Download KEY Midterm Exam 1 Sept.14, 1999 Chemistry 211 PAGE 1 0f 5

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KEY
Midterm Exam 1
Chemistry 211
Sept.14, 1999
PAGE 1 0f 5
Conversion factors:
1 in = 2.54 cm
o
K = o C + 273.15
o
C = (5/9)(o F - 32)
453.6 g = 1 lb
1 gallon = 3.78541 Liter Avogadro's number = 6.022 x 102 3
I. (80 pts.) Each question is worth 4 points (No partial credit). Use Scantron Sheet.
1. Isotopes are:
(a) atoms with different numbers of protons and neutrons.
(b) atoms with the same number of neutrons and electrons.
(c) atoms with the same atomic number and different mass numbers.
(d) atoms with the same mass number but different atomic numbers.
2. An element is:
(a) a pure substance which cannot be separated into simpler substances by
ordinary means.
(b) electrically charged, either positive or negative.
(c) a substance which has atoms of identical atomic number.
(d) a molecule.
(e) a part of a larger particle.
3. An alpha (α) particle is: (a) an electron.
(b) a helium nucleus.
(c) a proton.
(d) high energy radiation.
4. A chemical formula: (a) is a representation of a compound which indicates the proportions in which
atoms or
ions are combined.
(b) applies to mixtures.
(c) is a representation of an element.
(d) shows which elements are contained in the compound.
5. Boiling is: (a) the temperature at which a liquid changes to a gas.
(b) a process that liberates heat.
(c) a physical change.
(d) a chemical change.
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6. Which of the following is an example of a chemical property? (a) conductivity
(b) texture
(c) corrosiveness
(d) ductility
7. A physical change: (a) may alter the identity of a substance.
(b) is always accompanied by a chemical change.
(c) does not alter the identity of the substance.
(d) cannot be reversed.
8. Which of the following represents the greatest percent uncertainty?
(a) A thermometer reading of 100o C + 3o C.
(b) The specific heat of Cd is measured to be 0.197 J/g.
(the theoretical value is 0.231 J/g).
(c) The melting point of tin is measured to be 244o C (the theoretical
value is 232o C).
(d) A balance measured the mass of a 1.000 g standard as 0.9981 g.
9. Elements of the same group in the periodic table: (a) have equal numbers of electrons in their atoms.
(b) are either all metallic or non-metallic.
(c) are arranged in short or long horizontal rows in the table.
(d) are arranged in a vertical column in the table.
10. Stoichiometry is: (a) the set of rules governing the identity of products formed in a chemical reaction.
(b) the set of rules used for naming inorganic compounds.
(c) the set of relationships among the amounts of substances in a chemical
reaction.
(d) an approach to problem solving in which the units or dimensions of the quantities are
examined to see what conversions are required.
11. Sulfur trioxide is the name of a
(a) mixture
(b) compound
(c) element
(d) atom
(e) heterogeneous mixture
12.What is the correct representation for an entity containing 14 neutrons, 13 protons, and
14 electrons?
(a) 1 4Si
(b) 4 1Nb(c) 27Co
(d) 1 3Si+
(e) 2 7Al13.Which of the following is the best description of an atom?
(a) the smallest unit of a chemical compound
(b) a small positively charged nucleus surrounded by negatively charged electrons
(c) a large nucleus with no charge, surrounded by protons and electrons
(d) a small nucleus of heavy electrons, surrounded by protons and neutrons
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(e) a group of fig newtons, surrounded by a collection of lightweight morons
14. Ions are:(Choose the best answer)
(a) atoms with an excess of electrons
(b) atoms or groups of atoms with a net electrical charge other than zero
(c) formed by adding or removing neutrons
(d) atoms with a deficiency of electrons
(e) formed by adding or removing protons
15. Which of the following statements are false?
(i) Grams are a measure of mass.
(ii) Millimeters are a measure of volume.
(iii) Cubic centimeters are a measure of density.
(iv) (85 - 84)o F = (85 - 84)o C
(a) all of the above
(b) ii only
(c) ii,iii,&iv
(d) ii&iii
16. The answers to which of the following will have three significant figures?
(i) (6.022 x 102 3) (2.61 x 108 )
(ii) 6.314 x 10-4 + 6.94 x 10-1 + 2.6501 x 103
(iii) (9.42 x 102 + 1.62 x 104 ) / 4.1
(iv) 485 / 9.231
a) i&ii
b) iii&iv
c) i,ii&iv
d) i&iv
17. Which of the following are heterogeneous?
(i) gold
(ii) gasoline
(iii) a solution of ethanol and water
(iv) water
a) i&iii
b) ii&iii
c) i,ii,&iv
d) none of the above
Use the following table to answer Questions 18 & 19.
Density of various common substances at 20o C.
Substance
Density (g/cm3 )
Water
1.00
Magnesium
1.74
Aluminum
2.70
Lead
11.34
Mercury 13.6
18. At 20o C
(i) 1.0 ml of magnesium has a greater mass than 1.0 ml of aluminum.
(ii) 13.6 ml of water has a greater mass than 1.0 ml of lead.
