Law of Constant Proportion
As we know, compounds are composed of two or more elements. The proportion in which elements are present in a compound remains the same, irrespective of its method of preparation.
For example, pure water obtained from any source (well, river, lake, or sea) and from any country (India, Russia, America, etc.) will always contain two hydrogen atoms and one oxygen atom. Hydrogen and oxygen respectively combine together in the ratio of 1:8 by mass to form water. The ratio by the number of atoms for water will always be H : O = 2 : 1. Thus, 18 g of water contains 2 g of hydrogen and 16 g of oxygen.Similarly, in ammonia, nitrogen makes up 14/17 of the mass of ammonia while hydrogen makes up the remaining 3/17 of the mass. Irrespective of the source from which ammonia is obtained, it will always contain nitrogen and hydrogen in the ratio of 14: 3 by mass. Thus, 17 g of ammonia contains 14 g of nitrogen and 3 g of hydrogen, and 34 g of ammonia contains 28 g of nitrogen and 6 g of hydrogen. This led to the law of constant proportion.
Thus, according to the law of constant proportion, a chemical substance always contains the same elements in a fixed proportion by mass, irrespective of its source.
The law of constant proportion is also known as the law of definite proportion. This law, which was introduced by Proust, stated that ‘in a compound, the elements are always present in definite proportions by mass’.
Laws of Chemical Combination and Dalton’s Theory
The next challenge for the scientists was to come up with proper explanations for these laws. This was undertaken by a British chemist, John Dalton. He picked up the idea of divisibility of matter and said that ‘atoms are the smallest particles of matter, which cannot be divided further’. He provided a theory based upon the laws of chemical combinations and gave a successful explanation for the two laws.
The postulates of Dalton’s atomic theory are as follows.
All matter is made up of very tiny particles. These particles are called atoms.
An atom cannot be divided further i.e., atoms are indivisible.
Atoms can neither be created nor destroyed in a chemical reaction.
All atoms of an element are identical in all respects, including the mass, chemical properties, etc.
Atoms of different elements have different masses and chemical properties.
Atoms of different elements combine in small whole number ratios to form compounds.
In a given compound, the relative number and types of atoms are constant.
Explanation of the Law of Chemical Combination using Dalton’s atomic theory:
The law of conservation of mass: Matter is made up of atoms (postulate 1), which can neither be created or destroyed (postulate 3). Hence, matter can neither be created nor destroyed.
For example, 100 g of mercuric oxide, when heated in a closed test tube, decomposes to produce 92.6 g of mercury and 7.4 g of oxygen gas.
2 HgO (s) → 2Hg (l) + O2
Total mass of the reactant = 100 g
Total mass of the products = 92.6 + 7.4 g = 100 g
Hence, during the decomposition reaction, matter is neither created nor destroyed. Here, matter is made up of tiny mercuric (Hg) and oxygen (O) atoms. The given reaction shows that atoms can neither be created nor destroyed in chemical reactions.
The law of constant proportion: This law follows directly from the 6th and 7th postulates of Dalton’s atomic theory, which state that atoms of different elements combine in small whole number ratios to form a compound; and in a given compound, the relative number and types of atoms are constant.
Now, we know that a sample of carbon dioxide (no matter how it is prepared) is made up of carbon and oxygen. One carbon atom and two oxygen atoms combine to form a molecule of carbon dioxide. Thus, it obeys the law of constant proportion. The mass of carbon dioxide is 44 g. The mass of one oxygen and carbon atom is 16 u and 12 u respectively. Thus, in carbon dioxide, carbon and oxygen combine in the ratio of 3: 8 by mass.
t is difficult to determine the mass of an individual atom. However, relative atomic mass can be determined by comparing the mass of a particular atom with that of an atom of standard reference. The unit of the mass of an atom is atomic mass unit (amu). Initially,John Dalton suggested that the mass of a hydrogen atom be taken as a standard reference of atomic mass unit. Later, one-sixteenth of the mass of an oxygen atom was taken as the standard. In 1961, IUPAC (International Union of Pure and Applied Chemistry) adopted one-twelfth of the mass of a carbon-12 isotope as the standard unit to measure relative atomic masses. IUPAC named this unit as the unified atomic mass unit (u).
Thus, Atomic mass unit mass of a C-12 atom
1 u mass of a C-12 atom
Hence, the relative atomic mass of the atom of an element is defined as the average mass of the atom as compared to one-twelfth the mass of one carbon-12 units. For example, the atomic mass of oxygen is 16 u.
The atomic mass of some common elements is given in the following table.
Some more examples are given below to demonstrate the method of writing chemical formulae of compounds.
(a) Formula of calcium oxide
Symbol Ca O
Charge 2+ 2−
Thus, the chemical formula of calcium oxide is CaO..
(b)Formula of aluminium sulphate
Symbol Al SO4
Charge 3+ 2−
Thus, the formula of aluminium sulphate is Al2(SO4)3. Brackets are used when we have two or more of the same ions in a formula. Here, the bracket with a subscript 3 indicates that three sulphate groups are joined to two aluminium atoms.
The chemical formula can give us a lot of information about the compound. Let us discuss what all information a chemical formula can give us, taking an example of carbon dioxide. The chemical formula of carbon dioxide is CO2. Using this formula, we get the following information about carbon dioxide:
1. Two elements are present in CO2. They are carbon(C) and oxygen (O).
2. CO2 represents one molecule of carbon dioxide.
3. The valency of Carbon is twice than that of oxygen.
4. CO2 is a neutral molecule. It has no charge.
5. The ratio by mass between carbon and oxygen C: O is 12:32 i.e. 3:8.