1. Percent Composition, Molecular and Empirical Formulas … whew, say that 5 times fast…

2. 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

3. 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 XeF 4

4. Percent Composition Toluene: C 6 H 5 CH 3 What is the percent composition of carbon in this molecule? –% carbon = (mass of carbon)/(mass of molecule) Of hydrogen?

5. 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 : H 2 O 2 C 6 H 12 O 6 Empirical formula : HOCH 2 O

6. 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

7. 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?

8. 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

9. Molecular Formula A compound has an experimental molar mass of 78g/mol. Its empirical formula is CH. What is its molecular formula?