Relative formula mass

• Relative Formula Mass


  The sum of the relative atomic masses of the atoms in the numbers shown in the formula
• In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown
• Calculating percentage by mass in a compound


  Given the relative formula mass and the relative atomic masses
• Mass changes when a reactant or product is a gas
• The relative atomic mass is the weighted average of all naturally occurring isotopes.
• Relative molecular mass (Mr) = sum of relative atomic masses of atoms present
• Relative molecular mass (Mr) = sum of relative atomic masses of atoms in molecule
• The relative molecular mass (RM) of a molecule is calculated from the relative atomic masses of its constituent elements using the same method as that used to calculate the RAM of an atom.
• Average atomic mass = % abundance x Atomic mass
• Molecular mass (Mm) = sum of atomic masses of atoms present
• Relative formula mass (Mf) = sum of relative atomic masses of atoms in formula unit
• Average relative formula mass (Mf) = sum of relative atomic masses of atoms in formula units
• Atomic mass is the total number of protons and neutrons in an atom.
• Isotopes are different forms of the same element with the same number of protons but different numbers of neutrons.
• Periodic table


  Representation of the elements
• Finding the relative atomic mass of a compound

  1. Find the relative atomic masses of the elements in the compound

  2. Add up the relative atomic masses of the atoms in the compound
• Percentage mass of an element in a compound

  Calculated using the formula: (Relative atomic mass of element x Number of atoms of that element) / Relative molecular mass of compound x 100
• More complicated questions may require calculating the percentage mass of an element in a more complex compound