Relative atomic mass and relative formula mass. The A r of A r [Na] = 23 A r [Fe] = 56 A r [He] = 4 A r [Al] = 27 A r [Br] = ? A r [Be] = ? A r [Sn] =

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Presentation transcript:

Relative atomic mass and relative formula mass

The A r of A r [Na] = 23 A r [Fe] = 56 A r [He] = 4 A r [Al] = 27 A r [Br] = ? A r [Be] = ? A r [Sn] = ?

The A r of A r [Na] = 23 A r [Fe] = 56 A r [He] = 4 A r [Al] = 27 A r [Br] = 80 A r [Be] = ? A r [Sn] = ?

The A r of A r [Na] = 23 A r [Fe] = 56 A r [He] = 4 A r [Al] = 27 A r [Br] = 80 A r [Be] = 9 A r [Sn] = ?

The A r of A r [Na] = 23 A r [Fe] = 56 A r [He] = 4 A r [Al] = 27 A r [Br] = 80 A r [Be] = 9 A r [Sn] = 119

Relative atomic mass The relative atomic mass of an element (A r ) compares the mass of atoms of the element with the 12 C isotope. It is an average value for the isotopes of the element. We find the A r value on the periodic Table (the top number)

e. g. some A r values: A r [Na] = 23 A r [Fe] = 56 A r [He] = 4 A r [Al] = 27 A r [Br] = 80 A r [Be] = 9 A r [Sn] = 119

The M r of M r [H 2 ] = 2 M r [HF] = 20 M r [MgO] = 40 M r [H 2 SO 4 ] = 98 M r [CaS] = ? M r [SO 2 ] = ? M r [MgCl 2 ] = ?

The M r of M r [H 2 ] = 2 M r [HF] = 20 M r [MgO] = 40 M r [H 2 SO 4 ] = 98 M r [CaS] = 72 M r [SO 2 ] = ? M r [MgCl 2 ] = ?

The M r of M r [H 2 ] = 2 M r [HF] = 20 M r [MgO] = 40 M r [H 2 SO 4 ] = 98 M r [CaS] = 72 M r [SO 2 ] = 64 M r [MgCl 2 ] = ?

The M r of M r [H 2 ] = 2 M r [HF] = 20 M r [MgO] = 40 M r [H 2 SO 4 ] = 98 M r [CaS] = 72 M r [SO 2 ] = 64 M r [MgCl 2 ] = 95

Relative formula mass, M r The relative formula mass, M r, of a compound (or molecule) is the sum of the relative atomic masses of the atoms in the numbers shown in the formula. i.e. add together all the A r values in the formula

e.g. some M r values H 2 A r [H] = 1 M r [H 2 ] = (2 x 1) = 2 MgOA r [Mg] = 24 A r [O] = 16 M r [MgO] = ( ) = 40

H 2 SO 4 A r [H] = 1 A r [S] = 32 A r [O] = 16 M r [H 2 SO 4 ] = (2x1) (4x16) = 98 SO 2 A r [S] = 32 A r [O] = 16 M r [SO 2 ] = 32 + (2 x 16) = 64

Formulae with brackets If a formula has brackets, everything inside must be multiplied by the subscript that follows it (NH 4 ) 2 SO 4 A r [N] = 14 A r [H] = 1 A r [S] = 32 A r [O] = 16 M r (NH 4 ) 2 SO 4 = (2x14) + (8x1) (4x16) = x 4 = 8