CHEM 14A
Instructor: Dr. Laurence Lavelle
YOUR NAME (last name, first name) .....ANSWERS...........
STUDENT ID# ...............................
FALL 2011
MIDTERM
(Total number of pages = 10)
(Total points = 110)
(Total time = 110 minutes)
**Carefully remove the last two pages: Constants and Formulas, and Periodic Table.**
YOUR DISCUSSION SECTION ......................................
YOUR TA’s NAME ………………………………..
WRITE IN PEN (Show all your work on this paper, check units and significant figures.)
Box your final answer.
Good Luck
1
Do not write on this page.
QUESTION
SCORE
1
2
3
4
5
6
7
8
TOTAL (max 110)
2
Q1A. A compound found in tea is believed to be a stimulant. Its molar mass is 194.19 g.mol-1
and has a mass percentage composition of 49.47% C, 5.18% H, 28.86% N, and
16.49% O. What is its molecular formula?
(10pt)
Obtain actual masses of each element in a mole of the compound
194.19 g/mol x 0.4947 = 96.07 g C
1pt
194.19 g/mol x 0.0518 = 10.06 g H
1pt
194.19 g/mol x 0.2886 = 56.04 g N
1pt
194.19 g/mol x 0.1649 = 32.02 g O
1pt
Then calculate # of moles of each element
96.07 g
Mol of C atoms =
= 8.00 mol
12.01 g  mol -1
1pt
Mol of H atoms =
10.06 g
= 9.98 mol
1.008 g  mol -1
1pt
Mol of N atoms =
56.04 g
= 4.00 mol
14.01g  mol-1
1pt
Mol of O atoms =
32.02 g
= 2.00 mol
16.00 g  mol-1
1pt
Molecular formula ofthis compound is C8H10N4O2
2pt
Q1B. Calculate the volume needed to make 2.00 L of a 0.100 M K 2CrO4 from 1.75 M K2CrO4
stock solution.
(4pt)
2pt
2pt
-1sf
-1units
3
Q2.
Mix some sodium bicarbonate with lemon juice and cool frothy bubbles result:
NaHCO3(s) + C6H8O7(aq) → Na3C6H5O7(aq) + H2O(l) + CO2(g)
Will all the sodium bicarbonate (NaHCO3) be used up if 0.0100 moles of it reacts with 50.0
mL of a 0.0500 M solution of juice. If not, how many grams will be left over?
(12pt)
Balance the chemical equation:
3NaHCO3(s) + C6H8O7(aq) → Na3C6H5O7(aq) + 3H2O(l) + 3CO2(g)
5pt
Find moles of juice (C6H8O7):
Moles = Molarity x Volume (L) = 0.0500 M x 0.0500 L = 0.00250 mol C6H8O7
1pt
3:1 mole ratio between NaHCO3 and C6H8O7 means 0.00250 mol C6H8O7 will react with
0.0075 mol NaHCO3.
1pt
There is an excess of NaHCO3 since there is 0.0100 mol.
1pt
0.0100 mol – 0.00750 = 0.0025 mol NaHCO3 remaining
1pt
Molar mass NaHCO3: 84.00 g.mol-1
1pt
0.0025 mol NaHCO3 x
-1sf
84.00 g NaHCO 3
= 0.21 g NaHCO3 remaining
1mol NaHCO 3
-1units
4
2pt
Q3A. A newly discovered metal is determined to have a work function of 5.98 x 10 -18 J.
If the ejected electron is traveling at 2.01 106 m  s-1 , what is the wavelength of the
incident photon?
(12pt)
2pt
2pt
2pt
31
6
-1 2
hc 1
(9.109  10 kg)(2.01 10 m  s )
 mv 2   
 5.98  1018 J  7.82  10-18 J

2
2
2pt
2pt
2pt
8
-1
hc (6.626  10 J  s )(3.00  10 m  s )


