MCAT Chemistry 5 β Questions and Answers
Question 1: Which of the following describes the relationship between vapor pressure and temperature?
- Vapor pressure decreases as temperature increases
- Vapor pressure increases as temperature increases (Correct answer)
- Vapor pressure is independent of temperature
- Vapor pressure decreases linearly as temperature increases
Correct answer: Vapor pressure increases as temperature increases
As temperature increases, more molecules have sufficient kinetic energy to escape the liquid phase, increasing vapor pressure according to the Clausius-Clapeyron equation.
Question 2: In a mass spectrometer, the molecular ion peak (MβΊ) represents:
- The most abundant fragment in the spectrum
- The molecule after losing one electron, giving its molecular weight (Correct answer)
- The base peak with intensity set to 100%
- A fragment that has lost a methyl group
Correct answer: The molecule after losing one electron, giving its molecular weight
The molecular ion peak (MβΊ) appears when the molecule loses one electron without fragmenting, giving a peak at the molecular weight of the compound.
Question 3: Which of the following correctly describes Hess's Law?
- The enthalpy change of a reaction depends on the pathway taken
- The total enthalpy change for a reaction equals the sum of enthalpy changes for individual steps (Correct answer)
- Enthalpy changes are always negative for spontaneous reactions
- The entropy change determines whether a reaction is exothermic
Correct answer: The total enthalpy change for a reaction equals the sum of enthalpy changes for individual steps
Hess's Law states that since enthalpy is a state function, the total ΞH for a reaction is the sum of ΞH values for each step, regardless of the path taken.
Question 4: Which of the following best describes a sigma (Ο) bond?
- A bond formed by the side-to-side overlap of p orbitals
- A bond formed by the head-on overlap of orbitals along the internuclear axis (Correct answer)
- A bond that allows free rotation to be restricted
- A bond found only in aromatic compounds
Correct answer: A bond formed by the head-on overlap of orbitals along the internuclear axis
A sigma bond forms from head-on (end-to-end) overlap of orbitals along the internuclear axis and allows free rotation; all single bonds are sigma bonds.
Question 5: What is the effect of a catalyst on an equilibrium reaction?
- It shifts the equilibrium toward the products
- It shifts the equilibrium toward the reactants
- It lowers the activation energy of both forward and reverse reactions equally (Correct answer)
- It increases the equilibrium constant K
Correct answer: It lowers the activation energy of both forward and reverse reactions equally
A catalyst lowers activation energy for both the forward and reverse reactions equally, increasing the rate at which equilibrium is reached but not changing the equilibrium position or K.
Question 6: Which of the following pairs are diastereomers?
- Two molecules that are mirror images and non-superimposable
- Two molecules with the same molecular formula but different connectivity
- Stereoisomers that are not mirror images of each other (Correct answer)
- Molecules that differ only by rotation around a single bond
Correct answer: Stereoisomers that are not mirror images of each other
Diastereomers are stereoisomers with the same connectivity but different spatial arrangements that are NOT mirror images of each other, such as cis-trans isomers or epimers.
Question 7: Which technique would best separate and identify components of an amino acid mixture based on their charge at a given pH?
- Gas chromatography
- Mass spectrometry
- Gel electrophoresis (Correct answer)
- X-ray crystallography
Correct answer: Gel electrophoresis
Gel electrophoresis separates molecules by charge and size in an electric field; amino acids migrate differently based on their net charge at the given pH (related to their isoelectric point).
MCAT Chemistry 5
Which of the following describes the relationship between vapor pressure and temperature?