AP Chemistry Daily Live Review Experiment-Based Free Response Student Handout
M (s) + I2 (s) → MI2 (s)
To determine the molar mass of an unknown metal, M, a student reacts iodine with an excess of the metal to form
the water-soluble compound MI2 as represented by the equation above. The reaction proceeds until all of the I2 is
consumed. The MI2 (aq) solution is quantitatively collected and heated to remove the water, and the product is dried
and weighed to a constant mass. The experimental steps are represented below, followed by a data table.
Data for Unknown Metal Lab
Mass of beaker
110.630 g
Mass of beaker + Metal M
112.268 g
Mass of beaker + Metal M + I2
113.800 g
Mass of MI2
0060 mots
53kg
1.926 g
(a) Given that the metal M is in excess, calculate the number of moles of I2 that reacted.
(b) Calculate the molar mass of the unknown metal M.
1 638
g
l
0060moi laELM
molt
65.3 91m01
(c) Determine the mass of M that remains after the reaction is complete.
1.240
The student hypothesizes that the compound formed in the synthesis reaction is ionic.
(d) Propose an experimental test the student could perform that could be used to support the hypothesis. Explain
how the results of the test would support the hypothesis if the substance was ionic.
solve the compound inwaterthenrun anelectriccurrentthroughieuse a poeeneiomeeer ir ie cowards electricity it is ionic as
he pisassocineers ionswould allow forelectrical flow
(e) One potential source of error in this experiment is insufficient drying of the product. If the product were
insufficiently dried, would the calculated molar mass of metal M be greater than, equal to, or less than the
calculated molar mass of metal M from a completely dry product?
Greater
than
(f) A second potential source of error in this experiment is loss of product when transferring it from the test tube
where it is produced to the balance where its mass is measured. If some product is lost during transfer prior
to weighing, is the calculated molar mass of metal M greater than, equal to, or less than the calculated mass
would be if no product was lost prior to weighing?
Based on AP Chemistry FRQ 2015 #3
Less
than
A student is assigned the task of determining the mass percent of silver in an alloy of copper and silver by dissolving
a sample of the alloy in excess nitric acid and then precipitating the silver as AgCl.
a. The student is provided with a stock solution of 16 M HNO3, two 100 mL graduated cylinders that can be read
to ± 1 mL, a 100 ml beaker that can be read to ± 10 mL, safety goggles, rubber gloves, a glass stirring rod, a
dropper, and distilled water.
i.
Calculate the volume, in mL, of 16 M HNO3 that the student should use for preparing 50. mL of 6 M
HNO3.
ii.
Briefly list the steps of an appropriate and safe procedure for preparing the 50. mL of 6 M HNO3. Only
materials selected from those provided to the student (listed above) may be used.
iii.
Explain why it is not necessary to use a volumetric flask (calibrated to 50.00 mL ± 0.05 mL) to
perform the dilution.
iv.
During the preparation of the solution, the student accidentally spills about 1 ml of 16 molar HNO3 on
the benchtop. The student finds three bottles containing liquid sitting near the spill: a bottle of distilled
water, a bottle of concentrated Na2CO3 (aq) / NaHCO3 (aq) buffer solution, and a bottle of saturated
NaCl (aq). Which of the liquids is best to use in cleaning up the spill? Justify your choice.
19
mL
The student pours 25 mL of 6 M HNO3 into a beaker and adds a 0.7787 g sample of the alloy. After the sample
completely reacts with the acid, some saturated NaCl (aq) is added to the beaker, resulting in the formation of an
AgCl precipitate. Additional NaCl (aq) is added until no more precipitate is observed to form. The precipitate is
filtered, washed, dried, and weighed to constant mass in a filter crucible. The data are shown in the table below.
