1. Theory_English

[Audio] CHEMISTRY For JEE INTRODUCTION TO CHEMISTRY Atomic hypothesis: Keeping in view various laws of chemical combination, 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 are as follows: (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 in other words, atoms of a particular element are all alike but differ from atoms of other elements. (iii) Atoms of different elements possess different properties (including different masses). (iv) Atoms are indestructible in other words, atoms are neither created nor destroyed in chemical reactions. (v) Atoms of elements combine to form molecules and compounds are formed when atoms of more than one element combine. (vi) In a given compound, the relative number and kind of atoms is constant. Modern atomic hypothesis: The main modifications made in Dalton's hypothesis as a result of new discoveries about atoms are : (i) Atom is no longer considered to be indivisible. (ii) Atoms of the same element may have different atomic weights. E.g. isotopes of oxygen O-1-6--, O-1-7 and O18. (iii) Atoms of different element may have same atomic weights. E.g. isobars Ca40 and Ar40. (iv) Atom is no longer indestructible. In many nuclear reactions, a certain mass of the nucleus is converted into energy along with α, β and γ rays. (v) Atoms may not always combine in simple whole number ratios. E.g. in sucrose (C12H22O11), the elements carbon, hydrogen and oxygen are present in the ratio of 12 : 22 : 11 and the ratio is not a simple whole number ratio. Atomic & Molecular masses: Atomic mass: It is the average relative mass of atom of element as compared with times the mass of an atom of carbon-12 isotope. Average atomic mass: If an element exists in two isotopes having atomic masses 'a' and 'b' in the ratio m : n, then average atomic mass = . Atomic mass is expressed in amu. 1 amu = 1.66 × 10–24 grams One atomic mass unit (amu) is equal to of the mass of an atom of carbon-12 isotope. Gram atomic mass (G-A-M--): Atomic mass of an element expressed in grams is called Gram atomic mass or gram atom or mole atom. (i) Number of gram atoms = (ii) Mass of an element in g = No. of gram atoms × G-A-M.

[Audio] CHEMISTRY For JEE INTRODUCTION TO CHEMISTRY (iii) Number of atoms in 1 GAM = 6.02 × 1023 Number of atoms in a given substance = No. of gram atoms × 6.02 × 1023 = × 6.02 × 1023 (iv) Number of atoms in 1 grams of element = (v) Mass of one atom of the element (in g) = Molecular mass: Molecular mass of a molecule, of an element or a compound may be defined as a number which indicates how many times heavier is a molecule of that element or compound as compared with of the mass of an atom of carbon-12. Molecular mass is also expressed in amu. Actual mass of one molecule = Mol. mass (in amu) × 1.66 × 10–24 grams Molecular mass of a substance is the additive property and can be calculated by adding the atomic masses of atoms present in one molecule. Gram molecular mass (G-M-M--): Molecular mass of an element or compound when expressed in g is called its gram molecular mass, gram molecule or mole molecule. Number of gram molecules = Mass of substance in g = No. of gram molecules × GMM Average atomic mass and molecular mass (Average atomic mass) = (Average molecular mass) = Where A1, A2, A3 …… are atomic mass of species 1, 2, 3,…. et cetera with % as X1, X2, X3 …… et cetera Similar terms are for molecular masses. The Mole Concept One mole of any substance contains a fixed number 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 any ion, meaning respectively Avogadro's number of atoms, electrons or ions. Number of moles = = Note : 1 mole = 1 g-atom = 1 g-molecule = 1 g-ion. Properties of Gases The state of matter in which the molecular forces of attraction between the particles of matter are minimum, is known as gaseous state. It is the simplest state and shows great uniformity in behaviour. Characteristics of gases (1) Gases or their mixtures are homogeneous in composition. Page| 2.

[Audio] CHEMISTRY For JEE INTRODUCTION TO CHEMISTRY (2) Gases have very low density due to negligible intermolecular forces. (3) Gases have infinite expansibility and high compressibility. (4) Gases exert pressure. (5) Gases possess high diffusibility. (6) Gases do not have definite shape and volume like liquids. (7) Gaseous molecules move very rapidly in all directions in a random manner in other words, gases have highest kinetic energy. (8) Gaseous molecules collide with one another and also with the walls of container. (9) Gases can be liquefied, if subjected to low temperatures & high pressures. (10) Thermal energy of gases >> molecular attraction. (11) Gases undergo similar change with the change of temperature and pressure. In other words, gases obey certain laws known as gas laws. Measurable properties of gases The characteristics of gases are described fully in terms of four parameters or measurable properties: (i) The volume, V, of the gas. (ii) Its pressure, P (iii) Its temperature, T (iv) The amount of the gas (in other words, mass or number of moles). (1) Volume : (i) Since gases occupy the entire space available to them, the measurement of volume of a gas only requires a measurement of the container confining the gas. (ii) Volume is expressed in litres (L), millilitres (mL) or cubic centimetres (cm3), cubic metres (m3). (iii) 1 liters = 1000 mL; 1 mL = 10–3 L; 1 liters = 1 decimeters3 = 10–3 cube meters 1 cube meters = 103 decimeters3 = 106 centimeters3 = 106 mL = 103 liters (2) Mass : (i) The mass of a gas can be determined by weighing the container in which the gas is enclosed and again weighing the container after removing the gas. The difference between the two weights gives the mass of the gas. (ii) The mass of the gas is related to the number of moles of the gas in other words moles of gas (n) (3) Temperature : (i) Gases expand on increasing the temperature. If temperature is increased twice, the square of the velocity (v2) also increases two times. (ii) Temperature is measured in centigrade degree (ºC) or celsius degree with the help of thermometers. Temperature is also measured in Fahrenheit (ºF). (iii) S I unit of temperature is kelvin (K) or absolute degree. K = ºC plus 273 (iv) Relation between ºF and ºC is (4) Pressure : (i) Pressure of the gas is the force exerted by the gas per unit area of the walls of the container in all directions. Thus, Pressure (P) (ii) Pressure exerted by a gas is due to kinetic energy of the molecules. Kinetic.

[Audio] CHEMISTRY For JEE INTRODUCTION TO CHEMISTRY 1Pa = 1 Nm–2 = 1 kilogramsm–1s–2 (vi) C G S unit of pressure is dynes cm–2. (vii) M K S unit of pressure is Newton m–2. The unit Newton m–2 is sometimes called pascal (Pa). (viii) Higher unit of pressure is bar, kPa or MPa. 1 bar = 105 Pa = 105 Nm–2 = 100 KNm–2 = 100 KPa (ix) Several other units used for pressure are, Name Symbol Value bar bar 1 bar = 105 Pa atmosphere atm 1 atm = 1.01325 × 105 Pa Torr Torr 1 Torr = Pa = 133.322 Pa millimetre of mercury mm Hg 1 millimeters Hg = 133.322 Pa Ideal Gas Equation PV = nRT where, P : Pressure of gas ; V : Volume of gas ; n = Number of moles of gas T : Temperature of gas ; R : Universal gas constant. Values of R : 0.082 LatmK–1mol–1 ; 8.314 JK–1mol–1 ; 1.987 CalK–1mol–1 Prefixes used in the SI System Multiple Prefix Symbol 10–24 yocto y 10–21 zepto z 10–18 atto a 10–15 femto f 10–12 pico p 10–9 nano n 10–6 micro μ 10–3 milli m 10–2 centi c 10–1 deci d 10 deca da 102 hecto h 103 kilo k 106 mega M 109 giga G 1012 tera T 1015 peta P 1018 exa E 1021 zeta Z 1024 yotta Y.