13.3 CHAPTER 13. PHYSICAL AND CHEMICAL CHANGE
a water molecule, we see that94%of the mass of a water molecule is accounted for by
oxygen and the remaining6%is the mass of hydrogen. This mass proportion will be the
same for any water molecule.
This does not mean that hydrogen and oxygen always combine in a2 : 1ratio to form
H 2 O. Multiple proportions are possible. For example, hydrogen and oxygen may combine
in different proportions to form H 2 O 2 rather than H 2 O. In H 2 O 2 , the H:O ratio is1 : 1
and the mass ratio of hydrogen to oxygen is1 : 16. This will be the same for any molecule
of hydrogen peroxide.
Investigation: Law of constant composition
Aim: To investigate the ratio in which compounds combine.
Apparatus:- 0 , 1 M silver nitrate
(AgNO 3 ) - 0 , 1 M sodium chloride
(NaCl) - 0 , 1 M lead nitrate (PbNO 3 )
- 0 , 1 M sodium iodide (NaI)
- 0 , 1 M iron (III) chloride
(FeCl 3 ) - 0 , 1 M sodium hydroxide
(NaOH) - 9 large test tubes
- 3 propettes
Method:
Reaction 1:Prepare three test tubes with 5 ml, 10 ml and 15 ml of
silver nitrate respectively. Using a clean propette add 5 ml of sodium
chloride to each one and observe what happens.
Reaction 2:Prepare three test tubes with 5 ml, 10 ml and 15 ml of
lead nitrate respectively. Using a clean propette add 5 ml of sodium io-
dide to each one and observe what happens. Write a balanced equation
for this reaction.
Reaction 3:Prepare three test tubes with 5 ml, 10 ml and 15 ml of
sodium hydroxide respectively. Add 5 ml of iron(III) chloride to each
one and observe what happens.
Discussion and conclusion: Regardless of the amount of reactants
added, the same products, with the same compositions, are formed (i.e.
the precipitate observed in the reactions). However, if the reactants are
not added in the correct ratios, there will be unreacted reactants that
226 Chemistry: Chemical change