Everything Science Grade 10

CHAPTER 19. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE 19.2

n=Mm

nC =^39 ,^9 g

12 , 0 g·mol−^1

= 3, 325 mol

nH =^6 ,^7 g

1 , 01 g·mol−^1

= 6, 6337 mol

nO =^53 ,^4 g

16 , 0 g·mol−^1

= 3, 3375 mol

Step 3:Find the empirical formula

C : H : O

3 , 325 : 6 , 6337 : 3 , 3375

1 : 2 : 1

Empirical formula is CH 2 O

Step 4:Find the molecular formula

The molar mass of acetic acid using the empirical formula is 30 , 02 g·

mol−^1. However the question gives the molar mass as 60 , 06 g·mol−^1.

Therefore the actual number of moles of each element must be double

what it is in the empirical formula (^6030 ,,^0602 = 2). The molecular formula

is therefore C 2 H 4 O 2 or CH 3 COOH

Example 11: Waters of crystallisation

QUESTION

Aluminium trichloride (AlCl 3 ) is an ionic substance that forms crystals in the solid

phase. Water molecules may be trapped inside the crystal lattice. We represent this

as: AlCl 3 ·nH 2 O. Carine heated some aluminium trichloride crystals until all the

water had evaporated and found that the mass after heating was 2 , 8 g. The mass

before heating was 5 g. What is the number of moles of water molecules in the

Chemistry: Chemical change 347