Empirical formula Combustion analysis of a hydrocarbon showed that 64.00 g of the compound contained 47.98 g of carbon. Determine the empirical formula.

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Presentation transcript:

empirical formula Combustion analysis of a hydrocarbon showed that 64.00 g of the compound contained 47.98 g of carbon. Determine the empirical formula of the compound.

empirical formula of methane elements in compound carbon hydrogen mass in grams (or in %) 47.98 (74.9%) 16.02 (25.1%) number of moles 47.98/12.01 = 3.99 mole (74.9/12.01 = 6.24 moles) 16.02/1.01 = 16.02 moles (25.1/1.01 = 25.1 moles) ratio of moles 4 (6.25) 16 (25) most simple ratio of moles 4/4 = 1 16/4 = 4 empirical formula CH4

molecular formula Calculate the molecular formula of a compound with a molecular mass of 84g and an empirical formula of CH2.

molecular formula mass empirical formula = 14g molar mass of formula = 84g whole number ratio (n) of molar mass/empirical mass = 84 g/14 g = 6 molecular formula = empirical formula x whole number ratio: (CH2 ) x 6 = C6H12

molecular formula Naphthalene, best known as ‘mothballs’, is composed of carbon (93.71%) and hydrogen (6.29%). If the molar mass of the compound is 128g, what is the molecular formula of naphthalene?

AGAIN DO NOT ROUND UP OR DOWN BUT LOWEST WHOLE NUMBER RATIO molecular formula carbon hydrogen mass of element 93.71 6.29 number of moles 93.71/12 = 7.8 6.29/ 1 = 6.29 DO NOT ROUND UP OR DOWN most simple ratio 7.8/ 6.29 = 1.25 6.29/6.29 = 1 AGAIN DO NOT ROUND UP OR DOWN BUT LOWEST WHOLE NUMBER RATIO lowest whole number ratio = multiply by 4 5 4 empirical formula: C5H4 ratio molecular formula /empirical formula: 128g/ 64g = 2 molecular formula = 2 x C5H4 = C10H8

Empirical formula hydrated salt When 9.44g hydrated magnesium sulphate, MgSO4.nH2O, was heated until there was no further mass decrease, 4.58g of anhydrous magnesium was left behind. Determine the empirical formula.

Empirical formula hydrated salt compound MgSO4 H2O mass (in g or %) 4.58 4.86 molar mass 120.08 18.01 number of moles 4.58/120 = 0.039 4.86/18 = 0.27 most simple molar ratio 0.039/0.039 = 1 0.27/0.039 = 6.93 empirical formula MgSO4.7H2O