AP® CHEMISTRY
2006 SCORING GUIDELINES
Question 3
3. Answer the following questions that relate to the analysis of chemical compounds.
(a) A compound containing the elements C , H , N , and O is analyzed. When a 1.2359 g sample is burned
in excess oxygen, 2.241 g of CO2(g) is formed. The combustion analysis also showed that the sample
contained 0.0648 g of H.
(i) Determine the mass, in grams, of C in the 1.2359 g sample of the compound.
2.241 g CO2(g) ×
1 molCO 2
12.011 g C
1 mol C
×
×
1 mol C
1 mol CO2
44.01 g CO2
One point is earned for
the correct answer.
= 0.6116 g C
(ii) When the compound is analyzed for N content only, the mass percent of N is found
to be 28.84 percent. Determine the mass, in grams, of N in the original 1.2359 g sample of
the compound.
1.2359 g sample × 0.2884 = 0.3564 g N
One point is earned for the correct answer.
(iii) Determine the mass, in grams, of O in the original 1.2359 g sample of the compound.
Because the compound contains only C , H , N , and O ,
mass of O = g sample − ( g H + g C + g N )
= 1.2359 − (0.0648 + 0.6116 + 0.3564) = 0.2031 g
One point is earned for the
answer consistent with the
answers in parts (a)(i) and (a)(ii).
(iv) Determine the empirical formula of the compound.
Converting all masses to moles,
1 mol C
12.011 g C
1 mol H
0.0648 g H ×
1.0079 g H
1 mol N
0.3564 g N ×
14.007 g N
1 mol O
0.2031 g O ×
16.00 g O
0.6116 g C ×
= 0.05092 mol C
One point is earned for all
masses converted to moles.
= 0.06429 mol H
Note: Moles of C may be shown
in part (a)(i).
= 0.02544 mol N
= 0.01269 mol O
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AP® CHEMISTRY
2006 SCORING GUIDELINES
Question 3 (continued)
Divide all mole quantities by the smallest number of moles:
0.05092 mol
0.06429 mol
0.02544 mol
0.01269 mol
÷
÷
÷
÷
0.01269
0.01269
0.01269
0.01269
mol
mol
mol
mol
=
=
=
=
One point is earned for dividing by the
smallest number of moles.
4.013
5.066
2.005
1.000
One point is earned for the empirical
formula consistent with the ratio of moles
calculated.
Ÿ Empirical formula is C4H5N2O
(b) A different compound, which has the empirical formula CH2Br , has a vapor density of 6.00 g L− 1 at
375 K and 0.983 atm. Using these data, determine the following.
(i) The molar mass of the compound
PV = nRT Ÿ
PV
= n
RT
(0.983 atm)(1.00 L)
(0.0821 L atm mol 1K 1 )(375 K)
molar mass of gas (M) =
One point is earned for applying
the gas law to calculate n.
= 0.0319 mol
One point is earned for calculating
the molar mass.
6.00 g
= 188 g mol−1
0.0319 mol
OR
OR
M=
DRT 6.00 g L−1 × 0.0821 L atm mol−1 K −1 × 375 K
=
0.983 atm
P
= 188 g mol−1
Two points are earned for calculating
the molar mass using M =
DRT
P
(ii) The molecular formula of the compound
Each CH2Br unit has mass of 12.011 + 2(1.0079) + 79.90 = 93.9 g,
188 g
and
= 2.00, so there must be two CH2Br units per molecule.
93.9 g
Therefore, the molecular formula of the compound is C2H4Br2 .
One point is earned for the
molecular formula that is
consistent with the molar mass
calculated in part (b)(i).
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AP® CHEMISTRY
2007 SCORING GUIDELINES
Question 5
5 Fe2+(aq) + MnO4−(aq) + 8 H+(aq) → 5 Fe3+(aq) + Mn2+(aq) + 4 H2O(l)
The mass percent of iron in a soluble iron(II) compound is measured using a titration based on the balanced
equation above.
(a) What is the oxidation number of manganese in the permanganate ion, MnO4−(aq) ?
+7
One point is earned for the correct oxidation number.
(b) Identify the reducing agent in the reaction represented above.
Fe2+(aq)
One point is earned for the correct iron ion.
The mass of a sample of the iron(II) compound is carefully measured before the sample is dissolved in
distilled water. The resulting solution is acidified with H2SO4(aq). The solution is then titrated with
MnO4−(aq) until the end point is reached.
(c) Describe the color change that occurs in the flask when the end point of the titration has been reached.
Explain why the color of the solution changes at the end point.
