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ENVI 1106
Sample Midterm 1 ANSWERS
A periodic table sheet will also be provided and a calculator will be allowed. All numerical
answers should be given with the correct number of significant figures and units. All work
should be shown unless otherwise indicated.
This sample midterm would take 1 hour.
INFORMATION PROVIDED
Avogadro’s number = 6.022 x 1023 mol-1
T(K) = T(°C) + 273.15
Part I: Short answers: Fill in the blanks or answer as indicated, you do not need to show work.
1) (5 marks) Identify whether the following are Physical or Chemical changes:
C
Grape juice slowly ferments to form vinegar.
P
Hot water forms “snow” when sprayed into very cold air.
P
A helium balloon expands as it rises in the air until it pops.
C
Baking soda reacts with acidic ingredients in cake to help it rise.
P
Hot water is poured through coffee grounds to make a cup of coffee.
2) (1 mark) The result of the following calculation will have how many significant figures?
[(23.563 – 4.3)/(321.3 x 1.96)] x (1.689)
3) (1 mark) How many mm is 4.35 x 10-6 m?
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4.35 x 10-3 mm
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4) (1 mark) How many microlitres is 7.98 x 10-4 mL?
7.98 x 10-1 L
5) (1 mark) Which occupies a larger volume, 750 g of water (with a density of 0.995 g/mL) or
750 g of ethanol (with a density of 0.789 g/mL)
750 g ethanol
6) (1 mark) If a solution has a temperature of 355 K, what is its temperature in °C?
82 °C
7) (1 mark) The mass of an electron is 9.109 x 10-28 g. What is this in ng?
9.109 x 10-19 ng
8) (2 marks) Why do isotopes of the same element have the same atomic size? Answer briefly
in 1-2 sentences.
Atomic size (volume, radius) is determined by the electrons outside the nucleus. Since
isotopes have the same number of electrons, they have the same size.
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9) (4 marks) For the following atoms/ions:
Atom/Ion
#p
#n
#e
A
B
C
D
E
F
G
34
33
35
34
35
36
37
45
46
45
44
45
44
46
34
36
36
34
35
36
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Identify the following (by letter):
A&D a pair of isotopes
G
a cation
B or C an anion
A&B OR any pair of C, E, F two atoms that have the same mass number
10) (8 marks) Give the symbol or name for the following elements:
(a) potassium
K
(e) C carbon
(b) chromium
Cr
(f) Ne neon
(c) silver
Ag
(d) copper Cu
(g) Mg magnesium
(h) Si silicon
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PART II: Show your work for full credit.
[Final answer shown in red, but all work should be shown, like what it shows in purple. On a
test you should clearly indicate your final answer with an underline.]
1) (3 marks) An object having a mass of 35.6 g is placed in a graduated cylinder containing
32.5 mL water. The water level rises to 48.9 mL. Determine the density of the object in
g/cm3.
48.9 mL – 32.5 mL = 16.4 mL displaced volume
d = m/V = 35.6 g/16.4 mL = 2.17 g/mL = 2.17 g/cm3
3) (4 marks) Gas is sold for US$4.399 per gallon in Blaine, Washington. Your car needs 44.0 L.
How much will your credit card be charged in US dollars? (1 gallon = 3.79 L)
(44.0 L x 1 gal/3.79 L)($4.399/gal) = $51.07
(think about how you should express this number with the right number of significant figuresit’s not clear)
11) (4 marks) Earth’s oceans have an average depth of 3800 m and a total surface area of 3.63 x
108 m2. The average concentration of dissolved gold in the ocean is 3.63 x 10-9 g/L. How
many grams of gold are in the earth’s oceans?
V = (3800 m ) (3.63 x 108 m2) (106 cm3/1m3) (1 L/1000 cm3) = 1.38 x 1015 L
m = d . V = (3.63 x 10-9 g/L)( 1.38 x 1015 L) = 5.0 x 106 g gold
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12) (2 marks) What is the mass in grams of carbon of 25 carbon atoms?
(25 C atoms)(1 mol/6.022 x 1023 atoms)(12.01 g C/1 mol C) = 5.0 x 10-22 g C
13) (2 marks) What is the mass grams water of 5.98 x 10-3 mol H2O?
Molar mass of H2O = 2(1.01) + 16.00 = 18.02 g/mol
5.98 x 10-3 mol H2O x (18.01 g H2O/1 mol H2O) = 0.108 g H2O
14) (1 mark) How many moles is 4.5 g Cu?
4.5 g Cu x (1 mol Cu/63.55 g C) = 0.071 mol Cu
15) (8 marks)
a) Calculate the molarity of a solution made from 2.904 g KBr in 250.00 mL solution?
Molar mass of KBr = 119.00 g/mol
2.904 g KBr (1 mol KBr/119.00 g KBr) = 0.024403 mol KBr
Molarity = 0.024403 mol KBr/0.25000 L = 0.09761 M
b) How many grams KBr are needed to make 2.00 L of a 1.902 M solution?
M = n/V, n = M V = (1.902 M)(2.00 L) = 3.804 mol KBr
mass: (3.804 mol KBr )(119.00 g/mol) = 452.8 g KBr
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c) If a solution is made from 5.00 grams KBr and 500.0 g water, what is the percent by
mass KBr in the solution?
Total mass = 5.00 g + 500.0 g = 505.0 g
5.00 g
505.0 g
x 100 = 0.990 %
15) 15.00 mL of a 2.590 M NaOH solution is placed in a 300.0 mL volumetric flask and diluted
to the mark. Calculate the molarity of the new solution.
M1V1 = M2V2
M2 = M1V1 / V2 = 2.590 M (15.00 mL/300.0 mL) = 0.1295 M