Determining an Empirical Formula by Combustion Analysis Isopropyl alcohol, sold as rubbing alcohol, is composed of C, H, and O. Combustion of 0.255 g of isopropyl alcohol produces 0.561 g of C02 and 0.306 g of H20. Determine the empirical formula of isopropyl alcohol. The compound dioxane, which is used as a solvent in various industrial processes, is composed of C, H, and O atoms. Combustion of a 2.203-g sample of this compound produces 4.401 g CO2 and 1.802 g H2O. A separate experiment shows that it has a molar mass of 88.1 g/mol. Which of the following is the correct molecular formula for dioxane? (a) C2H4O (b) C4H4O2 (c) CH2 (d) C4H8O2.
Answer: Step 1 Combustion analysis can only determine the empirical formula of a compound; it cannot determine the molecular formula. However, other techniques can determine the molecular weight. Once we know this value, coupled with the empirical formulas, we can easily calculate what the molecular formula is. Empirical formula for a compound is the simplest integral ratio of all the number of atoms in a compound. For example the molecular formula of glucose is C6H12O6 but its empirical formula is CH2O . Thus, the empirical formula weight of this compound is the sum of atomic masses of one carbon atom, two hydrogen atoms and one oxygen atom. Step 2 The combustion of a compound containing carbon, hydrogen and oxygen atoms gives carbon dioxide and water as products. The weight of carbon dioxide formed in the combustion reaction is 4.401 g. Calculate the number of moles of carbon dioxide as follows: Number of moles of carbon dioxide=weight of carbon doixide/molar mass of carbon dioxide. =4.401 g/44.01g/mol. =0.1 mol The number of carbon atoms in one molecule of carbon dioxide is 1. Therefore, the number of moles of carbon dioxide is equal to number of carbon atoms in the compound, dioxane. Calculate the weight of carbon in the compound as follows: Weight of carbon =number of moles of carbon*atomic weight of carbon. =0.1 mol*12.01 g/mol. =1.201g Step 4 The weight of water formed in the combustion reaction is 1.802 g. Calculate the number of moles of water as follows: Number of moles of water =weight of water /molar mass of water =1.802g/18.02g/mol. =0.1 mol. The number of hydrogen atoms in one molecule of water molecule is 2. Therefore, the number of moles of hydrogen atoms in the compound, dioxane is twice the number of moles of water. Thus, number of moles of hydrogen atoms in dioxane is 0.2 moles. Calculate the weight of hydrogen in the compound as follows: Weight of hydrogen = number of moles of hydrogen *atomic mass of hydrogen. =0.2 mol*1.01 g/mol =0.202g.