MCAT Chemistry Practice Questions
Master MCAT Chemistry by working through targeted practice questions, learning key principles, and applying strategies to maximize your exam score.
(Note: This resource also appears in our MCAT Ultimate Guide.)
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Introduction
MCAT Chemistry Practice Passage #1
MCAT Chemistry Practice Passage #2
MCAT Chemistry Practice Passage #3
MCAT Chemistry Practice Questions (Standalone)
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Introduction
As a premed, you’ve undoubtedly had to wade your way through general chemistry and organic chemistry. Some of you may have even ventured into the murky waters of physical chemistry, inorganic chemistry, or advanced general chemistry. Your hard work in those classes will pay off as you begin studying for the MCAT.
While your study schedule should be optimized to strike an optimal balance between content and practice, you’ll still need to be aware of the important pieces of general chemistry and organic chemistry content.
The MCAT is a hard exam, so in addition to knowing the content, you’ll need to know how to solve practice problems. In fact, the test writers generally take a scientific article, include a few figures, and ask you questions that require you to draw on information from the passage and outside content knowledge.
So, what’s the best way to improve your MCAT score? Practice! Here, we’ll test your chemistry knowledge using MCAT-style passages. Use the following three chemistry passages and five standalone questions to test your readiness for MCAT-style passages. Each explanation for the passage-based questions will have suggestions for what you should review if you miss a question. Good luck!
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MCAT Chemistry Practice Passage #1
Most biochemical reactions depend on the pH value of the aqueous environment and many are strongly favored to occur in an acidic environment. A non-invasive control of pH to tightly regulate such reactions with defined start and end points is a highly desirable feature in certain applications.
Researchers report a novel optical approach to reversibly control a typical biochemical reaction by changing the pH and using acid phosphatase as a model enzyme. The reversible photoacid G-acid functions as a proton donor, changing the pH rapidly and reversibly by using high power UV LeDs as an illumination source in the experimental setup. The researchers found that the reaction can be tightly controlled by switching a light on and off, making the technology applicable to a wide range of other enzymatic reactions, thus enabling miniaturization and parallelization through non-invasive optical means. Figure 1 displays the structure of the G-acid.
Figure 1 Light activated deprotonation of G-acid.
In order to use the photoacid in a wide range of biochemical applications, the researchers ensured that it met certain requirements, such as solubility in water, low toxicity, and excitability with commonly available, inexpensive illumination sources, such as LEDs. LEDs have several advantages compared to more conventional illumination sources: They are more efficient, have a long life-time, a compact size, and high reliability.
The researchers chose an acid phosphatase as a model enzyme to further study the system. The activity optimum of APs typically lies at an acidic pH of 4.5–5.5. APs non-specifically catalyze the hydrolysis of monoesters to produce inorganic phosphate under acidic conditions. In experiment A, researchers used the acid phosphatase from potato (EC 3.1.3.2), which is active between pH 4–7, with an activity optimum at a pH of 5–5.3. At an alkaline pH of 8, the activity of EC 3.1.3.2. is several orders of magnitude lower.
To assay EC 3.1.3.2. activity, researchers used p-nitrophenyl phosphate (pNPP) assay. The pNPP assay is a colorimetric assay in which the substrate pNPP is hydrolyzed by acid phosphatase into the product p-nitrophenol (pNP) + Pi in purified HPLC grade water as shown by Figure 2.
Figure 2 Reaction scheme for pNPP assay.
1. Which of the following best describes the role of water in the deprotonation of the G-acid?
A) Acid
B) Catalyst
C) Base
D) Aprotic solvent
A) 2.0 x 1022
B) 1.5 x 1022
C) 1.0 x 1022
D) 0.5 x 1022
3. Which of the following functional groups is NOT found on the protonated G-acid?
A) Alcohol
B) Benzene
C) Sulfonate
D) Sulfite
4. Researchers are likely to observe which of the following during a pNPP assay for EC 3.1.3.2. when pH is lowered to 5?
A) Cleavage of EC 3.1.3.2.
B) An increase in inorganic phosphate levels
C) A decrease in sulfonate levels
D) Decrease in EC 3.1.3.2. activity
Answers and explanations for MCAT Chemistry Practice Passage #1
1. The correct answer is C. Water is a base as it accepts a proton from the G-acid (choice C is correct; choice A is incorrect). Water is not a catalyst in this reaction as it is not regenerated or used to lower the activation energy of the reaction. Water is a reactant (choice B is incorrect). Water is a protic solvent (choice D is incorrect).
Review acids and bases, catalysts, and protic versus aprotic solvents.
Review E = hv equation and MCAT math.
