Everything Science Grade 11

CHAPTER 6. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE 6.8

Example 17: Stoichiometric calculation 1

QUESTION

What volume of oxygenat S.T.P. is needed for the complete combustionof 2dm^3 of propane

(C 3 H 8 )? (Hint: CO 2 and H 2 O are the products in this reaction (and in all combustion reac-

tions))

SOLUTION

Step 1 : Write a balanced equation for the reaction.

C 3 H 8 (g) + 5O 2 (g)→ 3 CO 2 (g) + 4H 2 O(g)

Step 2 : Determine the ratio ofoxygen to propane thatis needed for the reaction.

From the balanced equation, the ratio of oxygento propane in the reactants is

5:1.

Step 3 : Determine the volumeof oxygen needed for the reaction.

1 volume of propane needs 5 volumes of oxygen, therefore 2 dm^3 of propane

will need 10 dm^3 of oxygen for the reaction to proceed to completion.

Tip

A closer look at the

worked example (stoi-

chiometric calculation 2)

shows that 5.6 g of iron

is needed to produce

8.79 g of iron (II) sul-

phide. The amount of

sulphur that is needed

in the reactants is 3.2

g. What would happen

if the amount of sulphur

in the reactants was in-

creased to 6.4 g but the

amount of iron was still

5.6 g? Would more FeS

be produced? In fact,

the amount of iron(II)

sulphide produced re-

mains the same. No

matter how much sul-

phur is added to the sys-

tem, the amount of iron

(II) sulphide will not in-

crease because there is

not enough iron to re-

act with the additional

sulphur in the reactants

to produce more FeS.

When all the iron is used

up the reaction stops. In

this example, the iron

is called the limiting

reagent. Because there

is more sulphur than can

be used up in the reac-

tion, it is called the ex-

cess reagent.

Example 18: Stoichiometric calculation 2

QUESTION

What mass of iron (II) sulphide is formed when 5.6 g of iron is completelyreacted with sulphur?

SOLUTION

Step 1 : Write a balanced chemical equation for the reaction.

Fe(s) + S(s)→ FeS(s)

Step 2 : Calculate the number of moles of iron that react.

n =

m

M

=

5. 6

55. 85

= 0. 1 mol

Step 3 : Determine the numberof moles of FeS produced.

From the equation 1 mole of Fe gives 1 mole of FeS. Therefore, 0.1 molesof iron

in the reactants will give0.1 moles of iron sulphide in the product.

Step 4 : Calculate the mass of iron sulphide formed