37 free MDCAT Prep Chemistry questions, each with the correct answer and a worked explanation. No sign-up and no payment — every answer on this page is visible to anyone who opens it.
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What is the molar mass of H2O?
Answer: 18 g mol^-1
Adding the relative atomic masses in H2O gives 18 g mol^-1.
1. Fundamentals — Moles and molar mass · foundation
E2 rate depends on
Answer: both substrate and base concentrations
Both substrate and base concentrations is characteristic of E1/E2 dehydrohalogenation.
15. Alkyl Halides — Elimination · standard
Deviation from ideal behavior increases when?
Answer: intermolecular attractions become significant
Intermolecular attractions become significant is the condition or assumption that best explains ideal or non-ideal behavior.
3. Gases — Real and ideal gases · standard
If the equilibrium constant for A <=> B is K, what is the constant for B <=> A?
Answer: 1/K
Reversing a balanced equilibrium expression inverts its equilibrium constant.
6. Chemical Equilibrium — Equilibrium calculations · advanced
For a reaction at a fixed temperature, Qc > Kc. In which direction will the system shift?
Answer: toward reactants
The reverse reaction is favored until Qc falls to Kc.
6. Chemical Equilibrium — Equilibrium calculations · advanced
For rate = k[A]^2[B], how does the rate change on doubling [B] only?
Answer: changes by a factor of 2
Apply the concentration exponents in the rate law; the rate changes by a factor of 2.
7. Reaction Kinetics — Concentration and rate · standard
In the balanced redox equation 2Fe3+ + Sn2+ -> 2Fe2+ + Sn4+, what is the coefficient ratio of the first listed reactant to the last listed product?
Answer: 2:1
Reading the balanced coefficients gives the ratio 2:1.
9. Electrochemistry — Balancing redox equations · advanced
A cation is usually.
Answer: smaller than its neutral atom
Smaller than its neutral atom is the expected trend after considering effective nuclear charge, shell number and electron repulsion.
11. S- and P-block Elements — Periodic trends · foundation
Which electronic configuration is correct for Na in its ground state?
Answer: 1s2 2s2 2p6 3s1
Na has 11 electron(s) in the stated species; applying Aufbau, Pauli and Hund rules gives 1s2 2s2 2p6 3s1.
2. Atomic Structure — Electronic configuration · standard
How many orbitals are present in a 3d subshell?
Answer: 5
The 3d subshell contains 5 orbital(s), each of which can hold two electrons.
2. Atomic Structure — Orbitals and subshells · standard
Which expression is the equilibrium constant Kc for CH3COOH(aq) <=> H+(aq) + CH3COO-(aq)?
Answer: [H+][CH3COO-] / [CH3COOH]
Kc is the product of equilibrium concentrations of products raised to stoichiometric powers divided by the corresponding reactant term; pure solids and liquids are omitted.
6. Chemical Equilibrium — Equilibrium constants · advanced
For rate = k[A]^2, how does the rate change on doubling [A]?
Answer: changes by a factor of 4
Apply the concentration exponents in the rate law; the rate changes by a factor of 4.
7. Reaction Kinetics — Concentration and rate · advanced
How does the enthalpy of products compare with reactants in an exothermic reaction?
Answer: products have lower enthalpy
Products have lower enthalpy follows from the direction of heat flow and the sign convention for ΔH.
8. Thermochemistry — Exothermic and endothermic · foundation
According to VSEPR theory, what is the molecular shape of BF3?
Answer: trigonal planar
BF3 has the electron-domain description AX3, which gives a trigonal planar molecular shape.
10. Chemical Bonding — VSEPR shapes · standard
What is the commonly assigned hybridization of carbonyl carbon?
Answer: sp2
The electron-domain geometry around carbonyl carbon is consistent with sp2 hybridization.
10. Chemical Bonding — Hybridization · standard
Which ground-state electronic configuration is correct for Cr?
Answer: [Ar] 3d5 4s1
After accounting for the 4s-before-3d filling pattern and removal of 4s electrons first in cations, Cr has [Ar] 3d5 4s1.
12. Transition Elements — d-block configurations · advanced
Which functional-group class is represented by R-CO-R?
Answer: ketone
The characteristic group R-CO-R defines the ketone class.
13. Organic Principles — Functional groups · foundation
Which functional-group class is represented by R-NH2?
Answer: amine
The characteristic group R-NH2 defines the amine class.
13. Organic Principles — Functional groups · foundation
Under kinetic molecular theory, classify this statement: All gas molecules at a given temperature have exactly the same speed.
