Some Basic Concepts in Chemistry

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Some Basic Concepts in Chemistry Physical Quantities and their Measurements Mass, length, time and temperature are physical quantities. These are expressed in numerals with suitable units.

1 Some Basic Concepts in Chemistry Physical Quantities and their Measurements Mass, length, time and temperature are physical quantities. These are expressed in numerals with suitable units. Units may be basic (fundamental) or derived. The units of mass (kg), length (m), time (s), electric current (A), temperature (K), luminous intensity (cd), and amount of substance (mol) are fundamental units. Derived units are basically derived from the fundamental units. e.g., unit of density (kg m ) is derived from units of mass (kg) and volume (m ). Precision and Accuracy The term precision refers for the closeness of the set of values obtained from identical measurements of a quantity. Precision is simply a measure of reproducibility of an experiment. Accuracy, a related term, refers to the closeness of a single measurement to its true value. Significant Figures A significant figure includes all those digits that are known with certainty plus one more which is uncertain. Rules for Reporting Significant Figures Read the number from left to right and count all the digits, starting with the first digit that is not zero. When adding or subtracting, the number of decimal places in the answer should not exceed the number of decimal places in either of the numbers. e.g., is reported as 7.. In multiplication and division, the result should be reported to the same number of significant figures as that in the quantity with least number of significant figures. e.g., is reported as 5 because 0. has only two significant figures. When a number is rounded off, the number of significant figures is reduced. The last digit retained is increased by only if the following digit is 5 and is left as such if the following digit is 4. e.g., 8.5 can be written as 8.4 and.9 can be written as.9. Outlines Physical Quantities and Their Measurements Precision and Accuracy Significant Figures SI Units Matter Laws of Chemical Combination Dalton s Atomic Theory Atomic and Molecular Masses Mole Concept Chemical Equations and Stoichiometry Avogadro's Hypothesis Equivalent Weight Percentage Composition Empirical and Molecular Formulae Get Discount Coupons for your Coaching institute and FREE Study Material at

2 Master JEE Main Chemistry in Just 40 s SI Units In October 960, the General conference of Weights and Measures adopted a particular choice now known as the International System of Units popularly known as SI units. The SI has seven base units (See Table given below) from whom all other units are derived. The Seven Base of Units Physical Quantity Unit Unit Symbol Length metre m Mass kilogram kg Time second s Temperature kelvin K Amount of substance mole mol Electric current ampere A Luminous intensity candela cd Plane angle* radian rad Solid angle* steradian sr *These are two other fundamental quantities with dimensionless units. A number of quantities must be derived from measured value of the SI base quantities. These are called derived units. e. g.,units of density ( kgm ) is derived from the units of mass and length. Dimensional Analysis Many of the calculations of general chemistry simple require that conversion of quantities from one set of units to another. This is done by using Conversion Factor (C.F.), A C.F. must always have the numerator and denominator representing equivalent quantities. Information sought Information given C.F. Matter Anything that occupies space and possesses mass is called matter. On the basis of chemical composition of substance, it is of three types. (i) Elements are the substances that cannot be decomposed into simpler substances by chemical change. (ii) Compounds can be decomposed into simpler substances by chemical changes. Compound is always homogeneous. (iii) Mixtures have variable composition and variable properties due to the fact that components retain their characteristic properties. Components of a mixture can be separated by applying physical methods. Laws of Chemical Combination (i) Law of conservation of mass (Lavoisier, 774) Total mass of reactants total mass of products. (ii) Law of constant composition/definite proportions (Proust, 799) For the same compound, obtained by different methods, the percentage of each element should be same in each case. (iii) Law of multiple proportions (Dalton, 804) An element may form more than one compound with another element. For a given mass of an element, the masses of other elements (in two or more compounds) are in the ratio of small integers. For example, in NH and N H 4, fixed mass of nitrogen requires hydrogen in the ratio :. (iv) Law of reciprocal proportions (Ritcher, 794) Weights of two different elements which are combining with a fixed weight of a third element, are also the weights with which they combine with one another or their multiples. e.g., CH 4, CO and H O. (v) Law of combining volumes (Gay-Lussac, 808) The volume of reactants and products in a large number of chemical reactions of gases are related to each other by small integers, provided the volumes are measured at the same temperature and pressure. Dalton s Atomic Theory John Dalton developed his famous theory of atoms in 80. The main postulates of this theory were : (i) Atom was considered as a hard, dense and smallest indivisible particle of matter. (ii) Atom is indestructible i.e., it cannot be destroyed or created. (iii) Atom is the smallest portion of matter which takes part in chemical combination. (iv) Atoms combine with each other, to form compound atom or molecule, in simple whole number ratio. Atomic Mass It is defined as the number which indicates how many times the mass of one atom of the element is heavier as compared to th part of the mass of one atom of C-. Since most of the elements have isotopes, so their actual atomic mass in the average of atomic masses of all the isotopes and hence generally in fraction. Average atomic mass is calculated as m r m r m r Mav + + r + r + r here, r, r and r relative abundances of the isotopes. Get Discount Coupons for your Coaching institute and FREE Study Material at

