Mrs for more complicated compounds

M_rs for more complicated compounds

We need to be careful with brackets and in particular water in crystallisation. For example, the compound below has 12 molecules of water per mole of compound

e.g. M_r (NH₄Fe(SO₄)₂.12H₂O) = 14+(4x1)+56+2(32+(4x16))+12(2+16) = 482

Percentage composition

Percentage composition means percentage composition by mass

e.g. work out %N in ammonium nitrate

Mr (NH₄NO₃) = 14+4+14+(3x16) = 80

so %N = ²⁸/₈₀ x 100 = 35%

Working out Formulas from masses

N.B. Formula of a compound is the simplest whole number ration of the elements in a compound

ratio of atoms of elements = ratio of MOLES of elements

Example : Work out the formula of compound which contains 3.400g of calcium and 6.035g chlorine

Work out number of moles ^3.400/₄₀ = 0.085 ^6.035/_35.5 = 0.170

work out ratio of moles C (divided 1 : 2 by smallest number)

Therefore the formula is CaCl₂

N.B. This is simplest whole number ratio of elements in a compound is called EMPIRICAL formula (formula from experiment)
N.B.2 If the masses are given as percentages, we should consider 100g of compound