Title: Relative mass formula, atomic mass, and empirical formula
1 Relative mass formula atomic mass and empirical formula 2 Relative formula mass Mr 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 H2O Therefore Mr for water 16 (2x1) 18 Work out Mr for the following compounds
H1 Cl35 so Mr 36 Na23 O16 H1 so Mr 40 Mg24 Cl35 so Mr 24(2x35) 94 H1 S32 O16 so Mr (2x1)32(4x16) 98 K39 C12 O16 so Mr (2x39)12(3x16) 138 3 More examples 4 Relative atomic mass
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
Try this neon-20 (90.9) neon-21 (0.3) and neon-22 (8.8)
5 Calculating percentage mass If you can work out Mr then this bit is easy 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
6 Calculating the mass of a product E.g. what mass of magnesium oxide is produced when 60g of magnesium is burned in air Step 2 WORK OUT the relative formula masses (Mr) 2Mg 2 x 24 48 2MgO 2 x (2416) 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
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
Mr 2Ca 2x40 80 2CaO 2 x (4016) 112 80g produces 112g so 10g produces (112/80) x 10 14g of CaO Mr 2Al2O3 2x((2x27)(3x16)) 204 4Al 4x27 108 204g produces 108g so 100g produces (108/204) x 100 52.9g of Al2O3 8 Another method Try using this equation Mass of product IN GRAMMES 4 6g 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 (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
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
10 Example questions
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
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 CaCO3 H20 and KMnO4 are all empirical formulae. Heres 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 0.040.06 is equivalent to 23 Step 3 Write the formula 2 iron atoms for 3 oxygen atoms means the formula is Fe2O3 12 Example questions
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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