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- 1. C5 GCSE Revision Reacting Masses 09/05/11
- 2. Atomic mass 09/05/11 He 2 4 SYMBOL PROTON NUMBER = number of protons (obviously) RELATIVE ATOMIC MASS, A r (“Mass number”) = number of protons + number of neutrons
- 3. Relative formula mass, M r 09/05/11 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
- 4. More examples 09/05/11 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
- 5. Calculating percentage mass 09/05/11 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 09/05/11 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. 09/05/11 <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 09/05/11 Try using this equation: Mass of product IN GRAMMES 6g 4 36 So mass of product = (4/36) x 6g = 0.66g of hydrogen 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?

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