1. Percent composition and Empirical formula LO: Explain what an empirical formula is (C) Calculate percentages and empirical formula (B) Use the idea of moles to investigate and calculate % composition, empirical formula and % yield (A*) Starter Work out the molecular weights of the following… KNO 3 CuCl 2 NaClNO 2

2. Percentage composition Percent = mass of an atom x 100 mass of the compound

3. K N O 3 Let’s say we want to work out the % of Nitrogen (N) in this molecule.

4. CuCl 2 % Copper NaCl% Sodium NO 2 % Nitrogen LiF% Lithium

5. ??????? Which of the following molecules contains the highest % of nitrogen ? KNO 3 (NH4) 3 PO 4 NH 4 NO 3

6. Empirical formula The simplest way that the formula can be written. C 6 H 6 could be written CH C 2 H 6 could be written CH 3 C 4 H 10 could be written ……….

7. In an exam you will have to take the percentage amount of each atom in a compound and turn it into an empirical formula. They might also give the mass of a substance present this number will work just the same. E.g. A hydrocarbon is 75% carbon and 25% hydrogen What is the empirical formula ? Write the symbols Write the % Write the RAM Divide % by RAM Divide by the smallest number

8. Practical CAUTION – WEAR GOGGLES – BUNSEN BURNERS USED – HOT !!! 1.Weigh the crucible and lid together (record this) 2.Put some magnesium in and weigh it all again (record this) 3.Heat over a bunsen as shown and lift the lid now and again to let a bit more oxygen in. 4.Wait until the glowing inside has stopped. 5.Turn off the bunsen and let it cool down.

9. Using your results Wt. of crucible + lid Wt. of crucible + lid + magnesium Wt. of crucible + lid + magnesium oxide Take away the Wt. of the crucible + lid to find out how much the magnesium and the magnesium oxide weigh. Magnesium = Magnesium oxide = % Magnesium = % Oxygen =

10. Write the symbols Write the % Write the RAM Divide % by RAM Divide by the smallest number

15. % Yield Suppose a reaction is supposed to make 50g of product and it only makes 10g. What is the % yield ?

16. We can judge how successful a reaction is using this. We can work out how much of a reactant is needed to make a certain amount of product. We can see which reactions make the most financial sense.