MCAT: General Chemistry

What does the principal quantum number (n) represent in atomic structure?

The principal quantum number represents the main energy level or shell in which an electron resides.

Why does atomic radius generally decrease from left to right across a period?

The atomic radius decreases because the increasing effective nuclear charge (Zeff) pulls the electron cloud closer to the nucleus.

How is the formal charge of an atom in a Lewis structure calculated?

Formal charge is calculated as: Valence Electrons - (Non-bonding electrons + 0.5 * Bonding electrons).

What is the systematic method for identifying the limiting reactant in a chemical reaction?

Divide the number of moles of each reactant by its stoichiometric coefficient; the lowest value identifies the limiting reactant.

State the Ideal Gas Law equation and define its variables.

PV = nRT, where P is pressure, V is volume, n is moles, R is the gas constant, and T is temperature.

How does the common ion effect influence the solubility of a sparingly soluble salt?

The presence of a common ion shifts the equilibrium toward the solid precipitate, thereby decreasing the salt's solubility.

By what mechanism does a catalyst increase the rate of a chemical reaction?

A catalyst provides an alternative reaction pathway with a lower activation energy (Ea) without being consumed.

In an exothermic reaction, how does an increase in temperature affect the equilibrium constant (Keq)?

Increasing the temperature of an exothermic reaction causes the Keq to decrease, favoring the reactants.

What is the pH of a buffer solution when the concentration of the weak acid equals the concentration of its conjugate base?

The pH of the buffer solution is equal to the pKa of the weak acid.

What does a negative value for Gibbs Free Energy (ΔG < 0) indicate about a reaction?

A negative ΔG indicates that the reaction is spontaneous in the forward direction.