CHAPTER-1 SOME BASIC CONCEPTS OF CHEMISTRY Topic 1 General Introduction, Laws of Chemical Combination, Dalton’s Atomic Theory, Atomic and Molecular Masses » Revision Notes Chemistry is defined as the branch of science which deals with the study of composition, structure and properties of matter. Chemistry is related with all the fields of our life, e.g., food, fuels, textiles, dyes, drugs, disinfectants, perfumes, building materials, paints, inks, fertilizers, insecticide, pesticide, soap, detergents, etc. Matter : Matter is anything that has mass and occupies space. For example, book, pen, pencil, water, air, all living beings etc. are composed of matter. Accuracy – Freedom from mistake or error : The quality or state of being accurate or the ability to work or perform without making mistakes. Accuracy = Mean value – True Value Precision : The quality, condition, or fact of being exact and accurate or the closeness of the set of values obtained from identical measurements of quantity. Precision = Individual Value – Arithmetic Mean Value Significant Figures : The digits in a properly recorded measurement or the total number of figures in a number including the last digit whose value is uncertain are called significant figures, e.g., 180.00 has five significant figures. Laws of Chemical Combination : Elements combine to form compounds in accordance with the following five basic laws, called the laws of chemical combinations : (i) Law of Conservation of Mass : Mass can neither be created nor destroyed. (ii) Law of Definite Proportions Compositions : A given compound always contains exactly the same proportion of elements by weight. (iii) Law of Multiple Proportions : If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers. (iv) Gay Lussac’s Law of Gaseous Volumes : When gases combine or are produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at same temperature and pressure. (v) Avogadro’s Law : At the same temperature and pressure, equal volumes of gases should contain the equal number of molecules. Dalton Atomic Theory : • All matter consists of tiny indivisible particles called atoms. • Atoms are indestructible and unchangeable. • All the atoms of a given element have identical properties including identical mass. Atoms of different elements differ in mass. • When different elements react to form compounds their atoms combine in simple fixed, whole number ratio. • In chemical reactions, atoms are neither created nor destroyed. They are reorganized in a chemical reaction. One atomic mass of an element is defined as a mass exactly equal to one-twelfth the mass of C-12 atom. Average Atomic Mass : The average atomic mass of an element refers to the atomic masses of the isotopes of the element, taking into account the abundances of the element’s isotopes. Average atomic mass = f1M1 + f2M2 + f3M3 + … + fnMn where, f is the fraction representing the natural abundance of the isotope and M is the mass number (weight) of the isotope. Molecular mass of a compound is the sum of atomic masses of all the constituent atoms. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together. 2 ] Oswaal NEET Solved Papers Chapterwise & Topicwise Revision Notes CHEMISTRY Topic 2 Mole Concept and Percentage Composition » Revision Notes Mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g (or 0.012 kg) of the 12C isotope. One mole of any element has same mass as the atomic weight of the element. 23 Avogadro constant is the number of entities present in 1 mole. It is denoted by ‘’NA’’ and is equal to 6.0221367 × 10 atoms/mol. Mass of substance (g ) Number of moles of a substance = Mass of 1 mole of substance (g ) Number of particles = 6.02× 1023 Volume of gas at NTP (in litres) Number of molecules of a substance =××6.02 1023 22.4 The mass of one mole of a substance in grams is called its molar mass. Mass of that element in ×100 the compound Mass percentage = Molar mass of the compound Topic 3 Empirical and Molecular Formulae, Chemical Equations, Stoichiometry » Revision Notes Empirical Formula : A formula that gives the simplest whole number ratio of various atoms present in a compound. Molecular formula : It is a formula that shows the exact number of atoms of different elements present in a molecule of a compound. Steps for calculation of empirical and molecular formula : (i) Conversion of mass percent to grams. (ii) Convert into number of moles of each element. (iii) Divide the mole value obtained above by the smallest number. (iv) Write empirical formula by mentioning the numbers after writing the symbols of respective elements. (v) Writing molecular formula : (a) Determine empirical formula mass by adding the atomic masses of various atoms present in the empirical formula. (b) Divide