Page 70 - Chemistry_Form_2

Chemistry
for Secondary Schools

Example 3.6

What is the empirical formula for a compound of mass 8.1 g if it consists of 4.9 g
of magnesium and 3.2 g of oxygen?
Solution
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Step 1: Obtain the mass of each element in the compound. These are already given:
Mass of magnesium = 4.9 g

Mass of oxygen = 3.2 g

Step 2: Divide the mass of each element by its R.A.M.
Magnesium, 4.9 =0.20
24
Oxygen, 3.2 = 0.20
16
Step 3: Divide by the lowest quotient.
Mg : O
0.20 : 0.20

0.20 0.20

Step 4: Obtain their whole number ratios directly or by approximation.
Mg : O

1 : 1

The empirical formula is MgO.

Example 3.7
Given that a certain compound is 69.59% barium, 6.09% carbon and the rest is
oxygen. Calculate the empirical formula of this compound.
Solution

Step 1: Assuming that you have 100 g of the compound, then the mass of each
element will be:

Ba = 69.59 g
C = 6.09 g

The mass of oxygen will be 100 g – (mass of barium + mass of carbon)

Student’s Book Form Two 63

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CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 63