3. The correct answer is D. An alcohol (-OH) group is present on the protonated G-acid (choice A is incorrect). A benzene ring (aromatic 6-carbon ring) is present within the protonated G-acid (choice B is incorrect). A sulfonate group contains a sulfur atom bound to an R-group (carbon in this case), two double bonds to oxygen, and a single bond to a negatively charged oxygen. There are two sulfonate groups present on the protonated G-acid (choice C is incorrect). A sulfite group contains three oxygens and two negative charges, which is not found on the protonated G-acid (choice D is correct).
Review functional groups.
4. The correct answer is B. When pH is lowered to 5, the EC 3.1.3.2. enzyme is in its optimal pH range according to the passage and activity would thereby increase (choice D is incorrect). Since EC 3.1.3.2. is an acid phosphatase, an increase in activity would lead to increased conversion of pNPP to pNP and inorganic phosphate, or Pi (choice B is correct). The passage does not indicate that the enzyme itself would be cleaved (choice A is incorrect). Sulfonate is attached to the G-photoacid and not directly involved in EC 3.1.3.2. activity (choice C is incorrect).
Review figures from the passage and draw out the trail of logic to arrive at the correct answer. An example trail of logic is: pH -> 5 -> EC 3.1.3.2. -> increased enzyme activity -> increased conversion of pNPP to pNP and Pi -> increased Pi
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MCAT Chemistry Practice Passage #2
Sesquiterpene lactones (STL) are a subclass of isoprenoids with many known bioactivities frequently found in the Asteraceae family. In recent years, researchers have determined the biosynthetic pathway of the core backbones of many STLs. In a new series of studies, researchers are aiming to discover decorating enzymes that can modify the core skeleton with functional hydroxy groups.
Investigators in a novel study used in vivo pathway reconstruction assays in heterologous organisms such as Saccharomyces cerevisiae and Nicotiana benthamiana to analyze several cytochrome P450 enzyme genes of the CYP71AX subfamily from Helianthus annuus clustered in close proximity to one another on the sunflower genome. The researchers found that one member of this subfamily, CYP71AX36, catalyzes the conversion of costunolide to 14-hydroxycostunolide. The chemical structures of costunolide and common derivates are shown in Figure 1.
Figure 3 Costunolide and common derivates.
The catalytic activity of CYP71AX36 may be of use for the chemoenzymatic production of antileukemic 14-hydroxycostunolide derivatives and other STLs of pharmaceutical interest. The chemical reaction of interest is shown in Figure 2.
Figure 4 Costunolide converted to 14-hydroxycostunolide via CYP71AX36.
Researchers also probed the reactivity of the terminal alkene group present on 14-hydroxycostunolide. The researchers formed -SH adducts using a glutathione or a cysteine.
1. A researcher uses IR spectroscopy to measure the progress of the reaction shown in Figure 2. Which of the following peaks will appear in the product spectrum without being present in the reactant spectrum?
A) 3200-3500 cm-1, broad
B) 3000 cm-1, narrow
C) 1600-1700 cm-1, broad
D) 1700 cm-1, narrow
2. Which of the following costunolide derivatives is a diol?
A) Taraxic acid
B) Eupatolide
C) 8B, 14-dihydroxycostunolide
D) 9-hydroxyparthenolide
3. Which of the following functional groups is NOT found in costunolide?
A) Ester
B) Alkyne
C) Carbonyl
D) Alkene
4. Researchers discover an active site E78L CYP71AX36 mutation that prevents the enzyme from converting costunolide to 14-hydroxycostunolide. Which of the following best describes the change in amino acid properties at position 78 in the enzyme mutant?
A) Positively charged to negatively charged
B) Hydrophobic to hydrophobic
C) Aromatic to negatively charged
D) Negatively charged to hydrophobic
Answer and explanations for MCAT Chemistry Practice Passage #2
Review IR spectroscopy and functional groups.
2. The correct answer is C. A diol is a molecule that contains two hydroxyl (-OH) groups. Figure 1 provides the composition of the four different R groups for different costunolide derivatives. 8B, 14-dihydroxycostunolide contains an OH at R2 and R4 according to Figure (choice C is correct). The other costunolide derivatives do not contain two OH groups (choices A, B, and D are incorrect).
Review functional groups and practicing interpreting Figure 1.
3. The correct answer is B. An alkyne contains a carbon-carbon triple bond, and this is not found in costunolide (choice B is correct). Costunolide does contain an ester, carbonyl, and alkene functional group (choices A, C, and D are incorrect).
Review functional groups.
4. The correct answer is D. The E78L mutation changes a negatively charged glutamate to a hydrophobic leucine (choice D is correct; choices A, B, and C are incorrect).
Review amino acids and properties.
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MCAT Chemistry Practice Passage #3
Carbon capture is essential for mitigating carbon dioxide emissions. Compared to conventional chemical scrubbing, electrochemically mediated carbon capture utilizing redox-active sorbents, such as quinones, has been increasingly studied by researchers. However, the practicality of such systems is hindered by the requirement of toxic, flammable organic electrolytes or costly ionic liquids.
