Everything Science Grade 10

CHAPTER 19. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE 19.4

Example 16: Stoichiometric calculation 2

QUESTION

What mass of iron (II) sulphide is formed when 5 , 6 g of iron is completely reacted

with sulphur?

SOLUTION

Step 1:Write the balanced equation

Fe (s)+S (s)→FeS (s)

Step 2:Calculate the number of moles

We find the number of moles of the given substance:

n=mM=^5 ,^6 g

55 , 8 g·mol−^1

= 0, 1 mol

Step 3:Find the mole ratio

We find the mole ratio between what was given and what you are

looking for. From the equation 1 mol of Fe gives 1 mol of FeS. Therefore,

0 , 1 mol of iron in the reactants will give 0 , 1 mol of iron sulphide in the

product.

Step 4:Find the mass of iron sulphide

m=n×M= 0, 1 mol× 87 , 9 g·mol−^1 = 8, 79 g

The mass of iron (II) sulphide that is produced during this reaction is

8 , 79 g.

Theoretical yield ESAGG

When we are given a known mass of a reactant and are asked to work out how much prod-
uct is formed, we are working out the theoretical yield of the reaction. In the laboratory,
chemists almost never get this amount of product. In each step of a reaction a small amount

Chemistry: Chemical change 355