Chemistry GCSE Revision

Formula Mass

    • Atoms are so small that their masses are not measured directly. One grain of sand contains millions of atoms, so atoms must be really small.

The mass of an element
    • The mass of an element is shown on the periodic table.
    • The mass of an element is called its atomic mass.
    • You find the atomic mass of an elements on the Periodic Table.

Calculating formula mass.
    • When we add elements together to make me compounds, we can calculate the formula mass.
                Ammonia NH3
                1 atom of nitrogen
                = 1x14
                3 atoms of hydrogen
                =3x1
                =3
        Formula mass of NH3 = 14+3=17

The rules
    • Write down the formula of the compound.
    • Write down which elements are present in the compound.
    • Write down how many atoms of each element are present.
    • Multiply the mass of each element by the number of atoms of that element by the numbers of atoms of that. element present.
    • Total up all the masses to give the Formula mass.

Example
Calculating the formula mass of Carbon Dioxide.
    • Each Oxygen has an atomic mass of 16
    • Each Carbon has an atomic mass of 12

Formula mass of Carbon Dioxide
    • Oxygen = 2 x 16 = 32
    • Carbon= 1 x 12 = 12
Total = 32 + 12 = 44

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