2024
AP Chemistry
®
Sample Student Responses
and Scoring Commentary
Inside:
Free-Response Question 1
☑ Scoring Guidelines
☑ Student Samples
☑ Scoring Commentary
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AP® Chemistry 2024 Scoring Guidelines
Question 1: Long Answer 10 points
(a) For the correct circled atom: 1 point
The rightmost hydrogen atom should be circled.
(b) For the correct calculated value: 1 point
1 mol
(10.22 g)
40.00 g
= 0.511 M
0.500 L
(c) For the correct pK a : 1 point
3.9 (acceptable range: 3.7 – 4.0)
(d) (i) For the X at the correct point: 1 point
The X should be at a point greater than or equal to 3 mL and less than 8 mL .
(ii) For a correct justification: 1 point
More acid particles are present than conjugate base particles, meaning that the titration
is before the half-equivalence point.
(iii) For a curve showing the correct equivalence point: 1 point
The equivalence point should be at 8 mL . See example response below.
For a curve with appropriate initial and final pH with a correct shape: 1 point
The drawn curve should begin at the same pH , gradually increase, rise sharply at a
volume different than 16 mL , and end at a pH similar to the first curve.
Total for part (d) 4 points
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AP® Chemistry 2024 Scoring Guidelines
(e) (i) For the correct calculated value: 1 point
q= mc∆T= (200.0 g)(4.2 J/(g ⋅ °C))(23.2°C − 20.0°C)= 2700 J
(ii) For the correct calculated value: 1 point
qrxn = −2700 J =
− qsoln = −2.7 kJ
qrxn −2.7 kJ
∆H rxn = = −54 kJ/molrxn
=
mol (0.100 L)(0.500 mol/L)
(iii) For the correct answer and a valid justification: 1 point
Agree. The heat lost from the system would result in a lower final temperature, which
results in values of ∆T , qsoln , and ∆H that are smaller than the actual value.
Total for part (e) 3 points
Total for question 1 10 points
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Sample 1A 1 of 4
Sample 1A 2 of 4
Sample 1A 3 of 4
Sample 1A 4 of 4
Sample 1B 1 of 4
Sample 1B 2 of 4
Sample 1B 3 of 4
Sample 1B 4 of 4
Sample 1C 1 of 4
Sample 1C 2 of 4
Sample 1C 3 of 4
Sample 1C 4 of 4
AP® Chemistry 2024 Scoring Commentary
Question 1
Note: Student samples are quoted verbatim and may contain spelling and grammatical errors.
Overview
Question 1 presents students with a series of questions concerning multiple experiments involving
lactic acid and sodium hydroxide.
Part (a) requires students to circle the hydrogen atom in the provided structural formula of lactic acid
that most readily participates in the chemical reaction with sodium hydroxide. The intent was for
students to identify and circle the hydrogen atom on the carboxyl group (Learning Objective SAP-
9.F/8.6.A, Skill 1.A from the AP Chemistry Course and Exam Description).
Part (b) requires students to calculate the molarity of a sodium hydroxide solution given the mass of
NaOH and volume of the solution (SPQ-3.A/3.7.A, 5.F).
Part (c) requires students to approximate the pKa of lactic acid based on the information from a weak
acid-strong base titration curve. The intent of the question was for students to identify the half-
equivalence point from the titration curve and recognize that the pH is equal to the pKa at this point
(SAP-9.E/8.5.A, 5.D).
Part (d)(i) requires students to draw an X on the area of the titration curve represented by a given
particle diagram. The intent of the question was for students to recognize that there were more lactic
acid particles compared to lactate ions (conjugate base) in the particle diagram and recognize that
the diagram represents an area of the curve prior to the half-equivalence point (SAP-9.E/8.5.A, 3.A).
Part (d)(ii) requires students to justify the location of the X drawn on the titration curve. The intent of
the question was for the students to explain the position of their X on the titration curve based on the
particle diagram containing more lactic acid particles than lactate ions (conjugate base), which
indicates that the pH is less than the pKa, a condition that is only true before the half-equivalence
point (SAP-10.A/8.7.A, 6.F).
Part (d)(iii) requires students to draw a weak acid-strong base titration curve for a second experiment
on top of the titration curve from the first experiment. Experiment 2 uses a sodium hydroxide
solution with twice the concentration, which would reach the equivalence point with half the volume
of NaOH(aq) required in experiment 1. The question’s intent was for students to adjust the expected
titration curve based on changing one variable of the experiment (SAP-9.E/8.5.A, 3.A).
Part (e)(i) requires students to calculate the quantity of heat produced when solutions of lactic
acid and sodium hydroxide are combined. This may be quantified by the equation = q mc∆T
(ENE-2.D/6.4.A, 5.F).
