Section II (Free-Response Questions)
Scoring Guidelines
One point deduction for mathematical error (maximum once per question)
One point deduction for error in significant figures* (maximum once per question)
*number of significant figures must be correct within +/– one digit
Part A: Question 1
- Given: () () () ()
.
C H NH aq H O C H NH aq OH aq
K for C H NHb 5 6 10
25 2 2 25 3
23 2
4
D
#
++,
=
+ -
(a) Given: 65.987 mL of 0.250 M C 2 H 5 NH 2
Restatement: Find [OH–]
Step 1: Rewrite the balanced equation for the ionization of ethylamine.
C H NH 25 2 ++H O 2 DC N NH 25 3 + OH-
Step 2:Write the expression for the base-dissociation constant.
K CHNH.
C H NH OH
b 56 10
2
3
25
(^254)
==#
- 6
7 7
@
A A
Step 3: Create a chart showing initial and final concentrations (at equilibrium) of the in-
volved species. Let xbe the amount of C 2 H 5 NH 3 +that forms from C 2 H 5 NH 2. Because
C 2 H 5 NH 3 +is in a 1:1 molar ratio with OH–, [OH–] also equals x.
Species Initial Concentration Final Concentration (at equilibrium)
C 2 H 5 NH 2 0.250 M 0.250 −x
C 2 H 5 NH 3 + 0 M x
OH– 0 M x
Step 4: Substitute the equilibrium concentrations from the chart into the equilibrium ex-
pression and solve for x.
..
()( )
K CHNH
C H NH OH
x
xx
b 2 56 10 0 250
3
25
(^254)
===# -
- 7 A 7 A -
Answers and Explanations for the Practice Test
- 7 A 7 A -