# Basic Concepts of Chemistry

Law of Definite Proportion: We know that mass is conserved. If we were to take a sample of a pure compound it will always contain the same elements combined in the same proportion by mass.

Law of Multiple Proportion: If one element can combine with another to form more than one compound then the mass ratios of the elements in the compounds are simple whole-number ratios of each other.

Law of Definite Proportion by Volume: The volumes of reactants and products, in chemical reactions, are related to each other by simple whole-numbers. Of course the measurements must be taken at the same temperature and pressure.

Law of Reciprocal Proportions: The ratio by mass, in which two elements combine with the fixed mass of a third element, is either the same or a simple multiple of the ratio in which they combine with each other.

Avogardo's Law: Equal volumes of all gases contain equal number of molecules under similar conditions of temperature and pressure.

One Atomic Mass Unit (amu): It is a mass unit equal to exactly one twelfth the mass of a carbon 12 atom.

Formula Mass of a Substance: It is the sum of the atomic masses of all atoms in a formula unit of a compound.

Molar Mass: The mass of 1 mole of any substance.

Molar Volume (Vm): The volume of 1 mole of the substance.

Mole (m):

• It is a number which is equal to 6.023 x 1023 .

• It is the amount of substance which contains as many entities (atoms, molecules, ions or other particles) as there are atoms in exactly 12 grams of Carbon 12 isotope.

• It is also known as Avogadro's Constant.

Number of Mole Method:

Number of moles of a substance n = w (in gms)/m

also n = volume (in lt) at NTP/22.4lt

also n = M x vol in lt (where M is Molarity = number of moles of solute per lt of solution)

M= (w/m) x (1000/ Vm )

Number of mole method is used only in the case of balanced chemical equations.

The coefficients of balanced equations represent the ratios in terms of number of moles in which the reactants react or products are formed.

Number of particles(ions, atoms or molecules) in a given number of moles = number of moles x 6.023 x 1023 .

Equivalent Weight of an Element (E): It is defined as the number of parts by weight of the element which combine with or displace from a compund 1 part by weight of Hydrogen, 8 parts by weight of Oxygen or 35.5 parts by weight of Chlorine.

E = M ( Molar Mass) / nf (n factor)

where:

nf = Valency in case of an atom

= Total positive or negative oxidation number of an atom in a molecule

= Basicity or Acidity.

= Change in oxidation number in case of a redox reacton.

Number of Gram Equivalents:

= weight in gms / E (Equivalent wt)

= Normality x vol in lt

Normality = (w/E) x (1000/Vm )

Number of Equivalents Method: In this method you need not balance the chemical equation. The basic principle is that the equivalents of each reactant which have dissapeared are equal to the number of equivalents of each of the products formed. The working tool for this is equivalent weight.

Number of equivalents = W in gms / Eq Wt = (W x nf ) / M

Dulong and Petit's Law:

Atomic wt (approx) = 6.4/specific heat(in calories)