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CHEM 115 EXAM #1
Practice, Fall 2011
Circle the correct answer to each question. (numbers 1 - 7, 2 points each)
1.
The term used to describe a substance composed of two or more elements chemically
combined in a fixed ratio is:
a. solution
2.
b. compound
d. homogeneous mixture
c. heterogeneous mixture
e. aggregate
What is not the same in different isotopes of the same element?
a. atomic number
b. atomic mass
d. number of neutrons
3.
Which of the following is a compound?
b. FeCl2
a. CH4
e. none of the above
4.
b. argon, Ar
c. cesium, Cs
d. iron, Fe
e. barium, Ba
b. sulfur, S
c. magnesium, Mg d. fluorine, F
e. potassium, K
Which of the following elements has chemical properties similar to those of lithium, Li?
a. Iron, Fe
7.
d. MgO
Which of the following elements has chemical properties similar to those of oxygen, O?
a. Iron, Fe
6.
c. CO2
f. all of the above
Which of the following elements is a transition metal?
a. sodium, Na
5.
c. number of electrons
e. mass number
b. oxygen, O
c. magnesium, Mg d. chlorine, Cl e. potassium, K
Which of the following is the smallest distance? [NOTE: all values have 3 sig. figs.]
a. 7.00 µm
b. 7.00 x 10-5 cm
c. 7.00 x 10-4 mm d. 7.00 x 103 nm e. 700 pm
(6 points)
8.
Specify the numbers of protons, neutrons, and electrons found in 56Fe and 56Fe3+.
#n
#p
#e
56Fe
30
26
26
56Fe3+
30
26
23
2
Fill in the blank with the appropriate word, phrase, or numerical answer.
(numbers 9 - 12, 2 points for each blank)
9.
Matter commonly is found in three physical states. List the two that would be expected
to have similar densities?
10.
and
.
mixture.
A scientist obtains the number 16.7249862 on a calculator. If this number actually has
five (5) significant figures, how should it be written?
12.
solid
A mixture that has different regions of composition and properties is known as a
heterogeneous
11.
liquid
16.725 or 1.6725 x 101
Round 0.0034169 to three significant figures and express in scientific notation. 3.42 x 10-3
(15 points)
13.
a. Lead (Pb) has a density of 11.34 g/cm3. Calculate the mass of a block of lead that has
dimensions 0.01000 m x 0.00800 m x 0.00400 m. I started by converting each length to
cm b/c this made life easy (just my personal opinion)
Volume = 1.000 cm x 0.800 cm x 0.400 cm = 0.320 cm3
mass = (11.34 g/cm3)*(0.320 cm3)
mass = 3.629 g
(Density)x(Volume) = mass,
b. Water has a density of 1.00 g/cm3. Find the volume of water has the same mass as the
lead block in part “a”.
volume = mass / density = (3.629 g)/ (1.00 g/cm3) = 3.629 cm3
c. Comment on the volume of lead and water (hint: it is helpful to look at the ratio of the
volume of water / volume of lead).
Volume Ratio = 3.629 / 0.320 = 11.34 cm3 of water / cm3 of lead
Since the water is 11.34 times less dense than lead, in order to have equal masses, a
volume of water must be 11.34 times the volume of lead.
2
3
(5 points)
14.
Define a compound and make sure to explain clearly how it differs from a homogeneous
mixture by giving examples of both.
A compound is a chemical combination of two or more elements in a fixed ratio (ex.
include NaCl, H2O, CuSO4 and many, many more) . A homogenous mixture is a
physical combination of two or more substances (which can be either elements or
compounds) that can be made in varying ratios (ex. include clean air, sugar completely
dissolved in water, HCl dissolved in water, gasoline, or Listerine mouthwash).
(10 points)
15. (a) 0.1761 grams of a gaseous compound composed of ONLY C and H is burned in pure oxygen
and all the water and carbon dioxide that forms is collected. If the masses collected are 0.5962
gram of CO2 and 0.1221 gram of H2O, what is the empirical formula of the compound?
[the molar masses (in g/mole) are C = 12.01, H = 1.008, CO2 = 44.01, and H2O = 18.02]
Find the moles of carbon and hydrogen from the mass of CO2 and H2O respectively.
Then divide by the smaller (if there is a smaller answer).
1 mole C
1 mole CO 2
x
x0.59621 g CO 2 = 0.01355 mole C
1 mole CO 2 44.01 g CO 2
2 mole H 1 mole H 2 O
x
x0.1221 g H 2 O = 0.01355 mole H
1 mole H 2 O 18.02 g H 2 O
The ratio is 1 : 1, so the empirical formulas is CH
(b) The molar mass of this compound is 78.11 g/mole, what is the molecular formula?
Show how you arrive at your answer.
The empirical unit has a mass of ~ 13.02 and the whole molecule has a mass of 78.11.
78.11/13.02 ≈ 6 . . . therefore, it takes six empirical units to build the molecule.
6 (CH) units can be written as C6H6
(8 points)
16. When you purchase your new computer you notice that the highest safe operating
temperature is listed as 320 K. Express this temperature in both ºC and ºF.
°C = K – 273.15 = 46.85 (rounds to 47°C)
°F = 1.8(°C) +32 = 116°F (note, I used 46.85 in this second calculation and then
rounded)
3
4
(10 points)
17. (a) Define and give an examples for the following:
Molecular formula: indicates the exact number of atoms of each type that make up a given
molecule. (ex. ethanol is made from 2 C, 6 H, and 1 O . .. the molecular formula is C2H6O)
Empirical formula: The empirical formula is the smallest whole number ratio of atoms that can
represent the composition of a compound. Sometimes this will be the same as the molecular
formula (see the example above) and sometimes it will not (see the example below).
(b) Describe the relationship between the mass of an empirical unit and a molecule (a specific
example will enhance your answer). The molecule will be made up of some whole number
multiple of the empirical unit. Well, I suppose the example from number 15 is as good as any.
C6H6 as a molecule can be represented as CH in terms of empirical formula.
18.
Write the names or formulas as needed. (24 points, 1.5 point each)
a. lithium perchlorate
LiClO4
b. phosphoric acid
H3PO4
c. calcium carbide
this is an unusual case, we can discuss it CaC2
d. copper (II) fluoride
CuF2
e. iron (III) sulfate
Fe2(SO4)3
f. diphosphorus pentoxide
g. ammonia
P2O5
NH3
h. water
H2 O
h. CoCl2·6 H2O
cobalt (II) chloride hexahydrate
i. Ni3(PO4)2
nickel (II) phosphate
j. CO
carbon monoxide
k. Fe2O3
iron (III) oxide
l. ZnSO4
zinc sulfate
m. SF6
sulfur hexafluoride
n. CH3CH2CH2CH3
butane
o. H2O2
hydrogen peroxide
4
(note: zinc is 2+)
5
BONUS (for up to 5 points)
Sulfur forms three compounds with fluorine: SF2, SF4, SF6.
Find the mass ratios F to S in each of these compounds and discuss how these numbers seem to
relate to one another. [F has a mass of 19.00 u and S is 32.06 u].
I will just round off the mass of S to 32 . . . we’ll still “get the message”
For SF2
the ratio F : S
= 38:32 = 1.19
For SF4
the ratio F : S
= 76:32 = 2.38
For SF6
the ratio F : S
= 114:32 = 3.56
Since the S has 2, 4, and 6 F bound to it in SF2, SF4, and SF6 respectively, the relative
mass of F in the compounds increases in a predictable way. The mass of F to S
doubles going from SF2 to SF4 and triples going from SF2 to SF6 (of course it only
increases by 50% going from SF4 to SF6).
5