Chem 150
Midterm
18 February 2016
Student Name:_______________________
1. (2.5 marks) Indicate if the statements 1a) to 1e) are True or False.
a) When comparing molecules of similar molecular mass the more polar molecule will likely
have the lower melting and boiling point.
b) When comparing molecules of vastly differing molecular mass the heavier molecule will likely
have the higher melting and boiling point.
c) Intermolecular forces are much weaker than the forces that hold together atoms in molecules
(chemical bonds).
d) If a molecule has polar covalent bonds it must be polar.
e) Heteronuclear diatomic molecules must always be at least slightly polar.
2. (2.5 marks) Indicate if the statements 2a) to 2e) are True or False.
a) A colligative property depends only on the amount of substance but not on its identity.
b) Equivalent Lewis structures contribute equally to the resonance hybrid.
c) Helium and sulfur form a homogeneous mixture at room temperature and ambient pressure.
d) Magnesium is more reactive toward water than Calcium.
e) In a mass spectrometer ions separate because of differences in the shape of the produced
ions.
3. (21 marks) Identify the electron group geometry (EGG)and the molecular shape of the
species below. Start by writing a valid Lewis structure and giving the number of electron groups
(#EG) around the central atom. Be sure to show all electrons and indicate any non zero formal
charges. Continue by drawing the EGG placing lone pairs and bonding pairs in the appropriate
positions. Name the EGG. Next draw the molecular shape. Here multiple bonds, formal charges
on atoms and lone pairs might be omitted for clarity. Name the molecular shape. Finally,
indicate if the molecule or ion is polar or not. Please refer to the example below (SO2) if in doubt
about any of the instructions.
Lewis Structure
# EG
EGG
molecular shape
polar (Yes or No)
_______________________________________________________________________________
O
S
O
S
3
O
S
O
trigonal planar
O
Yes
O
bent
SO2_____________________________________________________________________________
[SCN]-
________________________________________________________________________________
SiHCl3
________________________________________________________________________________
[IF4]-
4. a)(6 marks) Draw all valid Lewis structures for the nitrate ion. Remember to show all valence
electrons and represent lone pairs as pairs of dots and bonding pairs as solid lines. Show any
non-zero formal charges.
b)(2 marks) Are the structures you have drawn equivalent or non-equivalent Lewis structures?
5. (2 marks) Using the periodic table arrange the following in the expected order of increasing first
ionization energy: Al, C, Ba, Cs
6. (2 marks) Arrange the following atoms in order of increasing atomic radii: Se, As, S, F
7. (7 marks) Complete the following phrases:
Ionization energy ___________ from the top to the bottom in a group.
Electron affinity ____________ from the left to the right within a period.
Atomic radius ______________ from the top to the bottom in a group.
Electronegativity ____________ from the left to the right within a period.
The metallic character of an element _________ from the left to the right within a period.
Cations are ________________ than the atoms from which they form.
In isoelectronic anions the radius ____________as the charge increases (becomes more
negative).
8. (3 marks) Shown below are the perspective drawings of two simple organic compounds A and B.
Both compounds have similar molecular masses but differ hugely in their physical and chemical
properties. Identify the high and low boiling compound. In one or two sentences give the reason
for your choice. Use arguments involving intermolecular interactions.
H
H
C
C
H
H
H
H
H
C
S
H
Compound A
H
C
H
H
O
H
Compound B
9.
(3 marks) We have done little silicon chemistry in our Chem 150 class but even without any
substance knowledge you should be able to arrange the following compounds of silicon in order
of increasing boiling point. Hint: Write Lewis structures.
Zr[SiO4], SiH4, SiCl4, Si(CH3)4
10. (4 marks) Decide if the following pairs of molecules can form hydrogen bonds with each other:
H2O
O2
(Yes or No)
H2O
H2O2
(Yes or No)
H2O
H2
(Yes or No)
CH4
H2O
(Yes or No)
11. (3 marks) Draw two water molecules hydrogen bonded. Be sure to represent the correct
geometry of the molecules in your drawing and align the molecules properly. Draw covalent
bonds as solid lines (as usual) and the hydrogen bond as dashed line.
12. (3 marks) The solubility of Cl2 (g) and HCl (g) in water differ greatly. Which gas would you expect
to be more soluble in water and why?
13. (3 marks) Calculate the osmotic pressure of a 0.100 M sodium chloride solution at 22 °C.
14. (4 marks) 1.00 mol of carbon dioxide occupy a vessel of 3.00 L at 0°C. Calculate the real pressure
of the gas using the van der Waals equation. The van der Waals constants for carbon dioxide are
a = 3.62 x102 L2kPa/mol2 and b = 0.0427 L/mol
Figure A
Figure B
Figure C
15. (4 marks) The three gas mixtures depicted in the figures above (helium and nitrogen) are all at
the same temperature and in containers of the same volume. Rank the samples A, B and C
according to:
a) total pressure
b) partial pressure of helium
c) density
d) average kinetic energy
Use <, > or = for ranking (e.g. A>B=C)
16. Liquid carbon disulfide reacts with nitrogen monoxide gas to produce gaseous carbon dioxide,
nitrogen and solid sulfur. Spectacular blue light is also produced in this reaction .
A 5.00 L reactor is filled with 101.3 kPa of nitrogen monoxide at 20 °C and 2.50mL of carbon
disulfide are added. The reaction is initiated with a spark and is assumed to go to completion.
a)(3 marks) Write a balanced equation for this reaction.
b)(5 marks) Calculate the partial pressure of nitrogen after complete reaction (assume 20 °C and
assume further that volume occupied by the sulfur produced is negligibly small. The density of
carbon disulfide is 1.27 g/cm3) Note that there will be no part marks if the equation is wrong.
c)(3 marks) What mass of sulfur is produced?
d)(3 marks) What is the final pressure in the reactor (20 °C)? The vapour pressure of carbon
disulfide is 38 kPa at that temperature.
17. Radiator fluid for cooling the engine in most cars is made up of water and glycol (also known as
antifreeze because when added to water it significantly decreases the freezing point of the
mixture). The additional benefit of adding glycol is the effect it has on lowering the vapour
pressure of the mixture and thus increasing the boiling point.
a) (5 marks) Calculate the vapour pressure of a solution prepared by mixing 5.00 kg of water with
5.00 kg of glycol (HOCH2CH2OH) at 105° C. The vapour pressure of water at that temperature is
118 kPa and that of glycol is 19 kPa
b) (2 marks) What is the composition of the vapour at that temperature?
c) (2 marks) A young technician in an automotive shop is suggesting replacing the glycol with
ethanol (CH3CH2OH) as additive to radiator fluid because it is way les toxic. Without any
information on the physical constants of ethanol and just recalling on your experience from the
distillation lab comment if this would be a feasible idea (or not) and state your reason. WRITE A
SENTENCE! E.g. Yes or No is not an acceptable answer!
Formulas and Constants for the Midterm
pV = nRT
piVi(Ti)-1 = pfVf(Tf)-1
Ptotal = p1 + p2 + p3 + .... + pn
[p +
𝒏²𝒂
𝑽²
][V – nb] = nRT
Vmol = 22.4 L mol-1 at STP (1 atm = 101.325 kPa and 273.15 K)
R = 8.3145 L kPa mol-1 K-1
Avogadro’s number
Na = 6.022 x 1023 mol-1
Unified atomic mass unit
1 u = 1.660 x 10-27 kg
Π = MRT
PA = xAP°A
Sg = kpg