1. The atomic number tells us:
mass number
The atomic number tells us:
the number of protons in the nucleus of an atom.
The mass number tells us:
the mass of the atom (protons + neutrons).
atomic number
Sometimes, scientists want to find out the total mass of an element or a compound.
The total mass of an element is called the RELATIVE ATOMIC MASS (Ar).
The total mass of a compound is called the RELATIVE FORMULA MASS (Mr).

2. Task – 3 minutes!
05/11/2019
Find the relative atomic masses from the periodic table (inside front cover of your workbook):
C H
O S
Ca Al
Cl Fe N 12 1 16 32 40 27
35.5 56 14

3. Relative formula mass, Mr
The relative formula mass of a compound is 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

4. More examples CaCO3 40 + 12 + (3x16) 100 HNO3 1 + 14 + (3x16) 63 2MgO80
3H2O
3 x ((2x1) + 16) 54
4NH3
4x (14+(3x1)) 68
2KMnO4
2x (39+55+(16x4))
316
3C2H5OH
3x((2x12)+(5x1)+16+1))
138
Mg(OH)2
24+ ((16+1)x2) 58

5. Calculate the Percentage by mass
CO2 (Oxygen) %
Relative Formula Mass of CO2 = 12 + (2x16) = 44g/mol.
In CO2 there’s 2x Oxygen: 2x16 = 32.
To find percentage by mass: (32/44) x 100
= 72.7%

6. Calculate the Percentage by mass
CO2 (Oxygen) %
CaCl2 (Calcium) 36%
H2SO4 (Sulfur) %
Al2O3 (Aluminium) %
Fe(NO3)3 (Oxygen) %

7. Conservation of mass in reactions
In any reaction the total mass of products is the same as the total mass of the reactants
Example 1 – Magnesium oxide and hydrochloric acid Mg O H Cl
1 x magnesium, 1 x oxygen, 2 x hydrogen and 2 x chlorine atoms
Also 1 x magnesium, 1 x oxygen, 2 x hydrogen and 2 x chlorine atoms
Example 2 – Burning methane C H O

8. Calculating the mass of a product
E.g. what mass of magnesium oxide is produced when 60g of magnesium is burned in air?
Step 1: READ the equation:
2Mg + O MgO
ignore
Reacting ratio
2 Mg
makes
2 MgO
Step 2: WORK OUT the relative formula masses (Mr):
2Mg = 2 x 24 = MgO = 2 x (24+16) = 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

9. 1g of CaCl2 produced by (73/111)= 0.66
2) What mass of hydrochloric acid is needed to produce 11.1g calcium chloride?
Ca(OH)2 + 2HCl CaCl H2O
ignore
ignore
reacting ratio
2 HCl
makes
1 CaCl2
Mr: 2HCl = 2 x ( ) CaCl2 = 40 + (2 x 35.5)
= = 111
111g CaCl2 produced by 73g HCl
1g of CaCl2 produced by (73/111)= 0.66
so 11.1g produced by 0.66 x 11.1 = 7.3g of HCl

10. Mass of H2 and Cl2 needed to make 73g of HCl?
05/11/2019
Example…
H2 + Cl2  HCl
Mass of H2 and Cl2 needed to make 73g of HCl?
Atom
Number on Left
Number on Right H 2 1 Cl

11. Mass of H2 and Cl2 needed to make 73g of HCl?
05/11/2019
Example from sheet
H2 + Cl2  2HCl
Mass of H2 and Cl2 needed to make 73g of HCl?
Atom
Number on Left
Number on Right H 2 Cl

12. Example from sheet H2 + Cl2  2HCl 05/11/2019
Reactant/Product
Rel. Formula Mass (Mr)
Molar Ratio
Mr x Molar Ratio H2
2x1 = 2g/mol 1
2g/mol
Cl2
35.5x2 = 71g/mol
71g/mol
HCl
= 36.5g/mol 2
2 x 36.5g/mol = 73g/mol
What if I wanted to make only 7.3g of HCl?

13. Example from sheet H2 + Cl2  2HCl 05/11/2019
Reactant/Product
Molar Ratio
Mr x Molar Ratio H2 1
2g/mol
2 x 0.1 = 0.2g
Cl2
71g/mol
71 x 0.1 = 7.1g
HCl 2
2 x 36.5g/mol = 73g/mol
7.3/73 = 0.1
What if I wanted to make only 7.3g of HCl?
Divide the mass you want by (Mr x Molar Ratio) to find a factor to multiply by!

14. Workbook Qs!
05/11/2019 Pg