Answer: Incorrect
Kinetic molecular theory describes random motion, elastic collisions, negligible particle volume and temperature-dependent average kinetic energy; molecules have a distribution of speeds.
3. Gases — Kinetic molecular theory · standard
Which compound in the pair MgCl2 and NaCl is expected to have the greater lattice energy magnitude?
Answer: MgCl2
MgCl2 is expected to have greater lattice energy because Mg2+ has higher charge and smaller radius than Na+.
5. Solids — Lattice energy · advanced
Which unit is appropriate for the rate constant of an overall 0-order reaction when concentration is measured in mol dm^-3 and time in seconds?
Answer: mol dm^-3 s^-1
Units of k follow from rate = k(concentration)^0; for order 0, the stated unit is mol dm^-3 s^-1.
7. Reaction Kinetics — Rate constant · advanced
The salt bridge primarily
Answer: maintains electrical neutrality and completes the circuit
Maintains electrical neutrality and completes the circuit is the standard electrochemical convention.
9. Electrochemistry — Electrodes and SHE · standard
How many sigma and pi bonds are present in ethane, C2H6?
Answer: 7 sigma and 0 pi
Each single bond is one sigma bond; a double bond adds one pi and a triple bond adds two pi bonds. The count is 7 sigma and 0 pi.
10. Chemical Bonding — Sigma and pi bonds · advanced
What is the correct IUPAC name of CH2=CHCH2CH3?
Answer: but-1-ene
Choose the longest chain containing the multiple bond where relevant and assign the lowest locants; the name is but-1-ene.
14. Hydrocarbons — IUPAC nomenclature · foundation
The six carbon atoms in benzene are
Answer: sp2 hybridized and planar
Sp2 hybridized and planar follows from the molecular-orbital and resonance description of benzene.
14. Hydrocarbons — Benzene and resonance · standard
Steric hindrance slows SN2 by
Answer: blocking backside attack
Blocking backside attack explains the observed reactivity of alkyl halides.
15. Alkyl Halides — Structure and reactivity · standard
At the normal boiling point, vapor pressure equals.
Answer: 1 atm
1 atm follows from the kinetic molecular explanation of evaporation and boiling.
4. Liquids — Vapor pressure and boiling · standard
Complete the statement: ice is less dense than liquid water because
Answer: hydrogen bonds form an open lattice
The correct description is hydrogen bonds form an open lattice.
4. Liquids — Hydrogen bonding · standard
At equilibrium in a closed system
Answer: macroscopic concentrations remain constant
Macroscopic concentrations remain constant is a defining feature of chemical equilibrium.
6. Chemical Equilibrium — Dynamic equilibrium · standard
Why can reducing particle size of a solid affect reaction rate?
Answer: increases surface area-to-volume ratio
Increases surface area-to-volume ratio according to collision theory.
7. Reaction Kinetics — Factors affecting rate · foundation
What is the oxidation number of S in H2SO4?
Answer: +6
Assign standard oxidation numbers and require the total to equal the species charge; solving gives the stated value.
9. Electrochemistry — Oxidation numbers · standard
In 2Mg + O2 -> 2MgO, what role is played by O2?
Answer: oxidizing agent
O2 changes oxidation state in the direction that makes it the oxidizing agent.
9. Electrochemistry — Oxidizing and reducing agents · standard
Down a group, metallic character generally.
Answer: increases
Increases is the expected trend after considering effective nuclear charge, shell number and electron repulsion.
11. S- and P-block Elements — Periodic trends · foundation
SnO and PbO show.
Answer: basic or amphoteric behavior
Basic or amphoteric behavior reflects the chemistry and trends of Group 14 elements.
11. S- and P-block Elements — Group 14 reactions · standard
How many constitutional structures are commonly counted for C4H10?
Answer: 1
Systematic enumeration of distinct carbon skeletons, positions and functional groups gives 2 structure(s).
13. Organic Principles — Structural isomerism · advanced
What is the principal result of Ethene + HCl?
Answer: chloroethane
The alkene pi bond undergoes the stated addition or polymerization to give chloroethane.
14. Hydrocarbons — Alkene reactions · standard
Which statement is correct for sulphonation of benzene?
Answer: fuming H2SO4 and benzenesulphonic acid
Fuming h2so4 and benzenesulphonic acid is the expected reagent, product, intermediate behavior or directing effect.
14. Hydrocarbons — Electrophilic aromatic substitution · advanced
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