1. C2 3 How Much?
3.1 – 3.4 A Recap

2. O 16 8 C2 3.1 Mass Numbers Mass Number Proton Number Particle Mass
The
Mass Number is
MASSIVE!
Mass Number
Proton Number
How many protons in oxygen?
How many neutrons?
How many electrons? O 8 16
Particle
Mass
Proton 1
Neutron
Electron - 8 8 8

3. C2 3.2 Masses of Atoms & Moles6 12
Ar = Relative Atomic Mass
Relative to Carbon-12
Ar = usually the same as mass
number, accounts for isotopes.
Mr = Relative Formula Mass
Add the Ar of each atom
Mole = Relative Atomic Mass in grams
1 mole = Ar or Mr in grams
1 mole always contains 6.02x1023 particles
NaCl
= 58.5

4. C2 3.3 Percentages & Formulae
We can use Ar and Mr to work out the percentage of an element in a compound
In MgO, how much is Mg, how much is O?
Magnesium Ar = 24; Oxygen Ar = 16
Magnesium Oxide Mr = 40
(mass of Mg/total) x100
(24/40) x 100
= 60%
What is the percentage mass of hydrogen in methane, CH4?
x100
Mass of element in question
Total mass of compound
C=12, H=1
4/16=0.25
X100 = 25%

5. C2 3.3 Percentages & Formulae
Working out the formula of a compound from its percentage composition
aka the compounds Empirical Formula
If 9g Aluminium reacts with 35.5g Chlorine what is the empirical formula of the compound formed? Al 13 27
e.g. Water Hydrogen Peroxide
Molecular Formula H2O H2O2
Empirical Formula H2O HO 9g
27g
35.5g
Simplest whole number ratio:
3xCl, 1xAl
Al:
= 0.33 of a mole
Cl:
= 1 mole
AlCl3
Mass of 1 mole
Mass of 1 mole

6. C2 3.4 Equations & Calculations
We often need to know how much of each substance is involved in a chemical reaction.
Firstly, eqations must be balanced:
H2 + O2 → H2O
Not balanced
H2 + O2 → 2H2O
Still not balanced
2H2 + O2 → 2H2O
Balanced

7. C2 3.4 Equations & Calculations
Now, we can calculate the reacting masses:
E.g. sodium hydroxide reacts with chlorine to make bleach:
If we have a solution containing 100g NaOH, how much chlorine gas will we need?
2NaOH Cl → NaOCl NaCl H2O
Sodium hydroxide Chlorine Bleach Salt Water
We need 2 moles of NaOH for every 1 mole of Cl2
2NaOH Cl2
Cl2
35.5 x2
= 71
100g NaOH
40 (1 mole)
2.5 2
35.5 x2
= 2.5 moles
= 1.25 moles
= 40
= 71
So we will need:
= 81.75g
Of Chlorine!
1.25 moles x 71g

8. Questions: How many protons would there be in:
Li b) N c) Ne d) S e) Br
How many neutrons do the above have?
Mg + F2 → MgF2
How many moles of flourine react with 1 mole of magnesium atoms?
What is the Mr of MgF2?
How many times heavier is a magnesium atom than an oxygen atom?
What is the percentage mass of nitrogen in the fertiliser Ammonium Nitrate (NH4NO3)?
22.55% of the mass of a sample of phosphorus chloride is phosphorous. What is the formula of phosphorous chloride?
Hydrogen peroxide decomposes to form water & oxygen, write a balanced equation for this reaction.
Calcium reacts with oxygen: 2Ca + O2 → 2CaO
What mass of oxygen will react exactly with 60g of calcium?
1a)3; b)7; c)10; d) 16; e)35 / 2a)4; b)8; c)12; d)17; e)44 / 3a)1; b)62 / 4)1.5 times heavier / 5) 35% / 6) PCl3 / 7) H2O2 -> 2H2O + O2 / 8)24g

9. Answers a)3; b)7; c)10; d) 16; e)35 a)4; b)8; c)12; d)17; e)44
1.5 times heavier
35%
PCl3
H2O2 → 2H2O + O2
24g