Quantities in Chemical Reactions: Stoichiometry and the Mole Concept

Measurement Uncertainty

The experimental doubt or error present in every physical measurement, where the last recorded decimal digit represents an estimate.

Significant Figures

All the reliably known digits in a measured quantity plus one final uncertain or estimated digit.

Captive (Interior) Zeros

Zeros located between nonzero digits; they are always significant (e.g., $304.9$ contains 4 significant figures).

Leading Zeros

Zeros to the left of the first nonzero digit that serve only as place holders; they are never significant (e.g., $0.0093$ contains 2 significant figures).

Trailing Zeros

Zeros to the right of the last nonzero digit; they are significant if a decimal point is explicitly present, but not significant if no decimal point is shown.

Scientific Notation

A standard format for writing numbers as $M \times 10^n$, where $1 \le M < 10$ and $n$ is an integer indicating decimal shifts.

Isotopes

Atoms of the same element containing the same number of protons (atomic number) but differing numbers of neutrons (mass number).

Relative Atomic Mass

The mass of an atom compared against the international reference standard carbon-12 ($^{12}C$), which is assigned an exact mass of $12\,\text{amu}$.

Isotopic Abundance

The percentage by mass of each distinct isotope present in a representative, naturally occurring terrestrial sample of an element.

The Mole ($n$)

The fundamental SI base unit for the amount of substance, representing a collective count of exactly $6.02 \times 10^{23}$ representative entities.

Avogadro's Constant ($N_A$)

The number of representative entities in one mole of substance, defined as $6.02 \times 10^{23}\,\text{particles/mol}$.

Molar Mass ($M$)

The mass in grams of one mole of a substance, expressed in $g\,mol^{-1}$.

Percentage Composition

The percentage by mass of each constituent element in a chemical compound: $\frac{\text{mass of element}}{\text{mass of compound}} \times 100$.

Empirical Formula

The chemical formula representing the lowest whole-number ratio of atoms of each element present in a chemical compound.

Molecular Formula

The chemical formula representing the actual number of atoms of each element present in one molecule of a compound.

Stoichiometry

The quantitative study and calculation of the amounts of reactants consumed and products formed in a balanced chemical reaction.

Mole Ratio

A conversion factor derived from the coefficients of a balanced chemical equation that relates the mole amounts of any two species in the reaction.

Limiting Reactant

The reactant that is completely consumed first in a reaction, thereby stopping the reaction and dictating the maximum theoretical amount of product that can form.

Excess Reactant

A reactant present in a quantity greater than required to react completely with the limiting reactant, leaving unreacted material behind.

Theoretical Yield

The maximum quantity of product calculated to form through complete consumption of the limiting reactant under ideal conditions.

Actual Yield

The amount or mass of pure product experimentally isolated, recovered, and measured in the laboratory.

Percentage Yield

The mathematical ratio expressing the efficiency of a chemical reaction: $\text{Percentage Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100$.