1. Tuesday, 28 June 2016Tuesday, 28 June 2016Tuesday, 28 June 2016Tuesday, 28 June 2016 Lord Grey Science Faculty Empirical Formula Lesson Objectives Lesson Objectives Calculate the empirical formula of a compound Calculate the empirical formula of a compound Calculate relative formula masses (M r ) from balance equations. Calculate relative formula masses (M r ) from balance equations. Keywords Keywords Relative Formula Mass (M r ) Relative Formula Mass (M r ) Empirical formula Empirical formula Compound Compound

2. Definition The empirical formula shows the simplest whole-number ratio of the atoms of each element in a compound. The empirical formula shows the simplest whole-number ratio of the atoms of each element in a compound. Benzene is made from 6 carbon atoms and 6 hydrogen atoms (C6H6). It has the Empirical formula CH. Can you see why ?

3. Calculating an empirical formula The chemical formula for butane is C 4 H 10 Step 1 Find the highest common factor. Step 1 Find the highest common factor. The highest common factor for 4 and 10 is 2. Step 2 Divide by the highest common factor. Step 2 Divide by the highest common factor. Step 3 Write down the empirical formula Step 3 Write down the empirical formula C 2 H 5 C 2 H 5

4. Key Now try calculating the empirical formula of ethanoic acid from this diagram Step 1 Work out the number of each atom Step 2 Divide by the highest common factor Step 3 Write down the empirical formula CH 2 O C = 2, H = 4, O = 2

5. Ratio Sort Cards Example The proportion of black to white is 1/3 Ratio of black to white is 1:2

6. Ratio Sort Cards Instructions In pairs, sort the cards into 7 sets In pairs, sort the cards into 7 sets Each set should have one of each type of card Each set should have one of each type of card The proportion of ……… is ……. Ratio of …… to ……. is …….

8. Answers (1) Ethane C 2 H 6 Empirical formula = CH 3 Proportion of white is 3/4; Ratio of white to black is 3:1 Water H 2 O Empirical formula = H 2 O Proportion of white is 1/3; Ratio of white to black is 2:1 COLOURS ARE ON CIRCLE DIAGRAM NOT MODEL!

9. Answers (2) Hydrogen peroxide H 2 O 2 Empirical formula = HO Proportion of white is 1/2; Ratio of white to black is 1:1 Methane CH 4 Empirical formula = CH 4 Proportion of black is 80%; Ratio of white to black is 1:4 COLOURS ARE ON CIRCLE DIAGRAM NOT MODEL!

10. Answers (3) Propane C 3 H 8 Empirical formula = C 3 H 8 Proportion of white is 3/11; Ratio of white to black is 3:8 Ethanoic Acid CH 3 COOH Empirical formula = CH 2 O Proportion of grey is 1/4 Ratio of black:white:grey is1:2:1 COLOURS ARE ON CIRCLE DIAGRAM NOT MODEL!

11. Answers (4) Ethanol C 2 H 5 OH Empirical formula = C 2 H 6 O Proportion of grey is 1/9 Ratio of black:white:grey is 2:6:1 COLOURS ARE ON CIRCLE DIAGRAM NOT MODEL!

12. How to work out relative formula mass from equations. Balance chemical equations show the formulae and number of atoms in a chemical reaction. You can use e Mass cannot be created or destroyed. Show that mass is conserved in this reaction. ZnCO 3 ZnO + CO 2 You need to use the ideas of M r

13. How to work out relative formula mass from equations. Balance chemical equations show the formulae and number of atoms in a chemical reaction. Remember - Mass cannot be created or destroyed. Show that mass is conserved in this reaction. ZnCO3 ZnO + CO2 You need to use the ideas of Mr