Everything Science Grade 11

6.5 CHAPTER 6. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE

Example 11: Determining the formula of a compound

QUESTION

207 g of lead combineswith oxygen to form 239 g of a lead oxide. Use this information to

work out the formula ofthe lead oxide (atomic masses: Pb = 207 u and O = 16 u).

SOLUTION

Step 1 : Calculate the mass of oxygen in the reactants

239 − 207 = 32 g

Step 2 : Calculate the number of moles of lead and oxygen in the reactants.

n =

m

M

Lead

207

207

= 1 mol

Oxygen

32

16

= 2 mol

Step 3 : Deduce the formula ofthe compound

The mole ratio of Pb:Oin the product is 1:2, which means that for every

atom of lead, there willbe two atoms of oxygen. The formula of the compound

is PbO 2.

Example 12: Empirical and molecular formula

QUESTION

Vinegar, which is used in our homes, is a diluteform of acetic acid. A sample of acetic acid

has the following percentage composition: 39.9%carbon, 6.7% hydrogenand 53.4% oxygen.

1. Determine the empirical formula of acetic acid.


2. Determine the molecular formula of acetic acid if the molar mass of acetic acid is 60 g·
   mol−^1.