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Chemistry
for Secondary Schools

= 100 g – (69.59 g + 6.09 g) = 24.32 g

Note that the 100 g is arbitrary for simplifying the calculations.
Step 2: Divide the mass of each element in the sample by its R.A.M.
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69.59
Ba ! ! 0.51
137.3

6.09
C = = 0.51
12

24.32
O = = 1.52
16
Step 3: Divide each value by the smallest number.

0.51
= 1
0.51

0.51 = 1
0.51

1.52 = 2.98
0.51
Step 4: Obtain the whole number ratios.

Ba : C : O

1 : 1 : 3
The empirical formula is therefore BaCO .
3

Example 3.8
A compound contains 15.8% carbon and 84.2% sulfur. Calculate its empirical
formula. If its relative molecular mass is 76, what is its molecular formula?

Solution

Step 1: Assume that you have 100 g of the compound, then find the mass of
each element in grams.
C = 15.8 g
S = 84.2 g

64 Student’s Book Form Two

01/08/2025 11:20:56
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 64
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 64 01/08/2025 11:20:56