Chemistry 121
Chp. 4&5 Sample Test
Answer Key
You will be given a periodic table and the electronegativities of all elements that you will be required to
know. In addition a polyatomic ion chart will be provided.
SAMPLE TEST
1. Write the ions formed by the following elements. Bromine, Argon, Barium, Selenium, Rubidium.
Indicate on each whether it is a cation or an anion.
Br- , Se-2 anions
Ba2+ , Rb+ - cations
Ar – no ion formed
2. What one thing do each of the above ions have in common? In other words why do they form the
specific ions you have shown and no others? The ions formed occur due to the stability of
the valence shell which now looks like the nearest noble gas valence shell.
3. Write the formulas for the following compounds and indicate whether they are ionic, polar covalent or
non-polar covalent in their bonding nature.
a) Iron(III) bicarbonate - Fe(HCO3)3
b) Nitrogen monoxide - NO
c) Hydrogen iodide - HI
d) Carbon tetrachloride - CCl4
e) Barium phosphate - Ba3(PO4)2
4. Name the following compounds from their chemical formulas.
a) N2O5 - dinitrogen pentoxide
b) Al2(HPO4)3 - aluminum hydrogenphosphate
c) NH4C2H3O2 - ammonium acetate d) K2SO3 - potassium sulfite
e) PCl5 - phosphorus pentachloride
5. Draw the Lewis dot structure for the following compounds/ions. In each case there is only one way the
atoms can be connected.
a) NH4+
b) N2H2
H
H
:
N
:
+
H
H
H
N === N
H
c) BF4-
d) NO31-
:F :
:
:F
B
:O :
::
F:
:
:F :
N
:O
O:
6. Indicate the shapes of the molecules in 5 above that you have made Lewis dot structures for.
a. Tetrahedral
b. Trigonal planar
c. Tetrahedral
d. Trigonal Planar
Chemistry 121
Chp. 4&5 Sample Test
Answer Key
7. The H2S molecule is an analog to the water molecule since S and O are in the same column. Write the
electron dot structure, indicate the shape, indicate whether it is a polar molecule and show where the
partial charges (and indicate type of charge) if it is polar.
S
H
H
Non-polar due to non-polar bonds S – 2.5 H – 2.1 Difference is .4
8. What is the formula weight of C2H5OH? What is its molar weight?)
Formula weight and molar weights are numerically the same only having different
units.
(2 x 12.011 + 6 x 1.0079 + 15.9994) = 46.069
g/mol for M.W,
amu/molecule – for formula weight
9. Write the balanced equation for the following reactions.
a) The combustion of butane C4H10, producing water and carbon dioxide.
2C4H10 + 13O2
10H2O + 8CO2
b) The reaction of iron(III) chloride with sodium hydroxide to form iron(III) hydroxide and table salt.
FeCl3 + 3NaOH
Fe(OH)3 + 3NaCl
c) The decomposition of ammonium carbonate into ammonia gas, carbon dioxide gas and one other
product, which you should be able to determine.
(NH4)2CO3
2NH3 + CO2 + H2O
d) The reaction of zinc metal with copper(II) sulfate to form zinc(II) sulfate and copper metal.
Zn + CuSO4
ZnSO4 + Cu
10. Ethanol, C2H6O, can be added to gasoline to produce "gasohol", a fuel for automobile engines.
a) Write the equation for the complete combustion of ethanol with oxygen to produce water and carbon
dioxide.
C2H6O
865g
+
18.776 moles
3O2
1800g
56.33 mol
2CO2 +
1650g
37.55 mol
3H2O
1010g
56.33 mol
The first step is to determine the moles of ethanol in 865g, which is accomplished by
dividing by the molar mass. Then use the ratio’s of the balanced equation to
determine the moles of oxygen needed and carbon dioxide and water produced.
Once the moles have been determined, calculate grams by multiplying the molar
weight of each times the moles of each. Do not round values until calculations are
finished. To verify, grams on the left must equal grams on the right (the grams may
vary slightly at the last SF due to rounding)
Chemistry 121
Chp. 4&5 Sample Test
Answer Key
The m.w. are as follows 46.0688g/mol for the ethanol, 31.9988g/mol for oxygen,
44.010g/mol for carbon dioxide and 18.015g/mol for water.
b) Calculate how many grams of O2 are require for the combustion of 865 g of ethanol.
c) Also, calculate the number of grams of H2O and CO2 produced in the complete reaction.
11. According to VSEPR theory, if the valence electrons on a central atom are 3 bond pairs and one
nonbonding (lone) pair, the geometry (shape) at this atom will be?
A. linear
B. bent (angular)
C. trigonal planar
D. trigonal pyramidal
E. tetrahedral
12. What term describes a solution of a compound in water that conducts an electric current?
An electrolyte solution or an electrolytic solution.
13. Describe, in terms of energy and bond length how a double bond between two atoms, A and B is
different than a single bond between them.
A double bond between two atoms is shorter than a single bond between the same two atoms, has
a higher bond energy than a single bond between the same two atoms and consists of four
electrons shared between A and B.
14. What kind of bond results when electron transfer occurs between atoms of two different elements?
This is the definition (one of them) of an ionic bond.
15. The ammonia molecule, NH3, is polar. Why does this fact suggest that its shape is trigonal
pyramidal, rather than trigonal planar?
If the molecule were trigonal planar, the symmetry would result in a nonpolar molecule. The
centers of positive and negative charge would coincide, therefore since it is described as polar
it can not have a symmetric shape. In addition when the Lewis structure is done it is clearly
tetrahedral having three bonding pairs and one lone pair of electrons.
16. In the molecule AX2, the central atom A has two lone pairs of electrons in addition to the two bond
pairs in the A X bonds. What is the shape of this molecule?
The basic or electron pair shape is tetrahedral, the net or molecular shape that results
when one only views the atoms is bent or angular.
17. What is defined as the amount of energy needed to break a bond holding two atoms together?
This is the “Bond dissociation energy”.
18. What kind of bonding is present in substances which are nonelectrolytes?
If they are non-electrolytes it means they are not polar, this means the bonds must be
covalent in nature.
19. In what way is a polar covalent bond similar to a nonpolar covalent bond? In what way are they
different?
Both bonds involve the sharing of at least one pair of electrons. However the real difference
is that the polar bond involves two atoms with significantly different electronegativities. The result
is unequal sharing and an increase in electron density at one side of the molecule and a
corresponding decrease on the opposite side.