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.