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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 on theme: "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] ="— Presentation transcript:

1 Relative atomic mass and relative formula mass

2 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] = ?

3 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] = ?

4 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] = ?

5 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

6 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)

7 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

8 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 ] = ?

9 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 ] = ?

10 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 ] = ?

11 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

12 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

13 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] = (24 + 16) = 40

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

15 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) + 32 + (4x16) = 132 2 x 4 = 8


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