OSAT Chemistry key terms

The amount of a substance containing 6.022 × 10^23 representative particles, used as the counting unit that links particle-level quantities to measurable laboratory masses.

The value 6.022 × 10^23, equal to the number of particles in one mole of any substance; it converts between particle counts and moles.

The mass in grams of one mole of a substance, numerically equal to its formula mass in atomic mass units; used to convert between mass and moles.

The simplest whole-number ratio of atoms of each element in a compound, found by converting percentage composition or masses to mole ratios.

The actual number of atoms of each element in one molecule of a covalent compound; it is a whole-number multiple of the empirical formula.

The mass percentage of each element in a compound, calculated by dividing the total mass of that element per mole by the molar mass and multiplying by 100.

The quantitative relationship among amounts of reactants and products in a balanced chemical equation, used to predict masses, moles, or volumes consumed and formed.

The reactant that is completely consumed first and therefore determines the maximum amount of product that can form in a reaction.

The maximum mass of product calculated from stoichiometry when the limiting reactant is fully consumed, assuming no losses or competing reactions.

The actual experimental mass of product divided by the theoretical yield, multiplied by 100; it measures the efficiency of a reaction.

Concentration expressed as moles of solute per liter of solution, abbreviated M; it is used in dilution and solution stoichiometry calculations.

Concentration expressed as moles of solute per kilogram of solvent; unlike molarity it does not change with temperature because it uses mass, not volume.

Adding solvent to a solution to lower its concentration; the relationship M1V1 = M2V2 applies because the moles of solute stay constant.

A physical property of a solution that depends on the number of dissolved particles rather than their identity, such as boiling-point elevation, freezing-point depression, vapor-pressure lowering, and osmotic pressure.

The pressure required to prevent net movement of solvent across a semipermeable membrane; it equals iMRT for dilute solutions and is a colligative property.

The equation PV = nRT relating pressure, volume, moles, and absolute temperature of a gas, where R is the universal gas constant.

The pressure a single gas in a mixture would exert if it alone occupied the container; total pressure is the sum of partial pressures (Dalton's law).

The number of moles of one component divided by the total moles in a mixture; used with Dalton's law to find partial pressures.

The volume occupied by one mole of an ideal gas at standard temperature and pressure, approximately 22.4 liters.

A model stating that gas particles are in constant random motion, collide elastically, and have average kinetic energy proportional to absolute temperature.

The escape of gas molecules through a tiny opening into a vacuum; the rate is inversely proportional to the square root of molar mass (Graham's law).

When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, it shifts in the direction that partially offsets the disturbance.

The ratio of product concentrations to reactant concentrations, each raised to its balanced-equation coefficient, at equilibrium; its value depends on temperature.

The same ratio as the equilibrium constant but calculated with current, nonequilibrium concentrations; comparing Q with K predicts the direction of net change.

A species that donates a proton (hydrogen ion) to another species in an acid-base reaction; its partner after donation is the conjugate base.

Two species that differ by exactly one proton, such as NH3 and NH4+; every Brønsted-Lowry acid-base reaction involves two such pairs.

Describing a substance that can act as either an acid or a base depending on its partner, as water does when it donates or accepts a proton.

The negative base-10 logarithm of the hydrogen ion concentration in moles per liter; values below 7 are acidic and above 7 are basic at 25 °C.

A solution of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists large pH changes when small amounts of strong acid or base are added.

The loss of electrons by a species, accompanied by an increase in oxidation number; it always occurs together with reduction in a redox reaction.

A bookkeeping charge assigned to an atom in a compound using rules such as group number and electronegativity, used to identify redox changes.

A device that converts chemical energy into electrical energy (galvanic) or uses electrical energy to drive a reaction (electrolytic) through separated half-reactions.

The voltage of a half-reaction measured against the standard hydrogen electrode under standard conditions; more positive values indicate greater tendency to be reduced.

The heat content of a system at constant pressure; the change in enthalpy equals the heat absorbed or released in a reaction.

The total enthalpy change of a reaction is the same whether it occurs in one step or several steps, allowing enthalpy changes to be summed from known reactions.

The enthalpy change when one mole of a compound forms from its elements in their standard states; values are used with Hess's law to find reaction enthalpies.

The minimum energy colliding particles must possess for a reaction to occur; it corresponds to the energy barrier between reactants and the transition state.

A substance that increases reaction rate by providing an alternative pathway with lower activation energy without being consumed or changing the equilibrium position.

The relative ability of an atom in a bond to attract shared electrons; differences in electronegativity predict bond polarity and ionic character.

A covalent bond in which electrons are shared unequally because the bonded atoms have different electronegativities, producing partial positive and negative charges.

A strong intermolecular attraction between a hydrogen atom bonded to N, O, or F and a lone pair on a nearby electronegative atom; it raises boiling points.

A model predicting molecular shape by arranging electron domains around a central atom as far apart as possible to minimize repulsion.

The mixing of atomic orbitals to form equivalent new orbitals (sp, sp2, sp3) that explain observed molecular geometries and bond angles.

One of two or more compounds with the same molecular formula but different arrangements of atoms, giving different structures and properties.

A specific group of atoms within an organic molecule, such as hydroxyl, carbonyl, or carboxyl, that determines characteristic chemical behavior and properties.

A large molecule formed by covalently linking many repeating monomer units, such as polyethylene from ethylene or proteins from amino acids.