The solution in the flask changes from colorless to faint purple-pink
at the endpoint of the titration.
At the endpoint there is no Fe2+(aq) left in the flask to reduce the
colored permanganate ion, so when a small amount of permanganate
ion is added after the endpoint, the unreacted permanganate ion
present in the solution colors the solution faint purple/pink.
One point is earned for stating
that a faint pink color appears
(unless indication of acid-base
reaction).
One point is earned for a
correct explanation involving
excess MnO4− after all Fe2+
has reacted.
(d) Let the variables g, M, and V be defined as follows:
g = the mass, in grams, of the sample of the iron(II) compound
M = the molarity of the MnO4−(aq) used as the titrant
V = the volume, in liters, of MnO4−(aq) added to reach the end point
In terms of these variables, the number of moles of MnO4−(aq) added to reach the end point of the
titration is expressed as M × V. Using the variables defined above, the molar mass of iron (55.85 g
mol−1), and the coefficients in the balanced chemical equation, write the expression for each of the
following quantities.
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AP® CHEMISTRY
2007 SCORING GUIDELINES
Question 5 (continued)
(i) The number of moles of iron in the sample
mol Fe2+ = 5 × M × V
OR
mol Fe2+ =
5 mol Fe
One point is earned for either expression.
2+
1 mol MnO 4 −
×M×V
(ii) The mass of iron in the sample, in grams
mass Fe = 5 × M × V × 55.85 g mol−1
OR
One point is earned for the answer in part (d)(i)
multiplied by 55.85.
mass Fe = mol Fe2+ × 55.85 g mol−1
(iii) The mass percent of iron in the compound
mass % Fe =
5 × M × V × 55.85
× 100
g
One point is earned for the answer in part (d)(ii)
divided by g.
OR
mass % Fe =
mass Fe
× 100
g
One point is earned for converting to percent.
(e) What effect will adding too much titrant have on the experimentally determined value of the mass
percent of iron in the compound? Justify your answer.
The experimentally determined mass percent of iron in the
compound will be too large.
V is too large Ÿ expression in (d)(iii) above is too large
One point is earned for stating that the
mass percent is too large, with
justification.
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AP® CHEMISTRY
2008 SCORING GUIDELINES
Question 2
Answer the following questions relating to gravimetric analysis.
In the first of two experiments, a student is assigned the task of determining the number of moles of water in
one mole of MgCl2 ⋅ n H2O. The student collects the data shown in the following table.
Mass of empty container
22.347 g
Initial mass of sample and container
25.825 g
Mass of sample and container after first heating
23.982 g
Mass of sample and container after second heating
23.976 g
Mass of sample and container after third heating
23.977 g
(a) Explain why the student can correctly conclude that the hydrate was heated a sufficient number of times
in the experiment.
One point is earned for
the correct explanation.
No additional mass was lost during the third heating, indicating that
all the water of hydration had been driven off.
(b) Use the data above to
(i) calculate the total number of moles of water lost when the sample was heated, and
mass of H2O lost = 25.825 − 23.977 = 1.848 g
OR
25.825 − 23.976 = 1.849 g
1.848 g H2O ×
One point is earned for calculating
the correct number of moles of water.
1 mol H 2 O
= 0.1026 mol H2O
18.02 g H 2O
(ii) determine the formula of the hydrated compound.
mass of anhydrous MgCl2 = 23.977 − 22.347 = 1.630 g
1.630 g MgCl2 ×
1 mol MgCl 2
= 0.01712 mol MgCl2
95.20 g MgCl2
0.1026 mol H 2 O
= 5.993 ≈ 6 mol H2O per mol MgCl2
0.01712 mol MgCl2
Ÿ formula is MgCl2 x 6H2O
One point is earned for calculating
the correct number of moles of
anhydrous MgCl2 .
One point is earned for writing the
correct formula (with supporting
calculations).
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AP® CHEMISTRY
2008 SCORING GUIDELINES
Question 2 (continued)
(c) A different student heats the hydrate in an uncovered crucible, and some of the solid spatters out of the
crucible. This spattering will have what effect on the calculated mass of the water lost by the hydrate?
Justify your answer.
The calculated mass (or moles) of water lost by the hydrate
will be too large because the mass of the solid that was lost
will be assumed to be water when it actually included some
MgCl2 as well.
One point is earned for the
correct answer with justification.
In the second experiment, a student is given 2.94 g of a mixture containing anhydrous MgCl2 and KNO3 .