3 Some Basic Concepts in Chemistry The approximate atomic mass of solid elements except Be B, C and Si, is related to specific heat as 6.4 Average atomic mass specific heat This is called Dulong and Petit s method. Mass spectrometer is used to determine the atomic mass experimentally. Molar Mass Molar mass of an element is defined as mass of mole of that element, i. e., mass of entities or particles of that element, e. g., molar mass of oxygen g/mol, that means molecules of oxygen weigh g. Mole Concept Mole is the amount of substance which contains Avogadro s number ( N A ) of particles and has mass equal to gram-atomic mass or gram-molecular mass. mass ( g) Mol molar mass ( g mol ) mol atoms gram-atomic weight gram atom of element atoms. mol molecules gram-molecular weight gram mole of substance molecules. In gaseous state (at STP), T 7 K, p atm Gram molecular wt. mol.4 L amu or u u molecules mass of one carbon (C ) atom 0 Atomic mass in u wt. of one atom 4 g.66 0 g Molecular mass in u wt. of one molecule e.g., H O (mol wt. 8) He 4 4u atom of He 8u molecule of H O Chemical Equations and Stoichiometry A balanced chemical equation with suitable stoichiometric coefficients represents the ratio of number of moles of reactants and products. The equation provides qualitative and quantitative information about a chemical change in a simple manner. For example H ( g ) + O ( g) H O( g) Mole ratio mol mol mol Molecule ratio molecules Weight ratio 4g g 6g Volume ratio volume volume volume molecules molecules (Volume ratio is valid for gaseous state at same temperature and pressure.) Limiting Reagent The substance which is completely consumed in a reaction is called limiting reagent. It determines the amount of product. Reaction yield actual yield 00 theoretical yield Avogadro s Hypothesis Equal volumes of gases or vapours obeying gas laws under similar conditions of pressure and temperature contain equal number of molecules. Molecular weight (for gaseous phase only) vapour density Equivalent Weight Equivalent weight of an element or of a compound is the weight of an element or of a compound which would combine with or displace (by weight) part of hydrogen or 8 parts of oxygen or 5.5 parts of chlorine. atomic wt. or molecular wt. Equivalent wt. (Eq. wt.) n factor n factor for various compounds can be obtained as : (i) n Factor for Acids i.e., Basicity Number of ionisable H + per molecule is the basicity of acid. e.g., (a) Basicity of HCl (b) Basicity of HSO 4 (c) Basicity of HPO 4 (ii) n Factor for Bases i.e., Acidity Number of ionisable OH per molecule is the acidity of a base. e.g., (a) Acidity of NaOH (b) Acidity of Mg(OH) (c) Acidity of Al(OH) (iii) n Factor for Salt Total positive or negative charge of ions. (a) Na CO Na + CO + (b) NaHCO Na + HCO n n + (c) Al (SO ) Al + SO n 6 Get Discount Coupons for your Coaching institute and FREE Study Material at