 2.54  10 8 m  25.4 nm
E
7.82  10-18 J
34
-1sf
-1
-1units
Q3B. Which of these has the longer de Broglie wavelength?
A proton (mass = 1.672 x 10-27 kg) moving with velocity of 3.82 x107 m.s-1 or
an electron (mass = 9.109 x 10-31 kg) moving with velocity of 1.20 x 107 m.s-1?
Show your work.
Proton:
 = h/p = h/(mv)
 = (6.626 x 10-34J.s)/((1.672 x 10-27 kg)(3.82 x 107 m.s-1))
 = 1.04 x 10-14 m
1pt
1pt
1pt
Electron:
 = (6.626 x 10-34J.s)/((9.109 x 10-31 kg)(1.20 x 107 m.s-1))
 = 6.06 x 10-11 m
1pt
1pt
The electron has a longer de Broglie wavelength.
-1sf
-1units
5
1pt
(6pt)
Q4A. If you want to listen to an FM radio station that is broadcasting with radio wave of
wavelength 3.28 m, what frequency should you be tuning in to?
(Give your answer in MHz.)
(4pt)
1pt
1pt
1pt
1pt
8
1
c 3.00  10 m  s
v 
 91.5  106 s1  91.5 MHz
λ
3.28 m
-1sf
-1units
Q4B. Heating Copper (I) Chloride to about 1200 C results in blue fireworks.
The hot compound emits blue light with energy of 4.40 x 10-19 J per photon.
What is the wavelength of the blue light in nanometers?
ΔE  hν and v 
c
λ
1pt
λ
34
1pt
J  s)(3.00 108 m  s1 )
hc (6.626  10

ΔE
4.40  1019 J
-1sf
(4pt)
1pt
1pt
 4.50  10- 7 m  450. nm
-1units
Q4C. What is the minimum uncertainty in the speed of an electron confined to within the
diameter of a tin atom which has a diameter of 145 pm?
(4pt)
ΔpΔx 
h
4π
Δp  me Δv
Δx  145 pm  145  10-12 m
1pt
1pt
1pt
1pt
34
2
1
h
6.626  10
kg  m  s
Δv 

 3.99  105 m  s -1
31
-12
4πme Δx 4(3.142)(9.109  10 kg)(145  10 m)
-1sf
-1units
6
Q5.
Two structural isomers of disulfur difluoride, S2F2, are known. In each the two S atoms
are bonded to each other. In one isomer each of the S atoms is bonded to an F atom.
In the other isomer, both F atoms are attached to one of the S atoms.
Draw each isomer and a resonance structure for each isomer, and determine for each
isomer which structure is favored and why.
(16pt)
F
S
S
F
F
+1
-1
S
S
F
isomer 1
favored
F
+1
-1
S
S
F
S
S
isomer 2
F
F
favored
3pt for each structure (12pt)
1pt for correctly identifying lower energy isomer (2pt)
2pt for correct formal charge calculations or for saying lower FC.
7
Q6A. Write the ground-state electron-configuration for Sn2+ ___[Kr]4d105s2_____
(2pt)
Dark red copper(I)oxide (Cu2O) forms when copper metal is oxidized by air.
Write the ground-state electron-configuration for Cu+ ___[Ar]3d10_____
(2pt)
Q6B. Which compound is more ionic?
(4pt)
(a) Cl2O or Na2O
___________________
(b) InCl3 or SbCl3
___________________
(c) LiH or HCl
___________________
(d) MgCl2 or PCl3
___________________
(a) Na2O; (b) InCl3; (c) LiH; (d) MgCl2
Q6C. The structure for the amino acid tryptophan is given below with lone pairs omitted.
Also some of the hydrogen atoms attached to carbon atoms are omitted.
(8pt)
What is the hybridization of the carbon atoms in the ring structure? __sp2_____
What is the hybridization of the nitrogen atom pointing towards you? __sp3_____
What is the hybridization of the last oxygen atom on the far right? __ sp3_____
What is the hybridization of the carbon atom that is connected to the two oxygen atoms?
___sp2___
8
Q7.
Draw the lowest energy Lewis structure and determine the following information for the
species given.
(a)
Dichloromethanol: HOCHCl2
(6pt)
VSEPR formula:
AX4
molecular shape:
tetrahedral
hybridization of central atom:
sp3
polar or non-polar: polar
give one bond angle: 109.5o or ~109.5o
(b)
Phosphoric acid, H3PO4, can give off three protons to form the phosphate anion, PO43-.
Answer the following questions for PO43-.
(5pt)
VSEPR formula:
AX4
molecular shape:
tetrahedral
give one bond angle: 109.5o
-2pt for incorrect structure.
-1pt for each incorrect answer.
9
Q8.
Draw the lowest energy Lewis structure and determine the following information for the
species given.
(a)
Dimethyl sulfoxide (DMSO), (CH3)2SO, is an important solvent for polar and nonpolar
compounds. Answer the following questions for DMSO.
(6pt)
VSEPR formula:
AX3E
molecular shape: trigonal pyramidal
polar or non-polar: polar
give one bond angle: less than 109.5o
(b)
Nitric acid, HNO3, can give off one proton to form the nitrate anion, NO3-.
Answer the following questions for NO3-.
(5pt)
VSEPR formula:
AX3
molecular shape:
trigonal planar
hybridization of central atom:
give one bond angle:
-2pt for incorrect structure.
-1pt for each incorrect answer.
10
120o
sp2