture? (HINT: The fact that the sample loses 47% of its
mass does not depend on the original sample size, only on the relative amounts of the two
carbonates. So you can start with any size sample you’d like.)
© 2015 L.S. Brown
A9
A10
© 2015 L.S. Brown