2004 AP® CHEMISTRY FREE-RESPONSE QUESTIONS (Form B)
Answer EITHER Question 2 below OR Question 3 printed on page 8. Only one of these two questions will be
graded. If you start both questions, be sure to cross out the question you do not want graded. The Section II score
weighting for the question you choose is 20 percent.
2. Answer the following questions related to hydrocarbons.
(a) Determine the empirical formula of a hydrocarbon that contains 85.7 percent carbon by mass.
(b) The density of the hydrocarbon in part (a) is 2.0 g L–1 at 50°C and 0.948 atm.
(i) Calculate the molar mass of the hydrocarbon.
(ii) Determine the molecular formula of the hydrocarbon.
(c) Two flasks are connected by a stopcock as shown below. The 5.0 L flask contains CH4 at a pressure of 3.0
atm, and the 1.0 L flask contains C2H6 at a pressure of 0.55 atm. Calculate the total pressure of the system
after the stopcock is opened. Assume that the temperature remains constant.
(d) Octane, C8H18(l), has a density of 0.703 g mL–1 at 20°C. A 255 mL sample of C8H18(l) measured at 20°C
reacts completely with excess oxygen as represented by the equation below.
2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g)
Calculate the total number of moles of gaseous products formed.
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AP® CHEMISTRY
2004 SCORING GUIDELINES (Form B)
Question 2
2. Answer the following questions related to hydrocarbons.
(a) Determine the empirical formula of a hydrocarbon that contains 85.7 percent carbon by mass.
1 mol C
= 7.14 mol C
12.01 g C
1 mol H
= 14.3 g H ×
= 14.2 mol H
1.008 g H
nC = 85.7 g C ×
nH
1 point for moles of C and moles of H
7.14 mol C 14.2 mol H
:
7.14
7.14
1 mol C : 1.99 mol H
1 point for ratio of moles of C to moles of H
The empirical formula is CH2
1 point for correct formula
(b) The density of the hydrocarbon in part (a) is 2.0 g L–1 at 50°C and 0.948 atm.
(i) Calculate the molar mass of the hydrocarbon.
mass
RT
molar mass
mass
RT
RT
= density ×
molar mass =
×
V
P
P
L atm
× 323 K
0.0821
mol K
molar mass = 2.0 g L−1 ×
0.948 atm
−1
molar mass = 56 g mol
PV = nRT =
1 point for correct substitution
1 point for the answer
(ii) Determine the molecular formula of the hydrocarbon.
empirical mass × n = molar mass
empirical mass for CH2 = 14 g mol−1
14 g mol−1 × n = 56 g mol−1
1 point for correct formula
n = 4
The molecular formula is C4H8.
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AP® CHEMISTRY
2004 SCORING GUIDELINES (Form B)
Question 2 (cont’d.)
(c) Two flasks are connected by a stopcock as shown below. The 5.0 L flask contains CH4 at a pressure of
3.0 atm, and the 1.0 L flask contains C2H6 at a pressure of 0.55 atm. Calculate the total pressure of the
system after the stopcock is opened. Assume that the temperature remains constant.
Pf of CH4 =
Pf of C2H6 =
PV
(3.0 atm) (5.0 L)
i i
= 2.5 atm CH4
=
Vf
6.0 L
1 point for final pressure of CH4 or C2H6
PV
(0.55 atm) (1.0 L)
i i
= 0.092 atm C2H6
=
Vf
6.0 L
PT = Pf CH4 + Pf C2H6 = 2.5 atm + 0.092 atm = 2.6 atm
1 point for the total pressure
(d) Octane, C8H18(l), has a density of 0.703 g mL–1 at 20°C. A 255 mL sample of C8H18(l) measured at
20°C reacts completely with excess oxygen as represented by the equation below.
2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g)
Calculate the total number of moles of gaseous products formed.
n products = 255 mL C8H18 ×
0.703 g C8 H18
1 mol C8 H18
×
×
1 mL C8 H18
114 g C8 H18
34 mol products
= 26.7 mol products
2 mol C8 H18
1 point for substitution of any of these
conversion factors
1 point for the correct answer
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AP® CHEMISTRY
2004 SCORING COMMENTARY (Form B)
Question 2
Sample: 2A
Score: 10
This response earns a perfect score of 10 points: 3 point for part (a), 2 points for part (b)(i), 1 point for
part (b)(ii), 2 points for part (c), and 2 points for part (d).
Sample: 2B
Score: 8
No points are earned in part (c). The pressures are divided by volume when they should be multiplied, and
vice versa.
Sample: 2C
Score: 6
No points are earned in part (c) or in part (d).
Copyright © 2004 by College Entrance Examination Board. All rights reserved.
Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents).
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