Introduction
In the year 1801, Robert Boyle and Charles had deduced the gas laws and proved them. They had confirmed that equal volumes of all the gases behave similarly, under similar conditions of temperature and pressure and the volumes of reacting gases and their gaseous products are in ratio of small whole number. In 1811 Amedeo Avogadro was able to explain Boyle's and Charles' Law on the assumption that equal volumes of different gases, under the same conditions of temperature and pressure, contain the same number of particles.
Gay Lussac's Law
In 1808, Gay Lussac discovered a relationship between the volume of gases that react produce gaseous products, at constant temperature and pressure. He formulated a law, which is named after him.
Verification of Gay Lussac's Law
Two volumes of hydrogen and one volume of oxygen combine to form two volumes of steam.
Problems Based on Gay Lussac's Law
Carbon monoxide burns in oxygen to give carbon dioxide, as per the equation:
Avogadro's Law
Amedeo Avogadro introduced the term "molecule" and distinguished it from 'atom'. According to Avogadro, particles in the gaseous state do not exist as atoms but as molecules. In 1811 he proposed his famous hypothesis, now known as 'Avogadro's Law'.
Stoichiometric Terms
According to Avagadro the smallest particle of an element, which may or may not have independent existence, is called an atom, while the smallest particle of a substance, which is capable of independent existence, is called a molecule. Molecules are classified as homoatomic and heteroatomic. Homoatomic molecules are made up of the atoms of the same element while heteroatomic molecules are made up of the atoms of different elements.
Atomicity of Gases
Atomicity is the number of atoms of an element present in one molecule of that element. They can be monoatomic (He), diatomic (H2), triatomic (O3) and polyatomic (S8).
Relative Atomic Mass
An atom or a molecule is a very minute particle; the actual mass of an atom is extremely small. The mass of atoms extends from 1.67 x 10-24 g (hydrogen) to 4.0 x 10-22 g only. It is physically not possible to measure such small and light masses easily. In 1811, Avagadro suggested that atomic and molecular masses could be expressed on an atomic mass scale relative to that of a standard reference atom. In practice, a relative measurement scale with the mass of an atom of hydrogen as the reference unit had been set up, to compare the masses of other atoms. Since 1961, instead of an atom of hydrogen, 
th the mass of a carbon 
atom is accepted as the unit for comparison.
Gram Atomic Mass
The relative atomic mass of an element expressed in grams, is known as the gram atomic mass of that element.
Relative Molecular Mass
Relative molecular mass shows the number of times a molecule is denser than a hydrogen atom or one amu. It is expressed as the ratio of the mass of a molecule of a substance to the mass of one amu (or to one atom of hydrogen).
Gram Molecular Mass
Gram molecular mass of a substance is the relative molecular mass of the substance, expressed in grams.
Chemical Formulae
Every chemical substance is known by a specific name. But many a times these names are cumbersome, confusing and do not provide information about its chemical composition. To overcome this, each chemical compound is represented by a chemical formula that gives its composition (constituent elements present) and the number of elements of each type present. There are two types of chemical formula. They are molecular formula and empirical formula.
Applications of Avogadro's Law
Avogadro's law has been useful in substantiating a number of important laws and concepts. It has helped in the following areas:
* In explaining Gay Lussac's law of gaseous volumes.
* In determining the atomicity of gases.
Determining the Atomicity of Gases
Atomicity of Hydrogen and Chlorine
Let up take the example of the same reaction between hydrogen and chlorine. We saw earlier that;
Mole Concept
In chemistry the term mole represents a pile or mass of atoms, molecules, ions or electrons. Just as common man measures quantity in terms of kilograms or dozens, a chemical scientist deals with a 'mole' of atoms, molecules, ions or electrons.
Problems Based on Mole Concept
How many moles of sodium hydroxide are present in 3200 g of it?
(Na=23, O=16, H=1)
Molar Volume
Molar volume is the volume occupied by one mole of any gas at a definite pressure and temperature. It is denoted by Vm. Molar volume of the substance depends on temperature and pressure. The unit of molar volume is litre per mol or millilitre per mol.
Percentage Composition
Percentage composition is the percentage by mass of the atoms of an element present in one mole of a compound.
Empirical Formula from Percentage Composition
Empirical formula of a compound is a formula that shows the simplest whole number ratio between the atoms of the elements in the compound.
Problems Based on Determination of Empirical Formula
A substance was found to have the following percentage composition;
Ca = 40% C =12%, O = 48%. Calculate its empirical formula. (Relative atomic mass of Ca=40, C=12, O=16).
Molecular Formula from Percentage Composition
Molecular Formula of a substance gives the names and number of atoms of the various elements present and the molecular mass of it.
Problems Based on Determination of the Molecular Formula
An organic compound has the following percentage composition: C= 40%, H = 6.66%, O = 53.33%. Find its molecular formula (Relative atomic mass of C=12, H=1, O=16 and Relative molecular mass of the compound = 180).
Calculations Based on Chemical Equations
In a chemical reaction there is a rearrangement of particles. This rearrangement of particles, results in drastic changes in the properties of the substances. For e.g., sodium and chlorine are very reactive elements. When they combine, an electron is shifted from the sodium atom to the chlorine atom. An electron has an insignificant mass. Yet this tiny particle brings about a tremendous change in the properties of the particles. The properties of the compound sodium chloride are entirely different from those of either sodium or chlorine.
Information from a Chemical Equation
A chemical equation gives information regarding the following:
a) Molecular proportion of the reactants and the products.
b) The relative mass of the reactants and the products.
c) The relative volumes of gaseous substances - which may be reactants or products.
The above can be illustrate by the equation:
Problems Based on Mass - Mass Relationship
Calculate the mass of lead chloride formed, by treating an aqueous solution of 6.62g of lead nitrate, with excess of hydrochloric acid.
(Relative atomic mass of: Pb=207, Cl=35.5, H=1, O=16, N=14)
Problems Based on Mass - Volume Relationship
Calculate the volume of carbon dioxide formed at STP in 'ml' by the complete thermal decomposition of 3.125 g of pure calcium carbonate (Relative atomic mass of Ca=40, C=12, O=16).
Summary
Molecular mass is the average relative mass of a molecule of the substance as compared with mass of an atom of carbon (C-12) taken as 12 a.m.u.
