JEE Main & Advanced

Atomic, Molecular and Equivalent masses          (1) Atomic mass : It is the average relative mass of atom of element  as compared with an atom of carbon –12 isotope taken as 12.            \[\text{Atomic mass }=\frac{\text{Average mass of an atom}}{\text{1/12}\times \text{Mass of an atom of }{{C}^{12}}\,}\]          Average atomic mass : If an elements exists in two isotopes having atomic masses ‘a’ and ‘b’ in the ratio   m : n, then average atomic mass =\[\frac{(m\times a)+(n\times b)}{m+n}.\] Since the atomic mass is a ratio, it has no units and is expressed in amu, 1 amu = \[1.66\times {{10}^{-24}}g\]. One atomic mass unit (amu) is equal to \[\frac{1}{12}th\] of the mass of an atom of carbon-12 isotope.          Gram atomic mass (GAM) : Atomic mass of an element expressed in grams is called Gram atomic mass or gram atom or mole atom.            (i) Number of gram atoms =\[\frac{\text{Mass of an more...

Important hypothesis (1) Atomic hypothesis : Keeping in view various law of chemical combinations, a theoretical proof for the validity of different laws was given by John Dalton in the form of hypothesis called Dalton's atomic hypothesis. Postulates of Dalton's hypothesis is as followes, (i) Each element is composed of extremely small particles called atoms which can take part in chemical combination. (ii) All atoms of a given element are identical i.e., atoms of a particular element are all alike but differ from atoms of other element. (iii) Atoms of different elements possess different properties (including different masses). (iv) Atoms are indestructible i.e., atoms are neither created nor destroyed in chemical reactions. (v) Atoms of elements take part to form molecules i.e., compounds are formed when atoms of more than one element combine.  (vi) In a given compound, the relative number and kinds of atoms more...

  Laws of chemical combination   Various chemical reactions take place according to the certain laws, known as the Laws of chemical combination. (1) Law of conservation of mass : It was proposed by Lavoisier and verified by Landolt. According to this law, Matter is neither created nor destroyed in the course of chemical reaction though it may change from one form to other.  The total mass of materials after a chemical reaction is same as the total mass before reaction.            (2) Law of constant or definite proportion : It was proposed by Proust. According to this law, A pure chemical compound always contains the same elements combined together in the fixed ratio of their weights whatever its methods of preparation may be. (3) Law of multiple proportion : It was proposed by Dalton and verified by Berzelius. According to this law, When two elements more...

Units for measurement              The chosen standard of measurement of a quantity which has essentially the same nature as that of the quantity is called the unit of the quantity. Following are the important types of system for unit,            (1) C.G.S. System : Length (centimetre), Mass (gram), Time (second)            (2) M.K.S. System : Length (metre), Mass (kilogram), Time (second)            (3) F.P.S. System  : Length (foot), Mass (pound),      Time (second)            (4) S.I. System : The 11th general conference of weights and measures (October 1960) adopted International system of units, popularly known as the SI units. The SI has seven basic units from which all other units are derived called derived units. The standard prefixes which helps to reduce the basic units are now widely used. Dimensional analysis : The seven basic quantities lead to a number of derived quantities such more...

The mole concept            One mole of any substance contains a fixed number \[(6.022\times {{10}^{23}})\] of any type of particles (atoms or molecules or ions) and has a mass equal to the atomic or molecular weight, in grams. Thus it is correct to refer to a mole of helium, a mole of electrons, or a mole of \[N{{a}^{+}}\], meaning respectively Avogadro?s number of atoms, electrons or ions.             \[\therefore \] Number of moles \[=\frac{\text{Weight (grams)}}{\text{Weight of one mole (g/mole)}}\]                                   \[=\frac{\text{Weight}}{\text{Atomic or molecular weight}}\] 

Chemical stoichiometry            Stoichiometry (pronounced “stoy-key om-e-tree”) is the calculation of the quantities of reactants and products involved in a chemical reaction. That means quantitative calculations of chemical composition and reaction are referred to as stoichiometry.            Basically, this topic involves two types of calculations.            (a) Simple calculations (gravimetric analysis) and            (b) More complex calculations involving concentration and volume of solutions (volumetric analysis).            There is no borderline, which can distinguish the set of laws applicable to gravimetric and volumetric analysis. All the laws used in one are equally applicable to the other i.e., mole as well as equivalent concept. But in actual practise, the problems on gravimetric involves simpler reactions, thus mole concept is convenient to apply while volumetric reactions being complex and unknown (unknown simple means that it is not known to you, as it’s not possible for you to remember all possible reactions), equivalent concept more...

Significant figures   In the measured value of a physical quantity, the digits about the correctness of which we are surplus the last digit which is doubtful, are called the significant figures. Number of significant figures in a physical quantity depends upon the least count of the instrument used for its measurement. (1) Common rules for counting significant figures Following are some of the common rules for counting significant figures in a given expression                   Rule 1. All non zero digits are significant.            Example : \[x=1234\] has four significant figures. Again \[x=189\] has only three significant figures.                   Rule 2. All zeros occurring between two non zero digits are significant.            Example : \[x=1007\] has four significant figures. Again \[x=1.0809\] has five significant figures.                       Rule 3. In a number less than one, all zeros to the right of decimal point and to the left of more...