Example Question - 3

Question: 0.46 g of compound X, which contains carbon, hydrogen and oxygen only, is burned completely in oxygen. It forms 0.88 g of carbon dioxide and 0.54 g of water. Find the empirical formula of X. (A_r: C = 12, H = 1, O = 16)Step 1: moles of carbon. All the carbon in the CO₂ came from X, and each CO₂ contains one carbon atom.n(C) = n(CO₂) = 0.88 ÷ 44 = 0.020 molStep 2: moles of hydrogen. Each H₂O contains two hydrogen atoms.n(H₂O) = 0.54 ÷ 18 = 0.030 mol, so n(H) = 0.060 molStep 3: mass of oxygen by subtraction. The oxygen in the products comes partly from the air, so it cannot be used directly. Instead:mass of C = 0.020 × 12 = 0.24 g     mass of H = 0.060 × 1 = 0.060 g
mass of O = 0.46 - 0.24 - 0.060 = 0.16 g
n(O) = 0.16 ÷ 16 = 0.010 molStep 4: simplest ratio. Divide by the smallest value, 0.010.C : H : O = 2 : 6 : 1Answer: the empirical formula of X is C₂H₆O.If the M_r of X is 46, the molecular formula is also C₂H₆O, because the empirical formula mass is 46. This is the formula of ethanol.