# 162022 Relative mass formula atomic mass and empirical

- Slides: 13

1/6/2022 Relative mass formula, atomic mass, and empirical formula www. assignmentpoint. com

Relative formula mass, Mr 1/6/2022 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: Relative atomic mass of O = 16 Relative atomic mass of H = 1 Therefore Mr for water = 16 + (2 x 1) = 18 Work out Mr for the following compounds: 1) HCl H=1, Cl=35 so Mr = 36 2) Na. OH Na=23, O=16, H=1 so Mr = 40 3) Mg. Cl 2 Mg=24, Cl=35 so Mr = 24+(2 x 35) = 94 4) H 2 SO 4 H=1, S=32, O=16 so Mr = (2 x 1)+32+(4 x 16) = 98 5) K 2 CO 3 K=39, C=12, O=16 so Mr = (2 x 39)+12+(3 x 16) = 138 www. assignmentpoint. com

More examples Ca. CO 3 40 + 12 + 3 x 16 HNO 3 1 + 14 + 3 x 16 2 Mg. O 2 x (24 + 16) 3 H 2 O 3 x ((2 x 1) + 16) 4 NH 3 2 KMn. O 4 3 C 2 H 5 OH 4 Ca(OH)2 www. assignmentpoint. com 1/6/2022 100 80

Relative atomic mass 1/6/2022 • 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. 5 x 35)+(24. 5 x 37))/(75. 5+24. 5)=35. 5 • Try this: neon-20 (90. 9%), neon-21 (0. 3%), and neon-22 (8. 8%) www. assignmentpoint. com

Calculating percentage mass 1/6/2022 If you can work out Mr then this bit is easy… Percentage mass (%) = Mass of element Ar Relative formula mass Mr x 100% Calculate the percentage mass of magnesium in magnesium oxide, Mg. O: 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: 1) Hydrogen in hydrochloric acid, HCl 2) Potassium in potassium chloride, KCl 3) Calcium in calcium chloride, Ca. Cl 2 4) Oxygen in water, H 2 O www. assignmentpoint. com

Calculating the mass of a product 1/6/2022 E. g. what mass of magnesium oxide is produced when 60 g of magnesium is burned in air? Step 1: READ the equation: 2 Mg + O 2 2 Mg. O IGNORE the oxygen in step 2 – the question doesn’t ask for it Step 2: WORK OUT the relative formula masses (Mr): 2 Mg = 2 x 24 = 48 2 Mg. O = 2 x (24+16) = 80 Step 3: LEARN and APPLY the following 3 points: 1) 48 g of Mg makes 80 g of Mg. O 2) 1 g of Mg makes 80/48 = 1. 66 g of Mg. O www. assignmentpoint. com 3) 60 g of Mg makes 1. 66 x 60 = 100 g of Mg. O

1/6/2022 1) When water is electrolysed it breaks down into hydrogen and oxygen: 2 H 2 O 2 H 2 + O 2 What mass of hydrogen is produced by the electrolysis of 6 g of water? Work out Mr: 2 H 2 O = 2 x ((2 x 1)+16) = 36 2 H 2 = 2 x 2 = 4 1. 36 g of water produces 4 g of hydrogen 2. So 1 g of water produces 4/36 = 0. 11 g of hydrogen 3. 6 g of water will produce (4/36) x 6 = 0. 66 g of hydrogen 2) What mass of calcium oxide is produced when 10 g of calcium burns? 2 Ca + O 2 Mr: 2 Ca = 2 x 40 = 80 2 Ca. O = 2 x (40+16) = 112 80 g produces 112 g so 10 g produces (112/80) x 10 = 14 g of Ca. O 3) What mass of aluminium is produced from 100 g of aluminium oxide? 2 Al 2 O 3 4 Al + 3 O 2 Mr: 2 Al 2 O 3 = 2 x((2 x 27)+(3 x 16)) = 204 www. assignmentpoint. com 4 Al = 4 x 27 = 108 204 g produces 108 g so 100 g produces (108/204) x 100 = 52. 9 g of Al 2 O 3

Another method 1/6/2022 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: 2 H 2 O 2 H 2 + O 2 What mass of hydrogen is produced by the electrolysis of 6 g of water? Mass of product IN GRAMMES 6 g 4 36 So mass of product = (4/36) x 6 g = 0. 66 g of hydrogen www. assignmentpoint. com

Calculating the volume of a product 1/6/2022 At normal temperature and pressure the Relative Formula Mass (M r) of a gas will occupy a volume of 24 litres e. g. 2 g of H 2 has a volume of 24 litres 32 g of O 2 has a volume of 24 litres 44 g of CO 2 has a volume of 24 litres etc Q. When water is electrolysed it breaks down into hydrogen and oxygen: 2 H 2 O 2 H 2 + O 2 What VOLUME of hydrogen is produced by the electrolysis of 6 g of water? • On the previous page we said that the MASS of hydrogen produced was 0. 66 g • 2 g of hydrogen (H 2) will occupy 24 litres (from the red box above), • So 0. 66 g will occupy 0. 66/2 x 24 = 8 litres www. assignmentpoint. com

Example questions 1/6/2022 1) What volume of hydrogen is produced when 18 g of water is electrolysed? 2 H 20 2 H 2 + O 2 2) Marble chips are made of calcium carbonate (Ca. CO 3). What volume of carbon dioxide will be released when 500 g of Ca. CO 3 is reacted with dilute hydrochloric acid? Ca. CO 3 + 2 HCl Ca. Cl 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 1 g of magnesium with excess acid? www. assignmentpoint. com Mg + 2 HCl Mg. Cl 2 + H 2

Empirical formulae 1/6/2022 Empirical formulae is simply a way of showing how many atoms are in a molecule (like a chemical formula). For example, Ca. O, Ca. CO 3, H 20 and KMn. O 4 are all empirical formulae. Here’s how to work them out: A classic exam question: Find the simplest formula of 2. 24 g of iron reacting with 0. 96 g 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: www. assignmentpoint. com 2 iron atoms for 3 oxygen atoms means the formula is Fe 2 O 3

Example questions 1/6/2022 1) Find the empirical formula of magnesium oxide which contains 48 g of magnesium and 32 g of oxygen. 2) Find the empirical formula of a compound that contains 42 g of nitrogen and 9 g of hydrogen. 3) Find the empirical formula of a compound containing 20 g of calcium, 6 g of carbon and 24 g of oxygen. www. assignmentpoint. com

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