Mass of sample of copper-silver alloy
0.7787 g
Mass of filter crucible
28.7210 g
Mass of filter crucible and precipitate
(first weighing)
29.4016 g
Mass of filter crucible and precipitate
(second weighing)
29.3138 g
Mass of filter crucible and precipitate
(third weighing)
29.3137 g
b. Calculate the number of moles of AgCl precipitate collected.
001214
c. Calculate the mass percent of silver in the alloy of copper and silver.
517.29 Y Aa
d. One potential source of error in this experiment is moisture on the crucible at the beginning of the experiment.
If the crucible was not completely dry before being weighed empty, would the calculated percent silver be too
high, too low, or unaffected? Justify your choice.
toohighthemob silvercalculaeebwouldbetoohighthereforethe mass of silver calculated in the compound wouldbe
wouldbe too high
high
thus hewhy the
silver
e. Explain
student needed
to perform a third weighing step in this experiment.
In
order to ensure the precipitate wasfully dried and HAD reached a constant was
f. The student realizes after
performing the experiment that the written procedure called for adding only 20 mL
of 6 M HNO3, not the 25 mL they added. As a result of the student’s error is the calculated percent silver too
high, too low or unaffected? Justify your choice.
Unaffected
as
HMOs is
in excess the
Additional
moles present wouldnotread
it
Poe on the
Use
measure
he blather
To
cylinder
The Beaker
Iii The
the other
AND
it
inl
mL MNO ADD it to
accuracy to the
provides
of
the
figs
necessary
amount
of it is
sig
the Buffer A low
the
pour
stir
process
AND
AND
to
GRADUATED
measure 19
number
necessary
with
b
water
goggles
cylinder
graduated
Beaker
For the
if
A
31 mL
Use
connece
rubber gloves
acid
will safely read
conjugate
BASE
Based on AP Chemistry FRQ 2011 #2
A student conducts a titration experiment to determine the concentration of an unknown monoprotic acid, HA. The
student titrates a 20.0 mL sample of the acid with a solution of sodium hydroxide, NaOH solution. The titration was
monitored with a pH meter and the data plotted in a graph as shown below.
a. Place an X on the graph at the equivalence point.
b. Is the concentration of NaOH titrant the student used greater than, less than or equal to the concentration of
the unknown acid solution, HA, that was titrated? Justify your choice.
Less than
took 40mL
with Just 20 ML ofauthyl
oftiyrn.ve to
react
c. The student rinseditthe buret with distilled water immediately before
filling it with the NaOH titrant solution. Dd
this error increase, decrease or have no effect on the equivalence point volume in this experiment? Justify
your choice.
have Diluted
student
equivalence point volume the
the
It d. increase
Which of thethe
two particulate representations shown below, Figure 1 or Figure 2, best may
represents the
unknown acid HA? Use evidence from the student’s experiment to support your answer.
of the
to
titration
no
prior
be present HD't's presence
Figure
would
1
acid's
reaction
w
NaOH titra
solution
corrugate
is
water
simply
base
because
In a second experiment, the student performs a titration of a 10.0 mL sample of a sodium hydroxide solution, NaOH
(aq) using 0.107 M HCl as the titrant.
e. The images below show the buret before the titration begins (below left) and at the equivalence point (below
right). What should the student record as the volume of HCl (aq) delivered to the flask?
31.6mL
sit
34.3
f.
Based on the given information and your answer to part (e), what is the concentration of the NaOH (aq)
analyte that should include in their lab report?
34
M
g. In a second trial of the experiment, the student accidentally added more HCl (aq) to the flask than was
needed to reach the equivalence point, and then recorded the final volume. Would this error increase,
decrease, or have no effect on the calculated NaOH concentration for the second trial? Justify your answer.
It
would increase the
calculated
Based on AP Chemistry FRQ 2016 #7 and 2018 #5
calculate
all the
Moles
NaOH
concentration
a
serpentwoo
of 144 were necessary to react with
moles or NaOH Therefore
would calculate More
that more moles
per
liter
of NaOH
They