To determine the percentage by mass of MgCl2 in the mixture, the student uses excess AgNO3(aq) to
precipitate the chloride ion as AgCl(s).
(d) Starting with the 2.94 g sample of the mixture dissolved in water, briefly describe the steps necessary to
quantitatively determine the mass of the AgCl precipitate.
Add excess AgNO3 .
- Separate the AgCl precipitate (by filtration).
- Wash the precipitate and dry the precipitate completely.
- Determine the mass of AgCl by difference.
Two points are earned for all three major
steps: filtering the mixture, drying the
precipitate, and determining the mass
by difference.
One point is earned for any two steps.
(e) The student determines the mass of the AgCl precipitate to be 5.48 g. On the basis of this information,
calculate each of the following.
(i) The number of moles of MgCl2 in the original mixture
5.48 g AgCl ×
One point is earned for
calculating the number of
moles of AgCl.
1 mol AgCl
= 0.0382 mol AgCl
143.32 g AgCl
0.0382 mol AgCl ×
1 mol MgCl2
1 mol Cl
×
= 0.0191 mol MgCl2
2 mol Cl
1 mol AgCl
One point is earned for
conversion to moles of
MgCl2 .
(ii) The percent by mass of MgCl2 in the original mixture
0.0191 mol MgCl2 ×
95.20 g MgCl2
= 1.82 g MgCl2
1 mol MgCl 2
1.82 g MgCl2
× 100% = 61.9% MgCl2 by mass
2.94 g sample
One point is earned for
calculating the correct percentage.
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AP® CHEMISTRY
2008 SCORING GUIDELINES (Form B)
Question 5
The identity of an unknown solid is to be determined. The compound is one of the seven salts in the following
table.
Al(NO3)3. 9H2O
BaCl2. 2H2O
CaCO3
NaCl
BaSO4
Ni(NO3)2. 6H2O
CuSO4. 5H2O
Use the results of the following observations or laboratory tests to explain how each compound in the table may
be eliminated or confirmed. The tests are done in sequence from (a) through (e).
(a) The unknown compound is white. In the table below, cross out the two compounds that can be eliminated
using this observation. Be sure to cross out these same two compounds in the tables in parts (b), (c),
and (d).
Al(NO3)3. 9H2O
BaCl2. 2H2O
CaCO3
NaCl
BaSO4
Ni(NO3)2. 6H2O
CuSO4. 5H2O
One point is earned for each correctly crossed-out compound.
(b) When the unknown compound is added to water, it dissolves readily. In the table below, cross out the two
compounds that can be eliminated using this test. Be sure to cross out these same two compounds in the
tables in parts (c) and (d).
Al(NO3)3. 9H2O
BaCl2. 2H2O
CaCO3
NaCl
BaSO4
Ni(NO3)2. 6H2O
CuSO4. 5H2O
One point is earned for each additional correctly crossed-out compound.
(c) When AgNO3(aq) is added to an aqueous solution of the unknown compound, a white precipitate forms.
In the table below, cross out each compound that can be eliminated using this test. Be sure to cross out the
same compound(s) in the table in part (d).
Al(NO3)3. 9H2O
BaCl2. 2H2O
CaCO3
NaCl
BaSO4
Ni(NO3)2. 6H2O
CuSO4. 5H2O
One point is earned for crossing out Al(NO3)3. 9H2O or for crossing out Ni(NO3)2. 6H2O if it had not
been crossed out earlier.
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AP® CHEMISTRY
2008 SCORING GUIDELINES (Form B)
Question 5 (continued)
(d) When the unknown compound is carefully heated, it loses mass. In the table below, cross out each
compound that can be eliminated using this test.
Al(NO3)3. 9H2O
BaCl2. 2H2O
CaCO3
NaCl
BaSO4
Ni(NO3)2. 6H2O
CuSO4. 5H2O
One point is earned for crossing out NaCl or for crossing out either CaCO3 or BaSO4 if they had not been
crossed out earlier.
(e) Describe a test that can be used to confirm the identity of the unknown compound identified in part (d).
Limit your confirmation test to a reaction between an aqueous solution of the unknown compound and an
aqueous solution of one of the other soluble salts listed in the tables above. Describe the expected results
of the test; include the formula(s) of any product(s).
Mix an aqueous solution of BaCl2. 2H2O
with an aqueous solution of CuSO . 5H O.
4
The BaSO4 will precipitate.
2
One point is earned for describing a precipitation
reaction between the compound left in part (d)
and another compound given in the problem.
One point is earned for a correct identification of
a precipitate that would form upon the mixing of
the chosen solutions.
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