FORM FOUR SS1 CHEMISTRY

1. WEEK: 5
TOPIC: Symbols, Formulae and
Equations.
Laws of chemical combinations
The Dalton’s atomic theory provided a satisfactory
basis for the gravimetric laws of chemical
combinations by mass. His postulates gave rise to
these three laws:
1.Law of conservation of matters
2.Law of constant composition
3.Law of multiple proportions.

2. Law of conservation of
mass
The law was suggested by a French chemist
Lavoisier (1774). It states that: During the
course of a chemical reaction, matter is
neither created nor destroyed.

3. Experiment to verify the
law of conservation of
matter (mass)
The equation of the chemical reaction chosen for
study is as follows;
• Silver nitrate + sodium chloride → Silver chloride + Sodium trioxonitrate (v)
(White precipitate)
Method:
1.Put some sodium chloride solution in a conical
flask
2.Fill a small test tube with silver trioxonitrate (v)
solution of string, suspend it in a conical flask as
shown below:

4. 3.Insert the stopper and weight the whole apparatus
on a balance, note the mass of the whole system.
4.Mix the two liquids by pulling the string attached to
the bottom end of the small test tube.
5.Weigh the whole apparatus

5. • Expected Result: Within the limit of
experimental error, the total mass before
mixing (mass of reactants) and after mixing
(mass of products) will be equal. The same set-up
can be used to illustrate the law by the use of
precipitation reactions involving aqueous solutions
of barium chloride and dilute tetraoxosulphate (iv)
acid, or solutions of potassium iodide and lead(II)
trioxonitrate(V).
NOTE: reactions in which a solid or a gas is a
reactant cannot be used to demonstrate this law.

6. • CONCLUSION: Since there is no
overall change in mass when the
products are formed, we can infer that
matter is neither created nor destroyed
during the chemical reaction.
This is because it is difficult to weigh gaseous
reactants, while reactions involving solid reactants
are relatively slow.

7. Explanation of the law of conservation of
mass by the Dalton's atomic theory
when a reaction occurs in a closed vessel, all the
atoms present before the reaction are still present
after the reaction.Hence,the total mass of the
reactants will be equal to the total mass of the
products.
Consequently, chemical equations must be
balanced in order to conform to the law of
conservation of mass.

8. SummarySummary
 Matter is anything that has mass and occupies space.
 Mass reactants ═ Mass products
Checkpoint1.
•Mention another compound that could be used instead of silvertrioxo-
nitrate(v) with sodium chloride
•State the law of conservation of mass/matter.

9. Law Of Definite Proportion or
Constant Composition
• The law was suggested by Proust (1799). It states
that:
• All pure samples of the same compound contain the
same elements combined in the same proportion by
mass. Or
• All pure samples of the same compound have the
same percentage composition by mass.

10. It based on the fact that when elements combine to
form a given compound, they do so in fixed
proportions by mass, so that all pure samples of
that compound are identical in composition by
mass. Water for example: chemical analyses
showed that as long as it is pure, its composition is
always in the ratio of one mole of oxygen to two
moles of hydrogen. i.e. 32g of O to 4g of H.
Irrespective of whether the water comes from river,
sea, rain or anywhere.

11. e.g. H2O = 2H atoms = 1g by mass = 2g He.g. H2O = 2H atoms = 1g by mass = 2g H
1 O atom = 16g by mass =1 O atom = 16g by mass = 16g O16g O
18gs of H18gs of H22OO
2H : O2H : O
2 : 162 : 16
1 : 81 : 8
Checkpoint 2
Two samples that contains only carbon and oxygen
are compound. A 10.0g sample of substance A
contains 2.73g of carbon. 2.00g of substance B
contains 0.545g of carbon. Are they the same
compound?

12. SOLUTIONSOLUTION
A 2.73g × 100 ═ 27.3% C
10.0g 1
B 0.545 × 100 ═ 27.25% C
2.00g 1
Therefore,
% of oxygen = 100 – 27.3 = 72.70%

13. Sample A Sample B
Mass of substances 10.0g 2.00g
Mass of carbon 2.73g 0.545g
Mass of oxygen 7.27g 1.455g
% carbon 2.73g × 100
10.0g 1
═ 27.3% C
0.545 × 100
2.00g 1
═ 27.25% C
Checkpoint 2
Two samples that contains only carbon and oxygen are
compound. A 10.0g sample of substance A contains 2.73g
of carbon. 2.00g of substance B contains 0.545g of carbon.
Are they the same compound?