Chemistry 114
Fourth Hour Exam
Name:____________
(4 points)
Please show all work for partial credit
1. (12 points)
A. What is pH of .00673M HCl?
pH = -log(.00673)
=-(-2.172)
=2.172
B. What is pH of .0673M NaOH?
pOH=-log(.0673)
=-(-1.172)
=1.172
pH=14-pOH = 12.828
C. If a solution of HNO3 has a pH of 2.34, that is the concentration of HNO3?
[H+]=10-pH
= 10-2.34
=.0046M
D. If a solution of magnesium hydroxide has a pH of 8.5, what is the concentration of
magnesium hydroxide in that solution?
Magnesium hydroxide = Mg(OH)2
pOH = 14-8.5 = 5.5
[OH]=10-pOH = 10-5.5 = 3.16x10-6, but Mg(OH)2WMg2+ + 2OHso [Mg(OH)2] = ½ [OH-] = 3.16x10-6/2 = 1.58x10-6
2. (12 points) Lactic acid (CH3CHOHCOOH) has a Ka of 1.4x10-4. What is the pH of a
.0055M solution of lactic acid?
Weak acid Ka = [H+][A-]/[HA]
X = [H+]=[A-]
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Ka = X /([HA]-X) , Assume X small so Ka = X2/[HA]
1.4x10-4 = X2/.0055; .0055 x 1.4x10-4 = X2
X = [H+ ] = sqrt(.0055 x 1.4x10-4) = 8.77x10-4
pH = -log(8.77x10-4) = 3.0
Lactic acid had the following structure:
Draw the structure of the conjugate base of lactic acid.
What is the KB of the conjugate base of lactic acid?
KB = KW /KA = 1x10-14/1.4x10-4 = 7.14x10-11
3. (12 points) Identify the following compounds as acids, bases or neutrals
RbCl ____N_____________
CH3COOK
___B_________ (CH3COO- is conjugate base)
NH4ClO4
___A_________ (NH4+ is conjugate acid)
Cr(NO3)6
__A__________ (Cr+6 is polyvalent cation)
SO3
___A______________ (covalent oxide)
K2 O
___B______________ (ionic oxide)
4. (12 points) I am going to make a buffer by adding 45 mLs of .1M Acetic acid with 55mls
of .12 M sodium acetate. If the pKa of acetic acid is 4.74, what is the pH of this buffer?
M = mol/volume(L); mol = M x Volume(L)
Moles acid = .045L x .1M = .0045 moles
Mole Conjugate base = .055L x .12M = .0066 moles
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5. (12 points) Given that lithium has a melting point of 454K, a ÄHfusion of 2.99 kJ/mol, a
boiling point of 1615K, and a ÄH Vaporization of 134.7 kJ/mol
What is the ÄS fusion for Lithium
ÄS fusion= ÄH fusion/Tmelting
-1
= 2990 JAmol /454K
= 6.59 J/KAmol
What is the ÄS Vaporization For Lithium
ÄS Vaporization =ÄH Vaporization /Tboiling point
=134700JAmol-1 /1615K
=83.4 J/KAmol
6. (12 points) Define the following terms:
Thermal disorder - Entropy that comes from distributing energy between particles
Positional disorder - Entropy that comes from distributing particles in space
A spontaneous process - A process that takes place without input of energy from an
external source
The third law of thermodynamics - The entropy of a perfect crystal at 0K is zero.
An entropy favored reaction - A reaction that occurs because ÄS is positive
The van’t Hoff equation
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7. (12 points)
A. Calculate ÄGo for the reaction 2Na(s) + 2H2O(l) 62Na+(aq) + 2OH-(aq) + H2(g)
given the following ÄGof values:
Substance ÄGof (kJ/mol)
Na(g)
77.0
Na(s)
0.00
Na+(aq)
-261.9
H2O(g)
-228.6
H2O(l)
-237.1
OH-(aq)
-157.2
H2(g)
0.00
ÄG rxn0 = sum ÄGof products - sum ÄGof reactants
=[2(-261.9) + 2(-157.2) + 0.00] - [2(0.00) + 2(-237.1)]
=[-523.8 + -314.4] - [-474.2]
= -838.2 + 474.2
= -364.0 kJ/mol
B. What is the K for this reaction at 20oC? (If you have no answer for part A, just make
a wild guess and continue on!)
T(K) = 20o C + 273 = 293K
-364.0kJ/mol = -364000J/mol
ÄGrxn =-RTlnK
-364000 = -8.314 x 293 ln(K)
-364000/(-8.314 x 293) = lnK
+149=lnK
e149 =K
K=7.83x1064
8. (12 points) Below are the names of four alkanes. For each alkane:
A. Draw the structure of each alkane below the name.
B. If the name is correct, circle that name.
C. If the name in incorrect draw a line through it and, provide a correct name.
3,4-dimethylbutane
3-ethyl-2,2,4-trimethylpentane
3-methylpentane
2-methy-2-propylpentane
2,5-diethyl-2,4,6-trimethylheptane
4,4-dimethylheptane
3-ethyl-2,4,6,6-tetramethyloctane
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