Formulas

Relative mass formula, atomic mass, and empirical formula Visit www.teacherpowerpoints.com For 100’s of free powerpoints

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: ,[object Object],[object Object],[object Object],[object Object],[object Object],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

More examples 4Ca(OH) 2 3C 2 H 5 OH 2KMnO 4 4NH 3 3 x ((2x1) + 16) 3H 2 O 80 2 x (24 + 16) 2MgO 1 + 14 + 3x16 HNO 3 100 40 + 12 + 3x16 CaCO 3

Relative atomic mass ,[object Object],[object Object],[object Object],[object Object],[object Object]

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% ,[object Object],[object Object],[object Object],[object Object],[object Object],Percentage mass (%) = Mass of element A r Relative formula mass M r x100%

Calculating the mass of a product E.g. what mass of magnesium oxide is produced when 60g of magnesium is burned in air? ,[object Object],[object Object],[object Object],[object Object],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

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],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 ,[object Object],[object Object],[object Object],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

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 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?

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 ,[object Object],[object Object],[object Object],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?

Example questions ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

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

Example questions ,[object Object],[object Object],[object Object]

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