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In 1803, A British school teacher John Dalton provided the basic theory about the nature of matter which provides explanation for the law of conservation of mass and the law of definite proportions. According to Dalton�s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms The postulates of this theory may be stated as follows:  (i) All matter is made of very tiny particles called atoms. (ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction. (iii) Atoms of a given element are identical in mass and chemical properties. (iv)  Atoms of different elements have different masses and chemical properties. (v)  Atoms combine in the ratio of small whole numbers to form compounds. (vi) The relative number and kinds of atoms are constant in a given compound. Drawbacks of Dalton�s Atomic Theory: �  The atom is further subdivided into protons, neutrons and electrons. �  T...

MOLE CONCEPT [CBSE Class IX Chemistry] While performing a reaction, to know the number. of atoms (or) molecules involved, the concept of mole was introduced. The quantity of a substance is expressed in terms of mole. Definition of mole:  Mole is defined as the amount of substance that contains as many specified elementary particles as the number of atoms in 12g of carbon-12 isotope. One mole is also defined as the amount of substance which contains Avogadro number (6.023 x 10 23 ) of particles. MORE TO KNOW Avogadro number: Number of atoms or molecules or ions present in one mole of a substance is called Avogadro number. Its value is 6.023x10 23 . Therefore, one mole of any substance = 6.023 x 10 23 particles may be atoms, molecules, ions For example: One mole of oxygen atoms represents 6.023 x 10 23 atoms of oxygen and 5 moles of oxygen atoms contain 5 x 6.023x10 23 atoms of oxygen. Questions based on mole concept: 1. When the mass of the substance...

How and why elements combine and what happens when they combine. Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination. (a) The Law of conservation of mass was stated by Antoine L. Lavoisier in 1785 as� Mass can neither be created nor destroyed in a chemical reaction� [The Law of conservation of mass is the 2 nd  postulate of Dalton's atomic theory. It states that Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.] Example: �   Water forms by the union of hydrogen and oxygen. If we weigh the reactants (hydrogen and oxygen) before the chemical reaction, we find the weight of the product (water) equal to the combined weight of reactants. �   The weight of iron increases on rusting. The increase in weight is equal to the weight of oxygen added to iron. �   Carbon combines with Sulphur to form Carbon disulphide. The mass of reactants i.e. carbon and sulphur is...

Empirical formula  : The empirical formula is the simplest formula for a  compound  in which atoms of different elements are present in simple ratio. It shows the  relative  number of atoms of each element. For example CH 2 O is the empirical formula of Glucose C 6 H 12 O 6 Calculating Empirical Formula from Percentage Composition Assume 100g of sample Convert all percentages to a mass in grams, eg, 21% = 21g, 9% = 9g Find the relative atomic mass (r.a.m) of each element present using the  Periodic Table Calculate the  moles  of each element present: n = mass � r.a.m Divide the moles of each element by the smallest of these to get a mole ratio If the numbers in the mole ratio are all whole numbers (integers) convert this to an empirical formula If the numbers in the mole ratio are NOT whole numbers, you will need to further manipulate these until the  mole ratio is a ratio of whole numbers (integers) A compound is found to contain 47.25% copp...