Part (e)(ii) requires students to calculate the molar enthalpy of the reaction based on the information
and calculated response in part (e)(i), including the appropriate algebraic sign. The intent of the
question was to relate the heat (qsoln) absorbed by the solution in part (e)(i) to the molar enthalpy of
the reaction in kJ/molrxn (ENE-2.F/6.6.A, 5.F).
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AP® Chemistry 2024 Scoring Commentary
Question 1 (continued)
Part (e)(iii) requires students to agree or disagree with a student’s claim that if heat is lost from
the calorimeter to the surrounding air during the reaction, then the experimental value of the molar
enthalpy would be smaller due to a smaller observed change in solution temperature. The intent
of the question was to have students predict the effect heat loss would have on measurable
observations (ENE-2.D/6.4.A, 6.G).
Sample: 1A
Score: 10
The response earned 10 points. In part (a) the hydrogen atom on the carboxyl group is circled and
therefore earned the point. In part (b) the point was earned because the grams of sodium hydroxide
is correctly converted to moles of sodium hydroxide and divided by the volume in liters to calculate
the correct molarity of the solution. In part (c) the response gives a pKa value of 3.8, which is in the
proper range of 3.7-4.0 and earned the point. In part (d)(i) the X placement is in the correct range
(greater than or equal to 3 mL and less than 8 mL) and therefore earned the point. The point was
earned in part (d)(ii) for indicating the correct ratio of C3H6O3 to C3H5O3− and recognizing that the
reaction is one-third of the way to the equivalence point. In part (d)(iii) 1 point was earned for
indicating the correct equivalence point at 8 mL, and 1 point was earned for beginning the curve at
the correct pH, ending it at the correct pH, and having a shape like a characteristic weak acid-strong
base titration curve. The point was earned in part (e)(i) for the correct amount of heat produced, in
joules. The point was earned in part (e)(ii) for the correct molar enthalpy value calculated with the
negative sign indicated. The point was earned in part (e)(iii) because the response agrees with the
student’s claim that the observed change in temperature will be smaller because not all energy is
transferred to the water, resulting in a smaller molar enthalpy value.
Sample: 1B
Score: 5
The response earned a total of 5 points. In part (a) the point was earned for circling the correct H on
the carboxyl group. In part (b) the point was earned because the correct molarity is calculated with
the appropriate work shown. In part (c) the point was earned for a correct pKa value of 3.80, which is
within the acceptable range of 3.7-4.0. In part (d)(i) the point was earned for the correct placement of
X on the graph in a position greater than or equal to 3 mL and less than 8 mL. In part (d)(ii) the point
was earned because the response correctly identifies the amount of C3H6O3 as greater than C3H5O3−,
indicating that the position in the titration is before the half-equivalence point. In part (d)(iii) the first
point was not earned because the equivalence point is not at 8 mL, and the second point was not
earned because the curve starts at a pH of 4 (not ~2) and has the same equivalence point as
experiment 1. In part (e)(i) the point was not earned because the response uses 6.50 grams of NaOH
instead of 200.0 grams of solution to determine the heat produced in the reaction. In part (e)(ii) the
point was not earned because the response uses 0.1 mole instead of 0.0500 mole to determine the
molar enthalpy and also lacks the required negative sign. The point was not earned in part (e)(iii)
because the response incorrectly indicates that the magnitude of the experimental molar enthalpy of
reaction would be larger, not smaller, than the actual value.
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AP® Chemistry 2024 Scoring Commentary
Question 1 (continued)
Sample: 1C
Score: 2
The response earned a total of 2 points. In part (a) the point was earned for circling the correct H
atom on the carboxyl group. In part (b) the point was not earned because 500 mL is used to calculate
the molarity instead of the correct 0.500 L. In part (c) the point was not earned for the incorrect pKa
approximation of 5.5. In part (d)(i) the point was not earned because X is located at the equivalence
point of the titration curve instead of a position greater than or equal to 3 mL and less than 8 mL. In
part (d)(ii) the point was not earned because the response does not indicate that the ratio of weak
acid to conjugate base in the particulate diagram represents a scenario before the half-equivalence
point. In part (d)(iii) the first point was not earned because the equivalence point drawn is at 12 mL
instead of 8 mL, and the second point was not earned because the curve starts at an incorrect
pH of 5. In part (e)(i) the point was not earned because the response uses the incorrect mass of
100 grams of solution instead of 200.0 grams of solution to calculate the heat produced. In part (e)(ii)
the point was not earned because the value from part (e)(i) is not carried forward, nor does the end
result have a negative value. In part (e)(iii) the point was earned because the response agrees with
the student’s claim and states that the final temperature reading would be lower, making the change
in temperature “lower than it should be,” resulting in a smaller experimental molar enthalpy.
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