2.1.1 (e) Relative Molecular Mass

Syllabus

(e) use of the terms relative molecular mass, M_r, & relative formula mass & their calculation from relative atomic masses.

{For simple molecules, the term relative molecular mass will be used. For compounds with giant structures, the term relative formula mass will be used. Definitions of relative molecular mass and relative formula mass will not be required.}

What does this mean?

Adding up a formula mass is as GCSE a skill as you'll find at A level, but it will pick you up the odd mark here and there.

Simple molecules.

Simple Covalent molecules have a finite number of atoms.

The relative molecular mass refers to the mass of one single molecule.

All you have to do is add up their masses by looking up A_r's from a Periodic Table.

eg A_r(N) = 14, A_r(O) = 16, A_r(H) = 1, A_r(C) = 12, A_r(Ca) = 40, A_r(Na) = 23, A_r(Cl) = 35.5, A_r(N) = 14, A_r(Si) = 28

M_r(NO) = 14 + 16 = 30 g/mol

M_r(NO₂) = 14 + (16x2) = 46 g/mol

M_r(N₂O) = (14x2) + 16 = 44 g/mol

Brackets (in Ionic compounds) make things marginally more complicated

M_r Ca(NO₃)₂ = 40 + 2x(14+(3x16)) = 164 g/mol

M_r(NH₄)₂CO₃ = 2 x(14+4) +12+ (16x3) = 98 g/mol

Giant Structures.

All metals, Ionic substances and a few covalent substances have giant structures.

This means that they don't have a regular number of atoms.

One salt crystal has a different number of ions to the next, one lump of copper has a different number of atoms to the next.

So it's not useful to find the mass of a single "molecule".

Instead we use the empirical formula of the substance.

You should already know that this is the lowest whole-number ratio of elements in the substance from GCSE.

In other words, a Sodium Chloride crystal with the formula Na_{2345678900987654321}Cl_{2345678900987654321} has an empirical formula of NaCl and so does another Sodium Chloride crystal with a formula of Na_{123456789765432345678}Cl_{123456789765432345678}.

So although we can't have a useful relative molecular mass we can have a relative formula mass based on the empirical formula NaCl = 23 + 35.5 = 58.5 g/mol

The same is true of giant covalent structures like silica which has an empirical formula of SiO₂ = 28 + (2x16) = 60 g/mol

You'll recall that both Diamond and Graphite are giant covalent structures of the element Carbon.

So the empirical formula of each is just C. And the Relative formula mass of each is 12 g/mol.

This is true of all elements that have giant structures - and this includes all metals.

So the relative formula mass of Calcium is the relative atomic mass of Ca = 40 g/mol

And the relative formula mass of Sodium is the relative atomic mass of Na = 23 g/mol

Water of crystallisation

Many salts absorb water as they form crystals.

We can't know the actual formula of every crystal but we have to include the water in the empirical formula.

So Anhydrous Copper Sulphate is simply CuSO₄

But Hydrated Copper Sulphate is CuSO₄ . 5H₂O - which means every mole of Copper Sulphate absorbs 5 moles of water.

So the relative formula mass of Anhydrous Copper Sulphate = 63.5 + 32 + (4x16) = 159.5 g/mol

And the relative formula mass of Hydrated Copper Sulphate = 63.5 + 32 + (4x16) + (5x18) = 249.5 g/mol

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