The law of constant proportions which is also
known as the law of definite proportions was stated by Proust in 1799 as “In a chemical substance the elements are always
present in definite proportions by mass”.
[The Law of constant proportions is the 6th
postulate of Dalton's atomic theory. The relative number and kinds of atoms are
constant in a given compound.]
E.g. In a compound such as water, the ratio of the mass
of hydrogen to the mass of oxygen is always 1:8, whatever the source of water.
Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are
always obtained.
if the element ‘A’ and ‘B’ combine chemically to form the
compound AB, then in whatever manner AB is formed, it is always composed of
same two elements ‘A’ and ‘B’ combined together in the same fixed ratio or
proportion by mass.
For example: Sulphur dioxide can be obtained b following sources:
For example: Sulphur dioxide can be obtained b following sources:
(i). Sulphur is burnt in
air,
S + O2 ---------® SO2
(ii). Copper is heated with conc. sulphuric acid
Cu + 2H2SO4 ---------® CuSO4 + 2H2O + SO2
(iii). Dilute hydrochloric acid is added to sodium bisulphate
NaHSO3 + HCl ----------® NaCl + H2O + SO2
In each case, the ratio of sulphur and oxygen in the sulphur dioxide obtained is of 32: 32 or 1: 1 by mass.
S + O2 ---------® SO2
(ii). Copper is heated with conc. sulphuric acid
Cu + 2H2SO4 ---------® CuSO4 + 2H2O + SO2
(iii). Dilute hydrochloric acid is added to sodium bisulphate
NaHSO3 + HCl ----------® NaCl + H2O + SO2
In each case, the ratio of sulphur and oxygen in the sulphur dioxide obtained is of 32: 32 or 1: 1 by mass.
Question: Define one mole
The number
of atoms present in 12 grams of carbon 12 which is equal to 6.023 X 1023
. This is also known as Avogadro's constant.
One mole is defined as the amount of
substance of a system which contains as many atoms, molecules and ions
as there are atoms in 12 grams of carbon 12".
Question: State the law of conservation of mass.
2.8 g of nitrogen gas was allowed to react with 0.6 g of hydrogen gas to produce 3.4 g of ammonia. Show that these observations are in agreement with the law of conservation of mass. State the law of conservation of mass.
Nitrogen + Hydrogen --------------- > Ammonia
Mass of reactants before experiment = Mass of nitrogen + mass of hydrogen
= 2.8 g + 0.6 g = 3.4 g
Mass of product after experiment = 3.4 g of ammonia As the mass of reactants (3.4 g) is equal to the mass of product (3.4 g), therefore, these masses are in agreement with the law of conservation of mass.
According to this law of conservation of mass “mass can neither be created nor can it be destroyed in a chemical reaction”.
Nitrogen + Hydrogen --------------- > Ammonia
Mass of reactants before experiment = Mass of nitrogen + mass of hydrogen
= 2.8 g + 0.6 g = 3.4 g
Mass of product after experiment = 3.4 g of ammonia As the mass of reactants (3.4 g) is equal to the mass of product (3.4 g), therefore, these masses are in agreement with the law of conservation of mass.
According to this law of conservation of mass “mass can neither be created nor can it be destroyed in a chemical reaction”.
Question: Magnesium and oxygen combine in the ratio of 3 : 2 by mass to form magnesium oxide. How much oxygen is required to react completely with 12 g of magnesium?
According to the law of constant proportion “In a pure substance same elements are always present in a definite proportion by weight”.
3 g of magnesium reacts with oxygen = 2 g
12 g of magnesium reacts with oxygen = 12 X 2/3 = 8g
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