1. 27/05/2016 Relative mass formula, atomic mass, and empirical formula

2. 27/05/2016 Relative formula mass, M r The relative formula mass of a compound is blatantly the relative atomic masses of all the elements in the compound added together. E.g. water H 2 O: Therefore M r for water = 16 + (2x1) = 18 Work out M r for the following compounds: 1)HCl 2)NaOH 3)MgCl 2 4)H 2 SO 4 5)K 2 CO 3 H=1, Cl=35 so M r = 36 Na=23, O=16, H=1 so M r = 40 Mg=24, Cl=35 so M r = 24+(2x35) = 94 H=1, S=32, O=16 so M r = (2x1)+32+(4x16) = 98 K=39, C=12, O=16 so M r = (2x39)+12+(3x16) = 138 Relative atomic mass of O = 16Relative atomic mass of H = 1

3. 27/05/2016 More examples CaCO 3 40 + 12 + 3x16100 HNO 3 1 + 14 + 3x16 2MgO2 x (24 + 16)80 3H 2 O3 x ((2x1) + 16) 4NH 3 2KMnO 4 3C 2 H 5 OH 4Ca(OH) 2

4. 27/05/2016 Relative atomic mass The mass of an isotopic element relative to Carbon- 12. Example: chlorine occurs in isotope forms Cl-35 (75.5%) and Cl-37 (24.5%) Relative atomic mass = ((75.5x35)+(24.5x37))/(75.5+24.5)=35.5 Try this: neon-20 (90.9%), neon-21 (0.3%), and neon-22 (8.8%)

5. 27/05/2016 Calculating percentage mass If you can work out M r then this bit is easy… Calculate the percentage mass of magnesium in magnesium oxide, MgO: A r for magnesium = 24Ar for oxygen = 16 M r for magnesium oxide = 24 + 16 = 40 Therefore percentage mass = 24/40 x 100% = 60% Percentage mass (%) = Mass of element A r Relative formula mass M r x100% Calculate the percentage mass of the following: 1)Hydrogen in hydrochloric acid, HCl 2)Potassium in potassium chloride, KCl 3)Calcium in calcium chloride, CaCl 2 4)Oxygen in water, H 2 O

6. 27/05/2016 Calculating the mass of a product E.g. what mass of magnesium oxide is produced when 60g of magnesium is burned in air? Step 1: READ the equation: 2Mg + O 2 2MgO IGNORE the oxygen in step 2 – the question doesn’t ask for it Step 3: LEARN and APPLY the following 3 points: 1)48g of Mg makes 80g of MgO 2)1g of Mg makes 80/48 = 1.66g of MgO 3)60g of Mg makes 1.66 x 60 = 100g of MgO Step 2: WORK OUT the relative formula masses (M r ): 2Mg = 2 x 24 = 48 2MgO = 2 x (24+16) = 80

7. 27/05/2016 Work out M r : 2H 2 O = 2 x ((2x1)+16) = 36 2H 2 = 2x2 = 4 1.36g of water produces 4g of hydrogen 2.So 1g of water produces 4/36 = 0.11g of hydrogen 3.6g of water will produce (4/36) x 6 = 0.66g of hydrogen M r : 2Ca = 2x40 = 80 2CaO = 2 x (40+16) = 112 80g produces 112g so 10g produces (112/80) x 10 = 14g of CaO M r : 2Al 2 O 3 = 2x((2x27)+(3x16)) = 204 4Al = 4x27 = 108 204g produces 108g so 100g produces (108/204) x 100 = 52.9g of Al 2 O 3 1)When water is electrolysed it breaks down into hydrogen and oxygen: 2H 2 O 2H 2 + O 2 What mass of hydrogen is produced by the electrolysis of 6g of water? 3) What mass of aluminium is produced from 100g of aluminium oxide? 2Al 2 O 3 4Al + 3O 2 2) What mass of calcium oxide is produced when 10g of calcium burns? 2Ca + O 2 2CaO

8. 27/05/2016 Another method Try using this equation: Mass of product IN GRAMMES Mass of reactant IN GRAMMES M r of product M r of reactant Q. When water is electrolysed it breaks down into hydrogen and oxygen: 2H 2 O 2H 2 + O 2 What mass of hydrogen is produced by the electrolysis of 6g of water? Mass of product IN GRAMMES 6g 4 36 So mass of product = (4/36) x 6g = 0.66g of hydrogen

9. 27/05/2016 Calculating the volume of a product At normal temperature and pressure the Relative Formula Mass (M r ) of a gas will occupy a volume of 24 litres e.g. 2g of H 2 has a volume of 24 litres 32g of O 2 has a volume of 24 litres 44g of CO 2 has a volume of 24 litres etc Q. When water is electrolysed it breaks down into hydrogen and oxygen: 2H 2 O 2H 2 + O 2 What VOLUME of hydrogen is produced by the electrolysis of 6g of water? On the previous page we said that the MASS of hydrogen produced was 0.66g 2g of hydrogen (H 2 ) will occupy 24 litres (from the red box above), So 0.66g will occupy 0.66/2 x 24 = 8 litres

10. 27/05/2016 Example questions 1)What volume of hydrogen is produced when 18g of water is electrolysed? 2H 2 0 2H 2 + O 2 2)Marble chips are made of calcium carbonate (CaCO 3 ). What volume of carbon dioxide will be released when 500g of CaCO 3 is reacted with dilute hydrochloric acid? CaCO 3 + 2HCl CaCl 2 + H 2 O + CO 2 3)In your coursework you reacted magnesium with hydrochloric acid. What volume of hydrogen would be produced if you reacted 1g of magnesium with excess acid? Mg + 2HCl MgCl 2 + H 2

11. 27/05/2016 Empirical formulae Empirical formulae is simply a way of showing how many atoms are in a molecule (like a chemical formula). For example, CaO, CaCO 3, H 2 0 and KMnO 4 are all empirical formulae. Here’s how to work them out: A classic exam question: Find the simplest formula of 2.24g of iron reacting with 0.96g of oxygen. Step 1: Divide both masses by the relative atomic mass: For iron 2.24/56 = 0.04For oxygen 0.96/16 = 0.06 Step 2: Write this as a ratio and simplify: 0.04:0.06 is equivalent to 2:3 Step 3: Write the formula: 2 iron atoms for 3 oxygen atoms means the formula is Fe 2 O 3

12. 27/05/2016 Example questions 1)Find the empirical formula of magnesium oxide which contains 48g of magnesium and 32g of oxygen. 2)Find the empirical formula of a compound that contains 42g of nitrogen and 9g of hydrogen. 3)Find the empirical formula of a compound containing 20g of calcium, 6g of carbon and 24g of oxygen.