Percent Composition, Molecular and Empirical Formulas … whew, say that 5 times fast… Percent Composition • Have you ever seen CSI? – You know when the lab people take a chemical and put it into this “magical machine” and it give a print out with the name of the compound… yeah, that machine doesn’t exist Percent Composition • Scientist use percent composition as a way to identify some compounds – the percentage by mass of each element in a compound – For example, in 1962 scientist made a new compound that they found was 63.7% Xe and 36.7% F… • This let them figure out that the formula was XeF4 Percent Composition • Toluene: C6H5CH3 • What is the percent composition of carbon in this molecule? – % carbon = (mass of carbon)/(mass of molecule) • Of hydrogen? Empirical Formula • Data from the percent composition lets us figure out the empirical formula for a compound – Shows the simplest ratio of a compound. – Example: • Molecular formula : H2O2 • Empirical formula : HO C6H12O6 CH2O Empirical Formula • To find the empirical formula: – calculate the number of moles of each element in a compound – Find the ratio of the elements to each other – If you are given percentages to start (and not masses), assume a 100g sample Empirical Formula • A compound is 38.77g of Cl and 61.23g O. What is it’s empirical formula • A compound is made up of 18.0% C, 2.26% H, and 79.7% Cl. What is it’s empirical formula? Molecular Formula • If you know the molecular weight of a compounds (or the molar mass) and it’s empirical formula, you can then determine it’s Molecular formula – a chemical formula that shows the number and kinds of atoms in a molecule – It is always a whole-number ratio • n(empirical formula) = molecular formula Molecular Formula • A compound has an experimental molar mass of 78g/mol. Its empirical formula is CH. What is its molecular formula?