4 4 Master JEE Main Chemistry in Just 40 s (iv) n Factor for Ion In case of ion n factor is equal to charge of that ion. e.g., E Cl 5.5; E CO 0 7 E Al (v) n Factor for Redox Titration (a) FeSO 4 As reducing agent Fe As an oxidising agent Fe + (b) HCO 4 or C O (c) HI + + Fe + e n factor + e Fe( s) n factor 4 As reducing agent only 4 C O CO + e H C O CO + H +e 4 As reducing agent only + n factor HI I + H + + e n factor (d) KCrO 7 As oxidising agent only (acidic) + 7 Cr O + 6 e + 4 H Cr + H O (e) KMnO 4 + As oxidising agent in acidic medium MnO + 8H + 5e Mn + 4H O dditional Points n factor 6 n factor 5 As oxidising agent in alkaline medium 4 MnO + H O + e MnO + 4OH (f) NaSO (sodium thiosulphate) n factor As reducing agent in acidic medium SO S 4 O 6 + e n factor As reducing agent in alkaline medium 4 S O + 0 OH SO + 5H O + (g) HNO (nitric acid) As oxidising agent (conc. HNO ) + NO + H + NO + H O e As oxidising agent (dil HNO ) + NO + 4 H e NO + H O 8e n factor 8 n factor n factor Percentage Composition, Empirical and Molecular Formulae Mass % of an element mass of element in mole of compound 00 mass of mole of compound Dividing percentage by atomic mass gives molar ratio from which empirical formula is obtained. molecular mass n empirical formula mass Molecular formula n empirical formula Molar mass vapour density Get Discount Coupons for your Coaching institute and FREE Study Material at 4

5 Some Basic Concepts in Chemistry 5 Getting Ready Exercise ( Target JEE Main) 6.. The total number of electrons present in 8 ml of water (density of water is gml ) is (a) (b) (c) (d) If ml of water contains 0 drops, what is the number of water molecules in the one drop of water? (A Avogadro number) (a) 0.5 A 8 (b) 0.05 A (c) A (d) 0.5 A. The number of gram molecules of oxygen in CO molecules is (a) 0 g (c) g molecules (b) 5 g (d) 0.5 g molecules 4. The weight of 0 molecules of CuSO 5H O is (a) 4.59 g (c) 4.59 g 4 (b) 45.9 g (d) None of these 5. The sulphate of a metal M contains 9.87% of M. The sulphate is isomorphous with ZnSO 4 7HO. The atomic weight of M is (a) 40. (b) 6. (c) 4. (d). 6. Rearrange the following (I to IV) in the order of increasing masses and choose the correct answer (atomic mass; O 6, Cu 6, N 4,) I. molecule of oxygen II. atom of nitrogen III. 0 0 g molecular weight of oxygen IV. 0 0 (a) II < I < III < IV (c) II < III < I < IV g atomic weight of copper (b) IV < III < II < I (d) III < IV < I < II 7. One mole of calcium phosphide on reaction with excess of water gives (a) one mole of phosphine (b) two moles of phosphoric acid (c) two moles of phosphine (d) one mole of phosphorus pentoxide 8. The equivalent weight of HPO, when it disproportionates into PH and HPO 4, is (a) 8 (b) 6.5 (c) (d) Medical experts generally consider a lead level of 0µgPb per dl of blood to pose a significant health risk ( dl 0. L). Express this lead level as the number of Pb atoms per cm blood (Pb 07). (a) (b) (c) (d) The percentage of water of crystallisation in a sample of blue vitriol is (a) 4.07% (b) 5.07% (c) 6.07% (d) 7.07%. A 00 ml solution of 0. N HCl was titrated with 0. N NaOH solution. The titration was discontinued after adding 0 ml of NaOH solution. The remaining titration was completed by adding 0.5 N KOH solution. The volume of KOH required for completing the titration is (a) 70 ml (b) ml (c) 5 ml (d) 6 ml. How much of NaOH is required to neutralise 500 cm of 0. N HCl? (Na ) (a) 40 g (b) 4 g (c) 6 g (d) 60 g. 0 dm of N gas and 0 dm of gas X at the same temperature contain the same number of molecules. The gas X is (a) CO (b) CO (c) H (d) NO 4. The volume of water to be added to 00 cm of 0.5 N HSO 4 to get decinormal concentration, is (a) 00 cm (b) 450 cm (c) 500 cm (d) 400 cm Get Discount Coupons for your Coaching institute and FREE Study Material at 5

