Everything Science Grade 11

CHAPTER 6. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE 6.5

Example 10: Determining the empirical formula of a compound

QUESTION

A compound contains 52.2% carbon (C), 13.0%hydrogen (H) and 34.8%oxygen (O). Deter-

mine its empirical formula.

SOLUTION

Step 1 : If we assume that wehave 100 g of this substance, then we can convert each

element percentage intoa mass in grams.

Carbon = 52. 2 g, hydrogen = 13. 0 g and oxygen = 34. 8 g

Step 2 : Convert the mass of each element into number of moles

n =

m

M

Therefore,

n(carbon) =

52. 2

12. 01

= 4. 35 mol

n(hydrogen) =

13. 0

1. 008

= 12. 90 mol

n(oxygen) =

34. 8

16

= 2. 18 mol

Step 3 : Convert these numbers to the simplest mole ratio by dividing by thesmallest

number of moles

In this case, the smallestnumber of moles is 2.18. Therefore:

Carbon

4. 35

2. 18

= 2

Hydrogen

12. 90

2. 18

= 6

Oxygen

2. 18

2. 18

= 1

Therefore the empiricalformula of this substance is: C 2 H 6 O.