Fall 2002 Chemisty 131
Name _______________
Problem Solving Section
Please answer the following questions in the spaces provided. Each problem is worth 10
points. You must show all work, set-ups and use proper units and significant figures for full
credit.
1. You obtain a block of platinum. Knowing how valuable that block is, you decide to sell it.
However, before you do, you decide to run some quick experiments to determine the
approximate value, so you can get a good deal.
a) You find that the block displaces 0.3425 L of water. What is the mass of this block,
in g?
First, convert the volume from L to mL, and then multiply by the density of the metal to find the
mass.
1000 mL 21.4 g
For Pt: 0.3425 L x
x
= 7329.5 g or 7330 g (sig figs)
1L
mL
b) If Platinum is currently selling for $568 for 1.000 troy ounce, how much can you get
for your block of platinum?
For Pt: 7330 g x
1 tr. oz.
$568
x
= $133,848 = $134,000
31.1035 g 1 tr. oz.
2. NO2 is a major component of chemical smog. For a 1.000-L sample of air that contains
4.000 g of NO2, calculate:
a) The number of moles of NO2 in the sample.
First, find the molecular weight of NO2: N + 2 O = 46.0055 g/mol. Use this to determine the
number of moles equivalent to the g of NO2.
mol
= 0.08695 mol
For 4.000 g: 4.000 g x
46.0055 g
b) The number of oxygen atoms in the sample.
Use the number of moles found in part a, the ratio set up by the molecular formula, and
Avogadro’s number to find the number of O atoms.
2 mol O
6.022 x 10 23 atoms
x
= 1.047 x 10 23 atoms
For 4.000 g: 0.08695 mol NO 2 x
1 mol NO 2
mol
An alternate solution was to find the mass percent of O in NO2, the number of grams of O in
NO2, and then the atomic weight of O and NA to find the number of atoms.
2(15.994 g/mol)
x 100% = 69.55% O in NO2
Mass percent of O in NO2:
46.0055 g/mol
In the 4.000 g sample, that equals 4.000 g (0.6655) = 2.782 g O
mol
6.022 x 10 23 atoms
2.782 g O x
x
= 1.047 x 10 23 atoms
15.9994 g
mol
***Note! The atomic weight of oxygen is used, not the molecular weight of O2. Mass percents
always give the percent of the element.***
Exam 1
Page 1 of 4
Fall 2002 Chemisty 131
Name _______________
3. Calculate the mass percents of C, H and O in estradiol, C18H24O2, to three significant
figures.
First, calculate the MW of estradiol  272.386 g/mol
18(12.011 g/mol)
x 100%
% C = 79.4
272.386 g/mol
24(1.0079 g/mol)
x 100%
272.386 g/mol
% H = 8.88
2(15.9994 g/mol)
x 100%
272.386 g/mol
% O = 11.7
True or False
Mark true (T) or false (F) in the space provided. 2 points each.
F
The mass of an object is the same as its weight. Mass is an amount of matter,
weight is a force which describes the effect of gravity on the mass.
T
Color is a physical property of matter.
F
If 6526 is added to 15.0, the answer should contain 5 significant figures. The
answer should contain only four significant figures. When you add, you keep all
digits up to the ‘lowest’ decimal place of the two values, which is the ones’ place.
F
Hydrogen peroxide, H2O2, contains equal parts by mass of hydrogen and oxygen. It
contains equal number of H and O atoms, but those atoms have very different
masses.
T
All isotopes of a specific element have the same number of protons. Isotopes vary
in the number of neutrons, but any atom of a particular element will have a set
number of protons.
F
The molecular formula of a compound represents the simplest whole number ratio
of atoms. This statement describes the empirical formula, or formula unit.
T
Germanium, Z = 32, is a metalloid. However, Selenium (Z = 34) is not a metalloid.
T
101.07 g of ruthenium (Ru) has the same number of atoms as 79.904 g of bromine
(Br). Both of those values are the atomic weights of the corresponding element.
Since they both represent one mole of each element, they contain the same
number of atoms
Exam 1
Page 2 of 4
Fall 2002 Chemisty 131
Name _______________
F
Pb(HSO3)2 is the correct formula for lead (II) hydrogen sulfate. That is the correct
formula for lead (II) hydrogen sulfite.
T
When sulfur (S) forms an anion, the charge is typically –2. Sulfur is in group VIA,
so 6 – 8 = -2.
F
Lighting a candle is an example of a physical change. Any combustion process is a
chemical change.
Multiple Choice
For each of the following, write the letter for the best answer in the box provided. 4 points
each.
C
1. Which of the following is not a basic SI unit? It’s a kilogram- know your units!
a) mole
b) meter
c) gram
d) Kelvin
C
2. How many significant figures are used in expressing the measurement 0.8160 cm?
a) six
b) five
c) four
d) three
B
3. The formula for ammonia, NH3, represents a compound composed of
a) one atom of nickel and three atoms of hydrogen.
b) one atom of nitrogen and three atoms of hydrogen.
c) three atoms of nitrogen and three atoms of hydrogen.
d) one atom of nitrogen and three atoms of helium.
D
4. All of the following statements are correct EXCEPT
a) The conversion of compounds into elements is a chemical change.
b) The conversion of sugar to carbon and water is a chemical change.
c) The conversion of elements into compounds is a chemical change.
d) The conversion of liquid water to gaseous water is a chemical change.
C
5. What is the best answer to the following expression?
(12.125 + 0.530 + 71.4)
a) 84.055
b) 84.06
c) 84.1
d) 84.0
Exam 1
Page 3 of 4
Fall 2002 Chemisty 131
Name _______________
A
6. The melting point of mercury is 235 K. This corresponds to
a) –38°C.
b) –20°C.
c) 20°C.
d) 38°C.
D
7. The formula weight of aluminum oxalate (Al2(C2O4)3) is
a) 230 g/mol.
b) 278 g/mol.
c) 291 g/mol.
d) 318 g/mol.
A
9. The average velocity of oxygen molecules at 1000°C is 8 x 104 cm/s. Which of the
following calculations would convert this value to the velocity in miles per hour?
a)
C
C
C
Exam 1
1s
4
8 x 10 cm
x
2.54
1 in
x
1 ft
12 in
x
1 mi
5280 ft
x
3600 s
1 hr
10. The formulas of the hydroxide ion, the nitrate ion, and the phosphate ion are
represented, respectively, as
a) OH-, NO3-, PO43-.
b) H-, NO2-, P3-.
c) H-, NO3-, P3-.
d) OH-, NO2-, PO33-.
11. Choose the name–formula pair that does NOT match.
a) sodium sulfite, Na2SO3  correct!
b) calcium fluoride, CaF2
c) potassium permanganate, K2MnO4  KMnO4
d) aluminum oxide, Al2O3
12. How many atoms of carbon are there in 0.40 mole of procaine, C13H20N2O2, a “pain
killer” used by dentists?
a) 4.8 x 1023
b) 9.6 x 1023
c) 3.1 x 1024
d) 6.3 x 1024
Page 4 of 4