6 6 Master JEE Main Chemistry in Just 40 s 5. A metal oxide contains 5% metal and carbon dioxide contains 7% carbon. Assuming the law of reciprocal proportions, the percentage of metal in the metal carbide is (a) 75 (b) 5 (c) 7 (d) g of magnesium and 0 g of oxygen are reacted, then the residual mixture contains (a) 60 g of magnesium oxide only (b) 40 g of magnesium oxide and 0 g of oxygen (c) 45 g of oxygen and 5 g of magnesium oxide (d) 50 g of magnesium oxide and 0 g of oxygen 7. 5 ml of N HCl, 0 ml of N/ HSO 4 and 0 ml of N/ HNO are mixed together and volume made to L. The normality of resulting solution is (a) 0.45 (b) 0.05 (c) 0.9 (d) g of an oxide of a metal is converted to chloride completely and it yielded 5 g of chloride.the equivalent weight of the metal is (a).5 (b).5 (c) (d) 0 9. Assuming that the density of water to be g / cm, calculate the volume occupied by one molecule of water is (a) cm (b) cm (c) 400 ml (d) 8 cm 0. If 0.5 moles of BaCl are mixed with 0. moles of Na PO 4, the maximum number of moles of Ba (PO 4) that can be formed, is (a) 0.7 (b) 0.5 (c) 0.0 (d) 0.0. If 0 molecules are removed from 00 mg of CO, the number of moles of CO left are (a).88 0 (b) (c) (d) Which of the following pairs of gases contains the same number of molecules? (a) 6 g of O and 4 g of N (b) 8 g of O and g of CO (c) 8 g of N and g of CO (d) g of O and g of N. How many moles of KI are required to produce 0.4 moles of KHgI 4? (a) 0.4 (b) 0.8 (c). (d).6 4. How much AgCl will be formed by adding 00 ml of 5 N HCl to a solution containing.7 g AgNO (Ag 08)? (a) 0.45 g (b).45 g (c) 4.5 g (d) 4.5 g moles of Ag + and.00 0 moles of CrO 4 react together to form solid Ag CrO 4. Calculate the amount of AgCrO 4 formed (Ag CrO.7 g mol ). 4 (a) 0.68 g (c) 0. g (b) 0.66 g (d).66 g 6. What would be the weight of the slaked lime required to decompose 8.0 g of ammonium chloride? (a) 5.5 g (b). g (c) 5.5 g (d) 7.6 g Directions (Q. Nos. 7 and 8) The following is a crude but ffective method for estimating the order of magnitude of Avogadro's number using stearic acid (C8H 6O ). When stearic acid is added to water, its molecules collect at the surface and form a monolayer, ie, the layer is only one molecule thick. The cross sectional area of each stearic acid molecule has been measured to be 0. nm. In one experiment it is found that g of stearic acid is needed to form a mono layer over water in a dish of diameter 0 cm. (The area of a circle of radius r is πr.) 7. Based on these measurements value of Avogadro s number is (a) 0 (b) 6 0 (c) 4 0 (d) 0 8. What is the equivalent of g H in atomic mass unit for this value of Avogadro s number? (a) g (b). 0 4 g (c) g (d) 0 g Directions (Q. Nos. 9 to ) One gram of activated charcoal has a surface area of 0 m. One molecule of ammonia having a diameter of 0. nm is adsorbed completely on the surface of charcoal (unilayer adsorption). The activated charcoal is brought in contact with 00 ml of M NH solution. 9. The number of ammonia molecules adsorbed on the charcoal surface is (a).4 0 (b).4 0 (c).4 0 (d) The moles of HCl required to neutralise left ammonia solution after adsorption is (a) 0. (b) 0.45 (c) 0.5 (d) The volume of HCl(g) at STP required to clean up all ammonia molecules adsorbed on the surface is (a) 0.75 L (b) 0.0 L (c) L (d) 0.56 L Directions (Q. Nos. to 5) Each of these questions contains two statements : Statement I (Assertion) and statement II (Reason). Each of these questions also has four alternative choices, only one of which is the correct answer. You have to select one of the codes (a), (b), (c) and (d) given below (a) Statement I is true, Statement II is true; Statement II is a correct explanation of Statement I. Get Discount Coupons for your Coaching institute and FREE Study Material at 6

