1. Relative mass formula, atomic mass, and empirical formula
2. 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: <ul><li>HCl </li></ul><ul><li>NaOH </li></ul><ul><li>MgCl 2 </li></ul><ul><li>H 2 SO 4 </li></ul><ul><li>K 2 CO 3 </li></ul>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 = 16 Relative atomic mass of H = 1
3. More examples CaCO 3 40 + 12 + 3x16 100 HNO 3 1 + 14 + 3x16 2MgO 2 x (24 + 16) 80 3H 2 O 3 x ((2x1) + 16) 4NH 3 2KMnO 4 3C 2 H 5 OH 4Ca(OH) 2
4. Relative atomic mass <ul><li>The mass of an isotopic element relative to Carbon-12. </li></ul><ul><li>Example: chlorine occurs in isotope forms Cl-35 (75.5%) and Cl-37 (24.5%) </li></ul><ul><li>Relative atomic mass = </li></ul><ul><li>((75.5x35)+(24.5x37))/(75.5+24.5)=35.5 </li></ul><ul><li>Try this: neon-20 (90.9%), neon-21 (0.3%), and neon-22 (8.8%) </li></ul>
5. 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 = 24 Ar for oxygen = 16 M r for magnesium oxide = 24 + 16 = 40 Therefore percentage mass = 24/40 x 100% = 60% <ul><li>Calculate the percentage mass of the following: </li></ul><ul><li>Hydrogen in hydrochloric acid, HCl </li></ul><ul><li>Potassium in potassium chloride, KCl </li></ul><ul><li>Calcium in calcium chloride, CaCl 2 </li></ul><ul><li>Oxygen in water, H 2 O </li></ul>Percentage mass (%) = Mass of element A r Relative formula mass M r x100%
6. Calculating the mass of a product E.g. what mass of magnesium oxide is produced when 60g of magnesium is burned in air? <ul><li>Step 3: LEARN and APPLY the following 3 points: </li></ul><ul><li>48g of Mg makes 80g of MgO </li></ul><ul><li>1g of Mg makes 80/48 = 1.66g of MgO </li></ul><ul><li>60g of Mg makes 1.66 x 60 = 100g of MgO </li></ul>Step 2: WORK OUT the relative formula masses (M r ): 2Mg = 2 x 24 = 48 2MgO = 2 x (24+16) = 80 Step 1: READ the equation: 2Mg + O 2 2MgO IGNORE the oxygen in step 2 – the question doesn’t ask for it
7. <ul><li>Work out M r : 2H 2 O = 2 x ((2x1)+16) = 36 2H 2 = 2x2 = 4 </li></ul><ul><ul><li>36g of water produces 4g of hydrogen </li></ul></ul><ul><ul><li>So 1g of water produces 4/36 = 0.11g of hydrogen </li></ul></ul><ul><ul><li>6g of water will produce (4/36) x 6 = 0.66g of hydrogen </li></ul></ul><ul><ul><li>M r : 2Ca = 2x40 = 80 2CaO = 2 x (40+16) = 112 </li></ul></ul><ul><ul><li>80g produces 112g so 10g produces (112/80) x 10 = 14g of CaO </li></ul></ul>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 <ul><li>When water is electrolysed it breaks down into hydrogen and oxygen: </li></ul><ul><li>2H 2 O 2H 2 + O 2 </li></ul><ul><li>What mass of hydrogen is produced by the electrolysis of 6g of water? </li></ul>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. Another method Try using this equation: Mass of product IN GRAMMES 6g 4 36 So mass of product = (4/36) x 6g = 0.66g of hydrogen
9. 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 <ul><li>On the previous page we said that the MASS of hydrogen produced was 0.66g </li></ul><ul><li>2g of hydrogen (H 2 ) will occupy 24 litres (from the red box above), </li></ul><ul><li>So 0.66g will occupy 0.66/2 x 24 = 8 litres </li></ul>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?
10. Example questions <ul><li>What volume of hydrogen is produced when 18g of water is electrolysed? </li></ul><ul><ul><li>2H 2 0 2H 2 + O 2 </li></ul></ul><ul><li>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? </li></ul><ul><ul><li>CaCO 3 + 2HCl CaCl 2 + H 2 O + CO 2 </li></ul></ul><ul><li>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? </li></ul><ul><ul><li>Mg + 2HCl MgCl 2 + H 2 </li></ul></ul>
11. 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 = 0.04 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 Fe 2 O 3
12. Example questions <ul><li>Find the empirical formula of magnesium oxide which contains 48g of magnesium and 32g of oxygen. </li></ul><ul><li>Find the empirical formula of a compound that contains 42g of nitrogen and 9g of hydrogen. </li></ul><ul><li>Find the empirical formula of a compound containing 20g of calcium, 6g of carbon and 24g of oxygen. </li></ul>
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