The University of Lethbridge
Chemistry 1000
Instructor: Dr. R.T. Boeré
Department of Chemistry
Practice Test #1 Answers - 2004
Time: 50 minutes
No. of Pages = 5
Instructions: I do not have an old midterm available that covers exactly the same topical material as we have
this year. However, it is at least an example of the kinds of tests and questions I have written in the past. All
these questions are on material on this year’s curriculum, i.e. you should be able to do all of them.
1.
[14]
a) Complete the following table:
Complete symbol
Number of protons
Number of neutrons
Number of electrons
N
7
8
7
Ag +
47
60
46
V
23
28
23
He 2 +
2
2
0
15
7
107
47
51
23
4
2
b) Complete the following table with a name or formula, as required:
Formula
Name
NH4Cl
ammonium chloride
AgCN
silver(I) cyanide
ZnS
FeBr2
zinc sulfide
iron(II) bromide
c) What was incorrect about Mendeleev’s original concept of the periodic table?
He organized the elements by increasing average atomic mass, instead of increasing atomic number.
(He was unaware of the existence of isotopes.)
d) Name and give symbols for one element (no repetitions, please) that:
(i) is a metal
(ii) is a non-metal
your choice - see the periodic table
e.g.: Hf - Hafnium
your choice - see the periodic table
e.g.: At - astatine
(iii) is a metalloid
(iv) exists as a molecule in its most stable allotrope
your choice - see the periodic table
e.g.: As - arsenic
one of: N2, O2, F2, Cl2, Br2, I2, P4, S8, Se8
nitrogen, oxygen, fluorine, chlorine, bromine, iodine
phosphorus, sulfur, selenium
1
2.
Predict the course of the following equations and state the products. Balance the equation and classify
it as (i) acid-base, (ii) gas-forming, or (iii) precipitation.
[6]
2 H3PO4 (aq) → 2 KH2PO4 (aq)
a) K2CO3 (aq) +
gas forming rxn and/or acid-base reaction
+
CO2 (g)
+
H2O (l)
[K2HPO4 or K3PO4 (aq) are less likely, but acceptable products with appropriate balancing]
b) Pb(NO3)2 (aq)
Li2SO4 (aq) → PbSO4 (s)
+
+
2 LiNO3 (aq)
precipitation reaction
c) HCN (aq)
+
NH3 (aq)
Æ
NH4CN (aq)
acid-base
3.
Calculate the average atomic weight for the hypothetical element myprosium, My, for which there are
three naturally-occurring isotopes, as follows:
68
My atomic mass = 67.93 54.50% abundance
69
My atomic mass = 69.01 30.30% abundance
71
My atomic mass = 70.98 15.20% abundance
[4]
UMy
= 0.5450 × 67.93 + 0.3030 × 69.01 + 0.1520 × 70.98 = 68.72
units: EITHER u or g mol–1
4.
A chemist recently obtained a new compound known to contain only C, H and O. Its molar mass was
determined to be 143±4 g mol–1 by osmometry. The results of five combustion analyses were averaged
to give the following composition: C 50.40 % and H 5.52%.
[8]
a) Determine the most likely empirical formula
First, the % O = 100 – (50.40 + 5.52) = 44.08
50.40 ÷ 12.011 g mol = 4.19615
simplest ratio integer ratio
1.523
3
5.52 ÷ 1.00 g mol–1 = 5.47619
1.9877
4
44.08 ÷ 16.00 g mol–1 = 2.755
1
2
–1
b) Determine the most likely molecular formula
143
= 1984
.
≅2
72.065
∴ C6H8O4
FW = 144 g mol–1
2
5.
[10]
Sulfur tetrafluoride is a powerful fluorinating agent for both organic and inorganic compounds. It
reacts with iodine pentoxide according to the equation:
5 SF4 (g) + 2 I2O5 (s) → 4 IF5 (l) + 5 SO2 (g).
a) If 75.00 g of SF4 are reacted with 100.0 g of I2O5, how many grams of IF5 can be produced?
b) If from the reaction in part (a) 39.5 g of gaseous SO2 are recovered, what is the % yield of the
reaction, based on the limiting reagent.
5 SF4 (g)
MM (g mol ) 108.07
mass (g)
75.00
mol
0.69399
–1
+
2 I2O5 (s)
333.808
100.0
0.29957
→
4 IF5 (l)
221.904
123.2
0.5552
+
5 SO2 (g)
64.07
44.464
0.69399
if all SF4 used, 2/5 × 0.69399 = 0.2776 moles I2O5 required – this is possible
if all I2O5 used, 5/2 × 0.29957 = 0.7489 moles SF4 required - this is not possible
∴ SF4 is limiting
a) 4/5 × 0.69399 = 0.5552 moles IF5 can be made or 123.2 g
b)
39.5
× 100 = 88.8%
44.464
1
6.
Balance each of these equations and then classify each one as an acid-base reaction, a precipitation, or
a gas-forming reaction. Show states for the products (s, l, g, aq), and then balance the completed
equation. Write the net ionic equation.
[7]
(a) MnCl2(aq) + Na2S(aq) ÆMnS + NaCl
MnCl2(aq) + Na2S(aq) Æ MnS (s) + 2 NaCl (aq)
Precipitation reaction.
Mn2+(aq) + S2–(aq) Æ MnS(s)
(b) K2CO3(aq) + ZnCl2(aq) Æ ZnCO3 + KCl
K2CO3(aq) + ZnCl2(aq) Æ ZnCO3(s) + 2 KCl (aq)
Precipitation
CO32–(aq) + Zn2+(aq) Æ ZnCO3(s)
(c) K2CO3(aq) + HClO4(aq) Æ KClO4 + CO2 + H2O
K2CO3(aq) + 2 HClO4(aq) Æ 2 KClO4(aq) + CO2(g) + H2O(l)
Gas-forming (better than acid-base, because carbonate is a non-Arrhenius base)
CO32–(aq) + 2 H+ (aq) Æ CO2(g) + H2O(l)
7.
Predict the products of each precipitation reaction, and then balance the completed equation.
[3]
(a) Pb(NO3)2(aq) + KBr(aq) ?
Pb(NO3)2(aq) + KBr(aq) Æ PbBr2(s) + 2 KNO3(aq)
(b) Ca(NO3)2(aq) + KF(aq) ?
Ca(NO3)2(aq) + 2 KF(aq) Æ CaF2(s) + 2 KNO3(aq)
(c) Ca(NO3)2(aq) + Na2C2O4(aq) ?
Ca(NO3)2(aq) + Na2C2O4(aq) Æ CaC2O4(s) + 2 NaNO3(aq)
3