molar mass by empirical formula mass. (c) Multiply empirical formula by n obtained above to get the molecular formula. Number of gram equivalent of solute Normality, (N) = Volume of solution (l ) (Gram equivalent of solute = w2 /equivalent weight) Molality and mole fraction do not change with temperature. Number of parts of the component ×106 Parts per million = Total no. of parts of all the components of the solution Volume of the component ×100 Volume percent = Total volume of the solution Relation between molarity and molality : Oswaal NEET Solved Papers Chapterwise & Topicwise Revision Notes, CHEMISTRY [ 3 M× 1000 Molality (mm) = 1000×× d-M M2 where, M- molarity of the solution, M2 - molar mass of solute, d- density of the solution. Chemical equation : A chemical equation is the symbolic representation of a chemical reaction, e.g., AgNO+ NaCl → AgCl + NaNO 32 Reactants Products Stoichiometry and Stoichiometric Calculations : Stoichiometry is the calculation of relative masses of reactants and products involved in chemical reactions. Stoichiometric problems can be solved in just four simple steps : (i) Balance the equation. (ii) Convert units of a given substances to moles. (iii) Using the mole ratio, calculate the moles of substance yielded in the reaction. (iv) Convert moles of wanted substance to desired units. Limiting Reagent : In a chemical reaction, the reagent which is completely consumed and limits the amount of product formed is called as limiting reagent. Ways to express concentration in a solution : Mass of solute (1) Mass percent = × 100 Mass of solution (2) Mole fraction : In a binary solution, if a substance ‘A’ dissolves in substance ‘B’ and their number of moles are nA and nB, respectively, then nA Mole fraction of A= nn+ AB n Mole fraction of B= B nn+ AB (Mole fraction of A + Mole fraction of B =1) Number of moles of solute (3) Molarity (M) = Volume of solution (litres) Number of moles of solute (4) Molality (m) = Mass of solvent in kg Mass-Number of moles Relationship : Mass of g Number of Moles = Molar mass in g mol−1 Multiplied by Divided by Molar mass Avogadro's constant (g) Divided by Multiplied by Molar mass Avogadro's constant For dilution M1V1 = M2V2 where, M1 is initial molarity, V1 is initial volume, M2 is final molarity and V2 is final volume. ll 4 ] Oswaal NEET Solved Papers Chapterwise & Topicwise Revision Notes CHEMISTRY CHAPTER-2 STRUCTURE OF ATOM Topic 1 Sub-atomic Particles, Isotopes and Isobars, Bohr's Model, Photoelectric Effect and Atomic Spectra » Revision Notes Atom is the smallest particle that can take part in chemical reaction. Dalton’s atomic theory states that element is composed of small indivisible particles called atom and atom of different elements combine to form molecules of compound. The individual nature of atom was discarded when discovery of sub-atomic particles took place. Sub-Atomic Particles : An atom consists of three – sub-atomic particles called electron, proton and neutron. Discovery of Electron : Study of Cathode Rays : During the latter half of the nineteenth century, it was found that the passage of electricity through gases as studied by a number of physicists, particularly by Faraday, Davy, Crookes and J.J. Thomson. When a current of high voltage is passed through a gas of air kept at a very low pressure blue rays are seen emerging from the case. These rays are called “Cathode Rays’’. Characteristics of Cathode Rays : (i) They are produced by the negative electrode, or cathode, in an evacuated tube and travel towards the anode. (ii) They travel in straight line, in the absence of electrical or magnetic field. (iii) These rays themselves are not visible but their behaviour can be observed with the help of certain materials (fluorescent). (iv) They are deflected by electric and magnetic field and behave like negatively charged particles suggesting that they consists of electronic. (v) The characteristics of cathode rays do not depend upon the material of electrodes and the nature of gas present in the cathode ray tube. Sir J.J. Thomson (1897) who discovered electron) extended the cathode ray experiment for the determination of velocity of electrons and their charge/mass ratio by applying electrical and magnetic field perpendicular to each other as well as to the path of electrons. Charge to Mass Ratio of Electron : J.J. Thomson measured the charge to mass ratio of electron : e = 1.758820 × 1011 C kg–1 m e Charge on the Electrons : R.A. Millikan (1868-1958) with the help of his oil drop experiment determined the charge on electrons and it is found to be 1.6 × 10–19 C. Mass of the Electrons : Mass of electron was determined from the value of charge on electron and charge to mass ratio of electron as follows.