YOU WILL NEED YOUR PERIODIC TABLE

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Presentation transcript:

YOU WILL NEED YOUR PERIODIC TABLE relative formula mass 15/11/2018 Learning Objective: Understand how to work out Mr of a compound Learning Outcomes: Calculate Mr of a given formula (B) Calculate percentage composition of an element in a compound (A) YOU WILL NEED YOUR PERIODIC TABLE

counting atoms NaOH H2O CaCO3 H2SO4 Ca(OH)2 CONNECT What is the total number of atoms in the following formulae? Write answers in your notebook NaOH H2O CaCO3 H2SO4 Ca(OH)2

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 ACTIVATE 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

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

calculate the mr 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

Complete the exam style questions Question 1 = GRADE B CONSOLIDATE Complete the exam style questions Question 1 = GRADE B Question 2 = GRADE A Question 3 = GRADE A*

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.

relative formula mass Learning Objective: 15/11/2018 Learning Objective: Understand how to work out Mr of a compound Learning Outcomes: Calculate Mr of a given formula (B) Calculate percentage composition of an element in a compound (A)