Empirical Formulae The empirical formula of a compound is the simplest ratio of the different atoms in it. For example, for ethane (C2H6)it is CH3. You can work out the empirical formula of a compound if you know the mass of each element in it. This is really the reverse of finding the % mass of a compound from its formula.

Step 1 Find the relative atomic masses of the elements involved. Example ; What is the empirical formula of a compound that contains 27.3g carbon and 72.7g oxygen? Step 1 Find the relative atomic masses of the elements involved. Ar(C) = 12 and Ar(O) = 16 Step 2 Work out how many moles of each element this must be using; number of moles = mass Ar carbon = 27.3/12 = 2.275 oxygen = 72.7/16 = 4.55 Step 3 Divide both numbers of moles by the smallest number (2.275 in this case). carbon = 2.275/2.275 = 1 oxygen = 4.55/2.275 = 2 In the empirical formula, there are 1 carbon atom and 2 oxygen atoms, so it is CO2.

How much: copper oxide forms when 16 g of copper reacts with 4 g of oxygen? chlorine is needed if 23 g of sodium makes 58.5 g of sodium chloride? oxygen is needed if 20 g of hydrogen makes 180 g of water? carbon is needed if 6.4 g of oxygen makes 8.8 g of carbon dioxide? zinc is needed if 17.75 g of chlorine makes 34 g of zinc chloride?

C2H8O2N2 CH3COOH Cl2C2H6 CH3COOC2H5 2. What is the empirical formula of a compound with these molecular formulae? C4H8 C2H8O2N2 CH3COOH Cl2C2H6 CH3COOC2H5

[H = 1, C = 12, N = 14, Al = 27, O = 16, Mg = 24, Cl = 35.5, I = 127] 3. What is the empirical formula of the following compounds containing: [H = 1, C = 12, N = 14, Al = 27, O = 16, Mg = 24, Cl = 35.5, I = 127] 2 g hydrogen and 16 g oxygen. 0.05 g hydrogen and 0.8 g oxygen. 0.6 g carbon and 0.8 g oxygen. 0.6 g carbon and 1.6 g oxygen. 1.2 g magnesium and 3.55 g chlorine 5.4 g of aluminium and 76.2 g iodine

[H =1, C = 12, Ca = 40, Cl = 35.5, N = 14, Na = 23, O = 16, S = 32] 4. Calculate the empirical formula for these compounds from mass: [H =1, C = 12, Ca = 40, Cl = 35.5, N = 14, Na = 23, O = 16, S = 32] A compound containing 27 g of calcium, 21.6 g of oxygen and 1.35 g of hydrogen A compound containing 36.5 g of sodium, 25.4 g sulphur and 38.1 g oxygen A compound contains 14.2 g of chlorine, 4.8 g carbon and 0.8 g of hydrogen. It has a molar mass of 99 g/mol. What is the empirical formula? What is its molecular formula?