(iii) 0.1 ml of lead has a greater mass than 1.0 ml of water.
(iv) 10 ml of magnesium has a greater mass than 1.0 ml of mercury.
a) i&ii
b) i,ii,&iv
c) ii,iii,&iv
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d) all of the above
19. At 20o C
(i) 1 g of lead has a greater volume than 1 g of mercury.
(ii) 1 g of water has a greater volume than 1 g of air.
(iii) 1 L of air has a greater volume than 1 L of water.
(iv) 100 g of magnesium has a greater volume than 100 g of water.
a) i only
b) i&iii
c) ii&iv
d) iii only
c) iii only
d) ii only
20. The neutral atom 6 5Cu contains
(i) 29 neutrons.
(ii) 29 protons.
(iii) 65 neutrons.
(iv) 65 electrons
a) i&ii
b) ii&iv
II. (20 pts.)
- Show all work or NO CREDIT will be awarded.
- Work problems to proper number of significant figures.
- Be sure to include units in your answers.
- Circle your final answers.
- Put your name on ALL pages.
21. (10 pts.)
Djenkolic acid is a sulfur-containing amino acid that is isolated from the djenkol and other
beans and is possibly implicated in flatulence. It is 33.06% carbon, 5.55% hydrogen,
11.01% nitrogen, 25.16% oxygen, and 25.22% sulfur by mass. Each molecule contains two
atoms of sulfur. Determine the molar mass of djenkolic acid to three significant figures and
its molecular formula.
% mass is given. Assume 100 g of sample:
grams divided by Atomic weight equal moles
g C = 33.06 g 12.01 g/mol 2.753
g H = 5.55 g 1.01 g/mol 5.50
g N = 11.01 g 14.01 g /mol 0.7859
g O = 25.16 g 16.00 g/mol 1.573
g S = 25.22 g 32.07 g/mol 0.7864
C:H:N:O:S
2.753 : 5.50 : 0.7859 : 1.573 : 0.7864
Divide by 0.7859
3.502 : 6.998 : 1.000 : 2.001 : 1.001
Multiply by 2 to obtain whole numbers:
7 : 14 : 2 : 4 : 2
Empirical Formula : C7 H1 4N2 O4 S2
The empirical formula already contains 2 S atoms, thus, this is the molecular
formula.
Molecular Formula = C7H14N2O4S2
Molar mass
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= (7x AW of C) + (14 x AW of H) + (2 x AW of N) + (4 x AW of O) + (2 x AW of
S)
= (7 x 12.0) + (14 x 1.0) + (2 x 14.0) + (4 x 16.0) + (2 x 32.1)
= 254 g/mole
22. (10 pts.)
A newspaper article about the danger of global warming from the accumulation of
greenhouse gases such as carbon dioxide states that "reducing driving your car by 20 miles
a week would prevent release of over 1000 pounds of CO2 per year into the atmosphere."
Check whether this is a reasonable statement.
Assume:
52 weeks per year
average gas mileage of a car = 20 miles per gallon
gasoline is mostly octane (C8 H1 8) and that it is burned completely into CO2 and
H2 O
density of gasoline = 0.8 g/mL
Determine the reduction in the amount of gasoline used:
(20 miles/week) x (52 weeks/year) x (1 gallon/20 miles) = 52 gallons/year
Determine number of grams in 52 gallons of gasoline:
(52 gallons) x (3.786541 L/gallons) x (1000 mL/L) x (0.8 g/mL) = 157520 g
Determine number of moles (use C8 H1 8, molar mass = 114 g /mole):
157520 g x (1 mole / 114 g) = 1400 moles of gasoline
From the equation:
C8 H1 8 + (25/2)O2 ---- 8CO2 + 9H2 O
1 mole of octane produces 8 moles of CO2 , thus
1400 moles of gasoline will produce 11200 moles of CO2 .
Convert moles of CO2 into pounds:
1100 lbs. of CO2
The statement is reasonable.
11200 moles x (44g/mole) x (1 lb /453.6) =
III. Extra Credit (10 pts)
The inhabitants of a planet "Htrae" in a distant galaxy measure mass in units of "margs",
where 1 marg = 4.8648 grams (exactly). Their scale of atomic masses is based on the
isotope 3 2S (atomic mass on earth = 31.972 g/mole), so they define one "elom" of 3 2S as the
amount of sulfur atoms in exactly 32 margs of 3 2S. Furthermore, they define Nor, or
"Ordagova's number" (after their well-known scientist Oedema Ordagova), as the number of
atoms of sulfur in exactly 32 margs of 3 2S. Calculate the numerical value of Nor to four
significant figures.
32 margs of 3 2S (4.8648 grams/1marg) = 155.6736 grams of 3 2S
155.6736 grams of 3 2S ( 1mole/31.972 grams) = 4.8691 moles of 3 2S
4.8691 moles of 3 2S (6.022 x 102 3 atoms/mole) = 2.932 x 102 4 atoms of 3 2S
32 margs of 3 2S contain 2.932 x 102 4 atoms of 3 2S, thus
Ordagova's number = 2.932 x 1024
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