relative formula mass

• RELATIVE FORMULA MASS (Mr):

  The SUM of the RELATIVE ATOMIC MASSES (Ar) of all the atoms in a chemical formula.
• To calculate the relative formula mass:

  • Find the RELATIVE ATOMIC MASSES (Ar) of each element in the compound (usually listed in the periodic table).
  • Multiply the Ar of each element by the NUMBER OF ATOMS of that element in the formula.
  • Add these values together to get the Mr of the compound.
• The Mr of CaF2 is calculated as:

  Mr = 40 + (19 x 2) = 78

  
• In a balanced chemical equation:

  The sum of the relative formula masses of the reactants equals the sum of the relative formula masses of the products.
• To find the percentage mass of an element in a compound:

  • MULTIPLY the RELATIVE ATOMIC MASSES (Ar) of the element by the number of atoms of that element in the compound.
  • DIVIDE this by the RELATIVE FORMULA MASS (Mr) of the whole compound.
  • Multiply by 100 to convert to a percentage.
• (7 x 2 / (7 x 2) + 32 + (16 x 4)) x 100 = 12.7%

  
• Mr of Water (H2O)

  18
• Mr of Glucose (C6H12O6)

  180
• Mr of sodium chloride (NaCl)

  58.5
• Mr of magnesium oxide (MgO)

  40
• Mr of ethanol (C2H5OH)

  46
• Percentage of oxygen in CO2:

  72.73%
• Percentage of calcium in CaCO3

  40.00%
• Percentage of nitrogen in NH4

  82.35
• Percentage of hydrogen in H2O

  11.11%
• Percentage of sodium in NaCl

  39.34%