Compounds and Stoichiometry (4)

MCAT General Chemistry Review 2025-2026

Compounds

are substances composed of two or more elements in a fixed proportion

Molecular Weight

is the mass (in amu) of the constituent atoms in a compounds as indicated by the molecular formula

Molas Mass

is the mass of one mole (Avogadro’s number) of a compound; usually measured in grams per mole

6.022 × 10 ^ 23

Avogadro’s number

Gram Equivalent Weight

is a measure of the mass of a substance that can donate one equivalent of the species of interest

Normality

is the ratio of equivalents per liter; it is related to molarity by multiplying the molarity by the number of equivalents present per mole of compound

Equivalents

are moles of the species of interest; these are most often seen in acid-base chemistry (hydrogen ions or hydroxide ions) and oxidation-reduction reactions (moles of electrons or other ions)

Law of Constant Composition

states that any pure sample of a compound will contain the same elements in the same mass ratio

Empirical Formula

is the smallest whole-number ratio of the elements in a compound

Molecular Formula

is either the same as or a multiple of the empirical formula; it gives the exact number of atoms of each element in a compound

Percent Composition

to calculate this by mass, determine the mass of the individual element and divide by the molar mass of the compound

Combination Reactions

occur when two or more reactants combine to form one product

Decomposition Reactions

occur when one reactant is chemically broken down into two or more products

Combustion Reactions

occur when a fuel and an oxidant (typically oxygen) react, forming the products water and carbon dioxide (if the fuel is a hydrocarbon)

Displacement Reactions

occur when one or more atoms or ions of one compound are replaced with one or more atoms or ions of another compound

Single-displacement Reactions

occur when an ion of one compound is replaced with another element

Double-displacement Reactions

occur when elements from two different compounds trade places with each other to form two new compounds

Neutralization Reactions

are those in which an acid reacts with a base to form a salt (and, usually, water)

Least; More; Charge

balancing equations are determined using the following steps in order:

1. balancing the ______ common atoms

2. balancing the ______ common atoms (usually hydrogen and oxygen)

3. balancing _______, if necessary

Limiting Reagent

balanced equations can be used to determine this, which is the reactant that will be consumed first in a chemical reaction

Excess Reagents

the other reactants present are termed this

Theoretical Yield

is the amount of product generated if all of the limiting reactant is consumed with no side reactions

Actual Yield

is typically lower than theoretical yield

Percent Yield

is calculated by dividing actual yield by theoretical yield and converting to a percentage

Roman Numerals

are used for nonrepresentative elements to denote ionic charge

-ous

can be used to indicate lesser charge

-ic

indicate greater charge

-ide

ending for all monatomic anions

-ite

oxyanions that contain a lesser amount of oxygen

-ate

oxyanions with a greater amount of oxygen

Hypo-

oxyanion species with the fewest oxygens

Per-

oxyanion species with the most oxygens

Hydrogen or Bi-

polyatomic ions containing one hydrogen

Dihydrogen

polyatomic ions containing two hydrogens

Ionic Charges

are predictable by group number and type of element (metal or nonmetal) for representative elements, but are generally unpredictable for non-representative elements

Metals

form positively charged cations based on group number

Nonmetals

form negatively charged anions based on the number of electrons needed to achieve an octet

Electrolytes

contain equivalents of ions from molecules that dissociate in solution. The strength of these depend on its degree of dissociation or solvation