7 Some Basic Concepts in Chemistry 7 (b) Statement I is true, Statement II is true; Statement II is not a correct explanation of statement I. (c) Statement I is true; Statement II is false. (d) Statement I is false; Statement II is true.. Statement I When 0 g of CaCO is decomposed, 5.6 g of residue is left and 4.4 g of CO escapes. Statement II Law of mass conservation is followed.. Statement I In Fe +, there are 4 electrons and 0 neutrons and thus, ionic mass is 56. Statement II Ionic mass neutron + electrons in the neutral species. 4. Statement I On changing volume of the solution by 0%, molarity of solution also changes by 0%. Statement II Molar concentration or molarity of solution changes on dilution. 5. Statement I HBO is monobasic Lewis acid but salt Na BO exists. Statement II HBO reacts with NaOH to give Na BO g of AgCO and CaCO on heating gives a residue which required 00 ml of M / 5 HCl for complete neutralisation. The percentage of Ag in original mixture will be (a).% (b) 6.6% (c) 67.6% (d) 78.% 4. The mass of potassium dichromate crystals required to oxidise 750 cm of 0.6 M Mohr s salt solution is (Given, molar mass : potassium dichromate 94, Mohr s salt 9) (AIEEE 0) (a) 0.49 g (b) 0.45 g (c).05 g (d). g 4. A student performs a titration with different burettes and finds titre values of 5. ml, 5.5 ml and 5.0 ml. The number of significant figures in the average titre value is (IIT JEE 00) (a) (b) (c) (d) 4 4. In the reaction, Al ( s) + 6HCl ( aq) Al ( aq) + 6Cl ( aq) JEE & AIEEE Compendium + + H ( g ) (AIEEE 007) (a) 6L HCl (aq) is consumed for every L, H ( g ) produced (b).6 L H ( g) is produced regardless of temperature and pressure for every mole Al that reacts (c) 67. L H ( g) at STP, is produced for every mole Al that reacts (d). L H ( g) at STP, is produced for every mole HCl (aq) consumed 7. The molecular mass of an organic acid was determined by the study of its barium salt g of salt was converted to free acid by the reaction with.64 ml of M H SO 4. The barium salt was found to have two moles of water of hydration per Ba + ion and the acid is monobasic. What is the molecular weight of anhydrous acid? (a).6 (b) 40 (c).6 (d) A mixture of FeO and Fe O 4 when heated in air to constant weight, gains 5% in its weight. What is the percentage of Fe O 4 in mixture? (a) 7.87% (b) 6.% (c) 79.75% (d) 0.5% 9. Weight of L milk is.0 kg. It contains butter fat (density 865 kg m ) to the extent of 4% by volume/volume. The density of the fat free skimmed milk will be (a) 08.5 kg m (b) 0. kg m (c) 997 kg m (d) kg m 40. Sea water contains 65 0 g L of bromide ions. If all the bromide ions are converted to produce Br, how much sea water is needed to prepare kg Br? (a) 5.8 L (b) L (c) L (d) 76.9 L 44. How many moles of magnesium phosphate, Mg (PO 4) will contain 0.5 mole of oxygen atoms? (AIEEE 006) (a) 0.0 (b).5 0 (c).5 0 (d) If we consider that /6 in place of /, mass of carbon atom is taken to be the relative atomic mass unit, the mass of one mole of a substance will (AIEEE 005) (a) to be a function of the molecular mass of the substance (b) remain unchanged (c) increase two fold (d) decrease twice 46. Two solutions of a substance (non-electrolyte) are mixed in the following manner : 480 ml of.5 M of first solution with 50 ml of. M of second solution. The molarity of final solution is (AIEEE 005) (a).0 M (b).50 M (c).44 M (d).70 M 47. Which has maximum number of atoms? (IIT JEE 00) (a) 4g of C() (b) 56g of Fe(56) (c) 7g of Al(7) (d) 08g of Ag(08) Get Discount Coupons for your Coaching institute and FREE Study Material at 7

