rules recap! A small number just talks about the element it is behind. E.g. O2 = 2 x O Small numbers outside the bracket mean multiply everything inside the bracket by that number E.g. (CO3)3 = 3 x C, 9 x O

Ca mass number mass number 40 20 Where do we find this information? The mass number is the top number in the periodic table mass number 40 Ca 20

Carbon is given an Ar of 12 (as it has 6 protons and 6 neutrons). Relative Atomic Mass (Ar) tells us the mass of an atom compared to 1 atom of carbon. Carbon is given an Ar of 12 (as it has 6 protons and 6 neutrons). e.g. Ar of magnesium = Ar of fluorine = Ar of calcium = Blah blah blah

relative formula mass So the Mr for water = 16 + (2x1) = 18 The relative formula mass (Mr) of a compound is the mass of all the elements in the compound added together. EXAMPLE: Water Mass of oxygen = 16 O Mass of hydrogen = 1 H H So the Mr for water = 16 + (2x1) = 18

We can use relative atomic masses to work out the relative formula mass (Mr)/relative molecular mass of a molecule or compound. E.g. NaCl Ar of Na = 23 Ar of Cl = 35.5 So Mr of NaCl = 23 + 35.5 = 58.5 E.g.2 H2O Ar of H = 1 Ar of O = 16 So Mr of H2O = (2 x 1) + 16 = 18

HCl 1 + 35.5 = 36.5 NaOH 23 + 16 + 1 = 40 CaCO3 40 + 12 + (3x16) = 100 Calculate the relative molecular mass Mr / relative formula mass for the following compounds. Show your working HCl NaOH CaCO3 H2SO4 Ca(OH)2 1 + 35.5 = 36.5 23 + 16 + 1 = 40 40 + 12 + (3x16) = 100 (2x1) + 32 + (4x16) = 98 40 + (2x16) + (2x1) = 74

142.5 ADD chemical CONUNDRUMS! PROGRESS CHECK Mr of CO2 Mr of NaCl Mr of MgO 142.5

58 Mr of CuSO4 SUBTRACT Mr of Al2O3 chemical CONUNDRUMS! PROGRESS CHECK Mr of CuSO4 SUBTRACT Mr of Al2O3 58

chemical CONUNDRUMS! PROGRESS CHECK Mr of H2O ADD Mr of ammonia 35

Calculate the relative formula mass of the following compounds (showing your working!): (a) Carbon monoxide CO (b) Carbon dioxide (c) Sulphur dioxide SO2 (d) Calcium carbonate CaCO3 (e) Sodium hydroxide NaOH (f) Sulphuric acid H2SO4 (g) Hydrochloric acid HCl (h) Copper sulphate CuSO4 (i) Magnesium chloride MgCl2 (j) Sodium carbonate Na2CO3 (k) Lead nitrate Pb(NO3)2 (l) Calcium hydroxide Ca(OH)2

How much gold is in your mobile phone? using Mr DEMONSTRATE How much gold is in your mobile phone? Use the procedure to calculate the percentage composition of gold in a mobile phone

PERCENTAGE COMPOSITION PERCENTAGE = MASS OF ELEMENT X 100 Mr OF ENTIRE FORMULA

MARK SCHEME 1. (i) 160 1 (ii) 112 1 (iii) 70 1 do not carry forward errors [3] 2. (a) evidence of Fe = 56, O = 16 gains 1 mark but (56 × 2) + (16 × 3) gains 2 marks but 160 gains 3 marks (b) evidence of 112*/160 gains 1 mark but 70* gains 2 marks * (credit answers consistent with part (a)) [5]

QUESTION 3 Mr for strychnine is (12 x21) + (1x22) + (14 x 2) + (16x2) = 334 The percentage mass of carbon in strychnine is therefore: 252 x 100 = 75.4% 334 This is not the same as the percentage mass of carbon in the white powder – so the white powder is not strychnine.

Calculating percentage composition of compounds Percentage mass of an element in a compound = Atomic mass Ar x Number of atoms x 100 Molecular mass Mr Q. Find the percentage mass of sodium in sodium carbonate Na2CO3 Ar of sodium = Ar of carbon = Ar of oxygen = Mr of Na2CO3 = Percentage mass of an element in a compound =

Find the percentage of oxygen in Fe2O3

Find the percentage of nitrogen in NH4NO3

Theoretical Yield – What you should make We can use masses in a reaction to help us calculate the amount of reactant and product. Step 1 - Write out the equation for the reaction. Make sure it is balanced. Step 2 - Work out the relative masses of the substances needed in the calculation. Remember to multiply by the number of molecules that are present. Step 3 - Convert the relative masses into the units in the question. Step 4 - Find the ratio by dividing both numbers by the smallest relative mass. Step 5 - Find the mass of the unknown by multiplying the mass of the known by the ratio of the unknown.

X 100 Percentage Yield Percentage Yield = Actual Yield Theoretical Yield The Actual Yield is how much you have made from the reaction (from the question). The Theoretical Yield is how much you should make if you have no loss what so ever (100% efficient). X 100