1. 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

2. 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

3. 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

4. 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

5. 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

6. calculate the mr for the following compounds show your working
HCl
NaOH
CaCO3
H2SO4
Ca(OH)2
= 36.5
= 40
(3x16) = 100
(2x1) (4x16) = 98
40 + (2x16) + (2x1) = 74

7. 142.5 ADD chemical CONUNDRUMS! PROGRESS CHECK Mr of CO2 Mr of NaCl
Mr of MgO
142.5

8. 58 Mr of CuSO4 SUBTRACT Mr of Al2O3 chemical CONUNDRUMS!
PROGRESS CHECK
Mr of CuSO4
SUBTRACT
Mr of Al2O3 58

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

10. 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*

11. MARK SCHEME
1. (i) (ii) (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]

12. 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.

13. 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)