Relative mass formula, atomic mass, and empirical formula 24/11/2018 Relative mass formula, atomic mass, and empirical formula

Relative formula mass, Mr 24/11/2018 The relative formula mass of a compound is blatantly the relative atomic masses of all the elements in the compound added together. Relative atomic mass of O = 16 E.g. water H2O: Relative atomic mass of H = 1 Therefore Mr for water = 16 + (2x1) = 18 Work out Mr for the following compounds: HCl NaOH MgCl2 H2SO4 K2CO3 H=1, Cl=35 so Mr = 36 Na=23, O=16, H=1 so Mr = 40 Mg=24, Cl=35 so Mr = 24+(2x35) = 94 H=1, S=32, O=16 so Mr = (2x1)+32+(4x16) = 98 K=39, C=12, O=16 so Mr = (2x39)+12+(3x16) = 138

More examples CaCO3 40 + 12 + 3x16 100 HNO3 1 + 14 + 3x16 2MgO 24/11/2018 CaCO3 40 + 12 + 3x16 100 HNO3 1 + 14 + 3x16 2MgO 2 x (24 + 16) 80 3H2O 3 x ((2x1) + 16) 4NH3 2KMnO4 3C2H5OH 4Ca(OH)2

Relative atomic mass 24/11/2018 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%)

Calculating percentage mass 24/11/2018 If you can work out Mr then this bit is easy… Percentage mass (%) = Mass of element Ar Relative formula mass Mr x100% Calculate the percentage mass of magnesium in magnesium oxide, MgO: Ar for magnesium = 24 Ar for oxygen = 16 Mr for magnesium oxide = 24 + 16 = 40 Therefore percentage mass = 24/40 x 100% = 60% Calculate the percentage mass of the following: Hydrogen in hydrochloric acid, HCl Potassium in potassium chloride, KCl Calcium in calcium chloride, CaCl2 Oxygen in water, H2O

Calculating the mass of a product 24/11/2018 E.g. what mass of magnesium oxide is produced when 60g of magnesium is burned in air? IGNORE the oxygen in step 2 – the question doesn’t ask for it Step 1: READ the equation: 2Mg + O2 2MgO Step 2: WORK OUT the relative formula masses (Mr): 2Mg = 2 x 24 = 48 2MgO = 2 x (24+16) = 80 Step 3: LEARN and APPLY the following 3 points: 48g of Mg makes 80g of MgO 1g of Mg makes 80/48 = 1.66g of MgO 60g of Mg makes 1.66 x 60 = 100g of MgO

Mr: 2Al2O3 = 2x((2x27)+(3x16)) = 204 4Al = 4x27 = 108 When water is electrolysed it breaks down into hydrogen and oxygen: 2H2O 2H2 + O2 What mass of hydrogen is produced by the electrolysis of 6g of water? 24/11/2018 Work out Mr: 2H2O = 2 x ((2x1)+16) = 36 2H2 = 2x2 = 4 36g of water produces 4g of hydrogen So 1g of water produces 4/36 = 0.11g of hydrogen 6g of water will produce (4/36) x 6 = 0.66g of hydrogen 2) What mass of calcium oxide is produced when 10g of calcium burns? 2Ca + O2 2CaO Mr: 2Ca = 2x40 = 80 2CaO = 2 x (40+16) = 112 80g produces 112g so 10g produces (112/80) x 10 = 14g of CaO 3) What mass of aluminium is produced from 100g of aluminium oxide? 2Al2O3 4Al + 3O2 Mr: 2Al2O3 = 2x((2x27)+(3x16)) = 204 4Al = 4x27 = 108 204g produces 108g so 100g produces (108/204) x 100 = 52.9g of Al2O3

So mass of product = (4/36) x 6g = 0.66g of hydrogen Another method 24/11/2018 Try using this equation: Mass of product IN GRAMMES Mass of reactant IN GRAMMES Mr of product Mr of reactant Q. When water is electrolysed it breaks down into hydrogen and oxygen: 2H2O 2H2 + O2 What mass of hydrogen is produced by the electrolysis of 6g of water? Mass of product IN GRAMMES 4 6g 36 So mass of product = (4/36) x 6g = 0.66g of hydrogen

Calculating the volume of a product 24/11/2018 At normal temperature and pressure the Relative Formula Mass (Mr) of a gas will occupy a volume of 24 litres e.g. 2g of H2 has a volume of 24 litres 32g of O2 has a volume of 24 litres 44g of CO2 has a volume of 24 litres etc Q. When water is electrolysed it breaks down into hydrogen and oxygen: 2H2O 2H2 + O2 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 (H2) will occupy 24 litres (from the red box above), So 0.66g will occupy 0.66/2 x 24 = 8 litres

Example questions 24/11/2018 What volume of hydrogen is produced when 18g of water is electrolysed? 2H20 2H2 + O2 Marble chips are made of calcium carbonate (CaCO3). What volume of carbon dioxide will be released when 500g of CaCO3 is reacted with dilute hydrochloric acid? CaCO3 + 2HCl CaCl2 + H2O + CO2 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 MgCl2 + H2

A classic exam question: Empirical formulae 24/11/2018 Empirical formulae is simply a way of showing how many atoms are in a molecule (like a chemical formula). For example, CaO, CaCO3, H20 and KMnO4 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 = 0.04 For 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 Fe2O3

Example questions 24/11/2018 Find the empirical formula of magnesium oxide which contains 48g of magnesium and 32g of oxygen. Find the empirical formula of a compound that contains 42g of nitrogen and 9g of hydrogen. Find the empirical formula of a compound containing 20g of calcium, 6g of carbon and 24g of oxygen.

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