REVISION - Chemical Reactions - Calculations

Types of questions:
1. Molar mass calculation
2. Percentage composition
3. mass-mass reaction calculation.

1. Molar mass calculation.
Using the mass (big) numbers off the periodic table the mass of a sunstance can be calculated.
Exampe:
CaCO3
1. Total how many of each atom there are in the formula
Ca = 1
C = 1
O = 3

2. Multiply this amount by the mass number of that atom off the periodic table.
Ca = 1 x 40 = 40
C = 1 x 12 = 12
O = 3 x 16 = 48

3. Add all of these totals together to get the molas mass.
40+ 12+ 48 = 100

2. Percentage composition

This the percentage by mass of an element in a compound.
Once you have calculated the molar mass of a substance you can calculate what percentage each element contributes to the whole mass.

Example; CaCO3
Ca = 1 x 40 = 40
C = 1 x 12 = 12
O = 3 x 16 = 48
Total = 100

% Ca = 40/100 x 100
%Ca = 40%

%C = 12 / 100 x 100
%C = 12%

%O = 48/100 x 100
%O = 48%

3. Mass-Mass calculations
If you know the balanced equation for a reaction and you know either how much reactant is used or how much product is used you can use this information to calculate the mass of other chemicals involved.

Example:

2Mg + O2 --> 2MgO.

Molar mass of 2Mg = 2x24 = 48
Molar mass of O2 = 2x16 = 32
Molar mass of 2MgO = 2(24+16) = 60

The balanced equation tells us that:
2 Mg's react with 1 O2 to make 2 MgO's
OR Using masses:
48g Mg reacts with 32g O2 to make 60g of MgO.

Question:
But what if we only start with 10g of Mg, how much MgO will we make?

What we know:

24g Mg + 32g O2 --> 60g MgO
10g Mg --> ? MgO


10 / 24 X 60
= 25g MgO produced.

REVISION -Chemical reactions - Types

Types of chemical reactions.
The only types you need to know for this standard is:
Thermal decomposition
Oxidation - Reduction
Precipitation

Steps:
1. Look for any reaction that has only one reactant. This will be a Thermal decomposition reaction.
2. Any reaction that has two solutions going to a solid and a solution OR a reaction where all of the partners are swappping is a Precipitation reaction.
3. Any remaining reaction will be a Oxidation - Reduction reaction

Example:
Classify the reactions below:
(a) C (s) + O2 (g) → CO2 (g)
(b) Ca(OH)2 (s) → CaO (s) + H2O (g)
(c) Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)
(d) CuCl2 (aq) + K2CO3 (aq) → CuCO3 (s) + 2KCl (aq)

Step 1: Thermal decomposition:
(b) Ca(OH)2 (s) → CaO (s) + H2O (g)

Step 2: Precipitation:
(d) CuCl2 (aq) + K2CO3 (aq) → CuCO3 (s) + 2KCl (aq)

Step 3: Oxidation - Reduction
(a) C (s) + O2 (g) → CO2 (g)
(c) Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)