1. Chemicals of the Natural Environment.
Lesson 9

2. Learning objective: To calculate how much metal can be extracted from its mineral.
Must: Be able to calculate the relative formula mass for any compound.
Should: Be able to calculate the mass of a metal that can be extracted form its mineral.
Should: Be able to write balanced symbol equations.
Keywords: Atomic mass, relative formula mass, percentage composition, reactants and products.
Starter: What do these numbers mean

3. counting atoms NaOH H2O CaCO3 H2SO4 Ca(OH)2 CONNECT
What is the total number of atoms in the following formulae?
NaOH
H2O
CaCO3
H2SO4
Ca(OH)2

4. 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

5. Relative atomic masses

6. Relative atomic masses
Atoms of different elements have different masses
The relative atomic mass (Ar) is the mass of the elements compared to the mass of a hydrogen atom
Relative atomic masses

7. Relative Formula Mass
The relative formula mass (Mr) is the relative mass of a molecule compared to the mass of a hydrogen atom
Relative formula mass

8. Calculating relative formula mass.
Top Tips.
1. The relative atomic mass is usually at the top – it is always the highest number.
2. When calculating the relative formula mass,
a) Write out all the different atoms (Capital letter).
b) Write down the number of each atom.
c) Multiply the number of each type of atom by its atomic mass.
d) Add all the masses together.

9. An example. Calculate the RFM – for calcium carbonate. Formula CaCO3
Type, number and atomic mass of each atom;
Ca x 1 x 40 = 40
C x 1 x 12 = 12
O x 3 x 16 = 48
Total = 100 – Relative formula mass of CaCO3.

10. Relative formula mass
Relative formula mass

11. Relative formula mass, Mr
18/07/2019
The relative formula mass of a compound is blatantly the relative atomic masses of all the elements in the compound added together.
Relative atomic mass of O = 16
E.g. water H2O:
Relative atomic mass of H = 1
Therefore Mr for water = 16 + (2x1) = 18
Work out Mr for the following compounds:
HCl
NaOH
MgCl2
H2SO4
K2CO3
H=1, Cl=35 so Mr = 36
Na=23, O=16, H=1 so Mr = 40
Mg=24, Cl=35 so Mr = 24+(2x35) = 94
H=1, S=32, O=16 so Mr = (2x1)+32+(4x16) = 98
K=39, C=12, O=16 so Mr = (2x39)+12+(3x16) = 138

12. More examples CaCO3 40 + 12 + 3x16 100 HNO3 1 + 14 + 3x16 2MgO
18/07/2019
CaCO3
x16
100
HNO3
x16
2MgO
2 x ( ) 80
3H2O
3 x ((2x1) + 16)
4NH3
2KMnO4
3C2H5OH
4Ca(OH)2

13. 142.5 ADD chemical CONUNDRUMS! PROGRESS CHECK Mr of CO2 Mr of NaCl
Mr of MgO
142.5

14. 58 Mr of CuSO4 SUBTRACT Mr of Al2O3 chemical CONUNDRUMS!
PROGRESS CHECK
Mr of CuSO4
SUBTRACT
Mr of Al2O3 58

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

16. Calculating percentage mass
18/07/2019
If you can work out Mr then this bit is easy…
Percentage mass (%) =
Mass of element Ar
Relative formula mass Mr
x100%
Calculate the percentage mass of magnesium in magnesium oxide, MgO:
Ar for magnesium = 24 Ar for oxygen = 16
Mr for magnesium oxide = = 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

17. Calculating the mass of a metal that can be extracted from its mineral.
How much Fe could you get from 100kg of Fe2O3?
There are 2 steps;
1. Calculate the percentage mass (composition) for the element in this case Fe.
2. Multiply the percentage mass (from step 1) by the mass of the mineral

18. An example. Percentage composition of Fe in Fe2O3
Mass of Fe in Fe2O3 = 2 x 56 =112
Mass of Fe2O3 = 2 x x 16 = 160
Percentage Fe in Fe2O3 = 112 x 100 = 70%
160
Mass of Fe in 100 kg of Fe2O3 = 70 x 100
= 70 kg
Click here for more examples of percentage composition problems.

19. 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

20. More problems. 1. What mass of Al could be made from 1 tonne of Al2O3?
2. What mass of Na could be made from 2 tonnes of NaCl?
3. The main ore of chromium is FeCr2O4. What is the percentage mass of Cr in FeCr2O4?
tonnes of copper ore are dug out of the ground. Only 1% of this is the pure mineral, CuFeS2. What mass of the Cu could be made from 1000 tonnes of the ore?

21. Conservation of Mass
Show that mass is conserved in this reaction. ZnCO3 ZnO + CO2 You need to use the ideas of Mr

22. Simple chemical reactionsMg O H Cl
Magnesium oxide +
Hydrochloric acid
Magnesium chloride +
Water
MgO +
2HCl
MgCl2 +
H2O Mg O Cu S
Magnesium +
Copper sulphate
Copper +
Magnesium sulphate Mg +
CuSO4 Cu +
MgSO4

23. Balancing reaction equations
This sequence of slides includes two approaches to balancing reaction equations. Individual slides may be copied elsewhere or used as templates.
1 Using symbols and no text
2 Using text prompts to explain each step of the process
click here
click here
To return to this page, click the button to the top right of any slide.

24. 2
Mg + O2  MgO 2 
Symbols Mg + O2

25. 2
Al I2  AlI3 3 1½ 2 
Symbols Al + I2

26. Write a balanced equation
sodium hydroxide is ionic: what are the formulae of its ions?
sodium hydroxide is ionic: what are the formulae of its ions?
Na+ and OH-
sodium + water  sodium hydroxide + hydrogen
It’s balanced!
Check the equation is balanced: count the number of reactant and product elements
Add numbers in front of the ringed formula so that the total number of reactant symbols = total number of product symbols
Write the formula of each substance
as it is ringed above
Reactants: 2Na 4H 2O
Products: 2Na 4H 2O 2 Na
+ H2O 2
 NaOH 2
+ H2

27. References
First example, with symbols only: Frank Harriss of Malvern College
Second example, with text prompts: Steve Lewis

28. Some examples 2 2 3 2 3 Mg + O2 Zn + HCl Fe + Cl2 NaOH + HCl CH4 + O2
Mg O2
Zn HCl
Fe Cl2
NaOH HCl
CH O2
Ca H2O
NaOH H2SO4
CH3OH O2
MgO
ZnCl H2
FeCl3
NaCl H2O
CO H2O
Ca(OH) H2
Na2SO H2O 2

29. Balancing equations
Balancing equations