Relative Mass

Basics of Relative Mass

  • The relative atomic mass (Ar) of an element is the weighted average mass of the atoms of an element compared to 1/12 of the mass of an atom of carbon-12.
  • The relative isotopic mass is the mass of an atom of a specific isotope compared to 1/12 of the mass of an atom of carbon-12.
  • The relative molecular mass (Mr) of a substance is the sum of the relative atomic masses of the atoms in a molecule.
  • The relative formula mass is used in similar context to Mr but it is the sum of the relative atomic masses of the atoms in a formula unit of a compound, used primarily for network solids and ions.

Isotopic Abundance

  • The Ar for an element is determined by both the relative isotopic masses and their isotopic abundance.
  • For elements with multiple stable isotopes, the Ar is not a whole number.
  • Isotopic abundance is usually determined using mass spectroscopy methods.

Moles and Avogadro’s Number

  • One mole of any substance contains the same number of entities (atoms, molecules, ions etc.) as there are atoms in exactly 12g of pure carbon-12.
  • This number is known as Avogadro’s number, and is 6.02 x 10^23 mol^-1.
  • Thus, the relative atomic or molecular mass of a substance in grams is one mole of that substance.

Chemical Reactions and Stoichiometry

  • In a balanced chemical equation, the numbers in front of the reactants and products (stoichiometric coefficients) indicate the ratio of moles of each substance that react or are produced.
  • Relative atomic masses (Ar) can be used to calculate the masses of reactants and products in a chemical reaction.
  • The masses must be converted to moles using the equation: moles = mass/Ar or Mr.
  • The limiting reactant in a chemical reaction is the reactant that is completely consumed first.
  • The amount of product formed is directly proportional to the amount of limiting reactant.

Empirical and Molecular Formulae

  • The empirical formula of a compound shows the simplest whole-number ratio of the atoms of each element present in the compound.
  • The molecular formula shows the actual number of atoms of each element in a molecule of the compound.
  • From the empirical formula and the Mr of the compound, one can determine the molecular formula.