MCAT General Chemistry - Compounds and Stoichiometry

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compounds

pure substances composed of two or more elements in a fixed proportion; can be broken down by chemical means to produce their constituent elements or other compounds; characterized by describing their physical and chemical properties

molecule

combination of two or more atoms (same or different) held together by covalent bonds; smallest units of compounds that display their identifying properties

formula unit

represents the empirical formula of an ionic compound

formula weight

represents the molecular weight of an ionic compound; found by adding up the atomic weights of the constituent ions according to its empirical formula; units are amu per molecule.

atomic weight

a weighted average of the masses of the naturally occurring isotopes of an element

molecular weight

the sum of the atomic weights of all the atoms in a molecule; atomic mass units (amu) per molecule

mole

a quantity of any substance equal to the number of particles that are found in 12 grams of carbon-12; defined as Avogadro’s number

Avogadro’s number (N_A)

6.022 × 10²³ mol^–1.

molar mass

mass of one mole of a compound; expressed in g/mol

equivalents

how many moles of product one mole of a given starting material will produce

gram equivalent weight

This amount of a compound, measured in grams, that produces one equivalent of the product of interest

where n is the number of particles of interest produced or consumed per molecule of the compound in the reaction

Normality (N)

measure of concentration, given in the units equivalents/L; not equivalent to molarity

where n is the number of particles of interest produced or consumed per molecule of the compound in the reaction

structural formulas

skeletal representations of compounds that show the various bonds between the constituent atoms of a compound

law of constant composition

any pure sample of a given compound will contain the same elements in an identical mass ratio

ex. every sample of water will contain two hydrogen atoms for every one oxygen atom, or—in terms of mass—for every one gram of hydrogen, there will be eight grams of oxygen

empirical formula

simplest whole-number ratio of the elements in the compound

molecular formula

exact number of atoms of each element in the compound and is a multiple of the empirical formula

ex. An empirical formula of CH2O is indicative of a monosaccharide (glucose, fructose, galactose)

percent composition

the percent of a specific compound that is made up of a given element by mass

combination reaction

two or more reactants forming one product; more reactants than products:

ex. formation of water by burning hydrogen gas in air

decomposition

a single reactant breaks down into two or more products, usually as a result of heating, high-frequency radiation, or electrolysis; more products than reactants

ex. breakdown of mercury(II) oxide

combustion reaction

involves a fuel (usually a hydrocarbon, but can be sulfur or sugars) and an oxidant (normally oxygen) to form the two products of carbon dioxide and water

ex. methane

single-displacement reaction

an atom or ion in a compound is replaced by an atom or ion of another element; often redox

ex. solid copper metal will displace silver ions in a clear solution of silver nitrate to form a blue copper nitrate solution and solid silver metal

double-displacement/metathesis reactions

elements from two different compounds swap places with each other to form two new compounds

ex. solutions of calcium chloride and silver nitrate are combined, insoluble silver chloride forms in a solution of calcium nitrate

Neutralization reactions

specific type of double-displacement reaction in which an acid reacts with a base to produce a salt (and, usually, water)

ex. hydrochloric acid and sodium hydroxide will react to form sodium chloride and water

law of conservation of mass

The mass of the reactants consumed must equal the mass of products generated; the number of atoms of each element on the reactant side equals the number of atoms of that element on the product side

law of conservation of charge

the total electric charge in an isolated system never changes; the net quantity of electric charge, the amount of positive charge minus the amount of negative charge in the universe, is always conserved

Stoichiometric coefficients

the numbers placed in front of each compound, are used to indicate the relative number of moles of a given species involved in the reaction

limiting reagent/reactant

reactant that will be used up or consumed completely

excess reagent/reactants

reactants that remain after all the limiting reagent is used up

theoretical yield

the amount of product predicted; maximum amount of product that can be generated as predicted from the balanced equation, assuming that all of the limiting reactant is consumed, no side reactions have occurred, and the entire product has been collected

raw or actual yield

the amount of product actually obtained during a reaction

percent yield

The ratio of the actual yield to the theoretical yield, multiplied by 100 percent

oxyanions

polyatomic ions containing oxygen

most oxygen per-

more oxygen -ate

less oxygen -ite

least oxygen hypo-

oxidation states,

elements can even have several different charges

ex. transition metals

ionicity

the degree to which a compound exhibits ionic character, meaning the extent to which it contains ions and their behavior in terms of electrical conductivity and solubility

electrolytes

solutes that enable solutions to carry currents

strong - dissociates completely into its constituent ions

weak - ionizes or hydrolyzes incompletely in aqueous solution, and only some of the solute is dissolved into its ionic constituents

solvate

dissolve