8 8 Master JEE Main Chemistry in Just 40 s 48. Mixture X 0.0 mole of [Co(NH ) 5SO 4] Br and 0.0 mole of [Co(NH ) 5Br] SO 4 was prepared in L of solution. L of mixture X + excess AgNO Y L of mixture X + excess BaCl Z Number of moles of Y and Z are (IIT JEE 00) (a) 0.0, 0.0 (b) 0.0, 0.0 (c) 0.0, 0.0 (d) 0.0, What volume of hydrogen gas at 7K and atm pressure will be consumed in obtaining.6 g of elemental boron (atomic mass 0.8) from the reduction of boron trichloride by hydrogen? (AIEEE 00) (a) 89.6 L (b) 67. L (c) 44.8 L (d).4 L. 8 ml H O 8 g H O mol molecules electrons electrons. Q 8 ml 8 g of water contains 0 8 drops A molecules A A drop contains molecules molecules CO molecules 0 moles CO 0 g atoms of O 5 g molecules of O 4. Q molecules of CuSO 5H O 0 molecules of CuSO Answers 50. How many moles of electron weigh one kilogram? (IIT JEE 00) (a) (b) (c) (d) In an organic compound of molar mass 08 g mol C, H and N atoms are present in 9 : :.5 by weight. Molecular formula can be (AIEEE 00) (a) C H N 6 8 (b) C H N (c) C H N 5 6 (d) C H N 4 8. (c). (c). (b) 4. (c) 5. (c) 6. (a) 7. (c) 8. (c) 9. (a) 0. (c). (d). (c). (a) 4. (d) 5. (a) 6. (d) 7. (a) 8. (a) 9. (a) 0. (d). (a). (a). (b) 4. (b) 5. (b) 6. (a) 7. (a) 8. (b) 9. (a) 0. (d). (d). (a). (a) 4. (d) 5. (c) 6. (b) 7. (c) 8. (c) 9. (a) 40. (b) 4. (c) 4. (c) 4. (d) 44. (b) 45. (b) 46. (c) 47. (a) 48. (a) 49. (b) 50. (d) 5. (a) Hints & Solutions g 5 H O g 5. As the given sulphate, is isomorphous with ZnSO 4 7HO, its formula would be MSO 4 7HO. If m is the atomic weight of M, molecular weight of MSO 4 7HO m m + m Hence, % of M (given) m + or 00 m 9.87 m or 90. m 9.87 or m I. molecule of O g 5. 0 g 4 II. atom of N. 0 g g III. 0 0 g mol. wt. of oxygen 0 9 g 0. 0 IV. 0 0 g atomic weight of copper g Order of increasing mass is II < I < III < IV 7. Ca P + 6HO Ca(OH) + PH 8. HPO disproportionates as H PO PH + H PO Molecular wt. of HPO Eq. wt L 00 ml has Pb 0 mg 0 0 g mol of Pb Get Discount Coupons for your Coaching institute and FREE Study Material at 8

SOME BASIC CONCEPTS IN CHEMISTRY

SOME BASIC CONCEPTS IN CHEMISTRY CS 1 Syllabus : SOME BASIC COCEPTS I CHEMISTRY Matter and its nature, Dalton s atomic theory; Concept of atom, molecule, element and compound; Physical quantities and their measurement in Chemistry, precision