In a novel study, researchers tested whether rationally designed aqueous electrolytes with high salt concentrations could effectively resolve the incompatibility between aqueous environments and quinone electrochemistry for carbon capture. Reactive quinones can capture carbon as shown in Figure 1, but they may also undergo undesirable side reactions such as protonation or ion association.
Figure 5 Quinone-mediated carbon capture.
1. Which of the following best explains the decrease in carbon capture after 6 M NaCl?
A) The solution became too acidic.
B) The quinones associated with the ions first due to the high concentration of Na+.
C) The high concentration of Na+ sequestered the carbon instead of the quinones.
D) The quinones were protonated quickly due to the high salt concentration.
2. The addition of a carboxyl group to a molecule is known as:
A) Carbonylation
B) Carboxylation
C) Carbolyation
D) Carboxylic acidation
3. The researchers optimize the correct salt solution for their experiments, but they still measure a high amount of quinone protonation. Which of the following steps would most likely reduce quinone protonation?
A) Increase pH
B) Decrease pH
C) Introduce a strong nucleophile
D) Introduce a strong electrophile
4. Which of the following best describes the first forward step of the reaction shown in Figure 1?
A) Oxidation
B) Nucleation
C) Combustion
D) Reduction
Answers and explanations for MCAT Chemistry Practice Passage #3
Review Figure 1 and salts in detail.
2. The correct answer is B. The addition of a carboxyl group to a molecule is known as carboxylation (choice B is correct; choices A, C, and D are incorrect).
Review carboxylation.
3. The correct answer is A. As pH decreases, the amount of protonation increases as more protons are available in solution (choice B is incorrect). As pH increases, the amount of protonation would decrease as fewer protons would be available to protonate the quinone molecules (choice A is correct). A strong nucleophile or electrophile would not affect this reaction (choices C and D are incorrect).
Review pH and its relationship with protonation. Review nucleophiles and electrophiles.
4. The correct answer is D. In the first forward step of the reaction shown in Figure 1, two electrons are added to a molecule. The addition of electrons and therefore negative charge is known as reduction (choice D is correct; choices A, B, and C are incorrect).
Review redox chemistry.
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MCAT Chemistry Practice Questions (Standalone)
1. Which of the following elements is MOST likely to be a strong nucleophile?
A) Hydroxide ion
B) Water
C) Ethanol
D) Tert-butanol
A) HCl(aq)
B) NaOH(aq)
C) H2O(aq)
D) Na2SO4(aq)
3. A researcher performs an organic reaction in which a benzene with an alcohol substituent turns into a benzene with an aldehyde substituent. Which of the following techniques can be used to determine the progression of the reaction?
A) Thin layer chromatography
B) Dissolution in an organic solvent
C) Protonation using an activated water
D) Treatment with an oxidizing agent
4. Which of the following is NOT an assumption used for ideal gases?
A) The molecules have no attraction or repulsion from one another
B) The molecules undergo inelastic collisions
C) Gases are made up of discrete, small particles called molecules
D) The volume of the particles is small compared to the volume of the container
A) 42He
B) 01e
C) 0-1e
D) 24He
Answers and explanations for MCAT Chemistry Practice Questions (Standalone)
1. The correct answer is A. Charge, electronegativity, H-bonding capacity, and steric bulk are important in determining how strong a nucleophile is. Here, hydroxide ions are negatively charged (OH-), strongly electronegative, and have no steric hindrance (choice A is correct). Water and ethanol are not very strong nucleophiles (choices B and C are incorrect). Tert-butanol has a lot of steric hindrance (choice D is incorrect).
2. The correct answer is B. Hydroxide ions (OH-) are very efficient at hydrolyzing (also known as saponifying) triglycerides due to the high nucleophilic capacity of these ions (choice B is correct). It is possible for water to do this, but it would occur much more slowly since water is not as good of a nucleophile (choice C is incorrect).
3. The correct answer is A. Thin layer chromatography (TLC) separates molecules on the basis of polarity. Aldehyde is a nonpolar functional group, whereas alcohol is a polar functional group. Species with an alcoholic functional group would be attracted to the polar stationary phase, and thus produce a lower Rf than species with an aldehyde. This can be used to determine whether or not the product has formed (choice A is correct). B, C, and D are procedural steps you can complete in the lab, but they are not commonly used techniques for determining the progression of a reaction.
4. The correct answer is B. The molecules undergo elastic collisions, not inelastic collisions. Answer choices A, C, and D are assumptions that are made for ideal gases.
5. The correct answer is A. An alpha particle is shown by answer choice A. The numerical orientation is incorrect in answer choice D. Answer choices B and C describe beta particles.
