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ICSE Class X Notes 2027 : Chemistry (St. Xaviers Institution (SXI), Panihati, Kolkata)

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Suryasish Sarkar
St. Xavier's Institution (SXI), Panihati, Kolkata

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MOLE CONCEPT AND STOICHIOMETRY Stoichiometry measures quantitative relationships, and is used to determine the amount of products/reactants that are produced/ needed in a given reaction. Describing the quantitative relationships among substances as they participate in chemical reactions is known as reaction stoichiometry. Gay-Lussac's Law of Combining Volumes According to this law, "when gases react, they do so in volumes which bear a simple ratio to one another and to the volume of the gaseous product, provided that all the volumes are measured at the same temperature and pressure". Note: Gay-Lussac's law is applicable only for gases. The volumes of liquids and solids are considered to be zero. The law may be illustrated by the following examples involving gases or vapours: 1. Reaction of nitrogen and hydrogen to form ammonia Experimentally it is noticed that one volume of nitrogen combined with three volumes of hydrogen gives two volumes of ammonia at the same temperature and pressure. N + 3H2 -> 2NH3 Thus, the ratio 1:3:2 is simple. 2. Formation of carbon dioxide Two volumes of carbon monoxide on combustion with one volume of oxygen gives two volumes of carbon dioxide at the same temperature and pressure. 2CO + O2 -> 2CO2 Thus, the ratio 2:1:2 is simple. Avogadro's Law Avogadro's law states that "equal volumes of all gases under similar conditions of temperature and pressure contain the same number of molecules." Atomicity The number of atoms in a molecule of an element is called its atomicity. (a) Monoatomic Monoatomic molecule is composed of only one atom. Examples: Inert gases like Helium, Neon, Argon, etc. (b) Diatomic Diatomic molecule is composed of two similar atoms. Examples: H2,O2, Cl2 ,N2, etc. (c) Triatomic Triatomic molecule is composed of three similar atoms. Example: Ozone gas O3 (d) Tetratomic Tetratomic molecule is composed of four similar atoms. Example: Phosphorus (P4) (e) Octatomic Octatomic molecule is composed of eight similar atoms. Example: Sulphur (S8) Molecules made up of same type of atoms are homoatomic molecules, e.g. phosphorus (P), ozone (O3) etc. while molecules made up of different types of atoms are hetero-atomic molecules, e.g. HCl, NH3, etc. NUMERICALS: 1. (a) Calculate the volume of oxygen at STP required for the complete combustion of 100 litres of carbon monoxide at the same temperature and pressure. 2CO + O2 -> 2CO2 (b) 200 cm3 of hydrogen and 150 cm3 of oxygen are mixed and ignited, as per following reaction, 2H2 + O2 -> 2H2O What volume of oxygen remains unreacted? 2. 24 cc Marsh gas (CH4) was mixed with 106 cc oxygen and then exploded. On cooling, the volume of the mixture became 82 cc, of which, 58 cc was unchanged oxygen. Which law does this experiment support? Explain with calculations. 3. What volume of oxygen would be required to burn completely 400 mL of acetylene [C2H2]? Also, calculate the volume of carbon dioxide formed. 2C2H2 + 5O2 -> 4CO2 + 2H2O(l) 4. 112 cm3 of H2S(g) is mixed with 120 cm3 of Cl2(g) at STP to produce HCl(g) and sulphur(s). Write a balanced equation for this reaction and calculate (i) the volume of gaseous product formed (ii) composition of the resulting mixture. 5. 1250 cc of oxygen was burnt with 300 cc of ethane [C2H6]. Calculate the volume of unused oxygen and the volume of carbon dioxide formed: 2C2H6 + 7O2 -> 4CO2 + 6H2O 6. What volume of oxygen at STP is required to affect the combustion of 11 litres of ethylene [C2H4] at 273 C and 380 mm of Hg pressure? C2H4 + 3O2 -> 2CO2 + 2H2O 7. Calculate the volume of HCl gas formed and chlorine gas required when 40 mL of methane reacts completely with chlorine at STP. CH4 + 2Cl2 CH2Cl2 + 2HCl 8. What volume of propane is burnt for every 500 cm3 of air used in the reaction under the same condition? (Assuming oxygen is 1/5th of air) C3H8 + 5O2 -> 3CO2 + 4H2O 9. 450 cm3 of nitrogen monoxide and 200 cm3 of oxygen are mixed together and ignited. Calculate the composition of resulting mixture. 2NO + O2 -> 2NO2 10. If 6 litres of hydrogen and 4 litres of chlorine are mixed and exploded and if water is added to the gases formed, find the volume of the residual gas. 11. Ammonia may be oxidised to nitrogen monoxide in the presence of a catalyst according to the following equation. 4NH3 + 5O2 -> 4NO + 6H2O If 27 litres of reactants are consumed, what volume of nitrogen monoxide is produced at the same temperature and pressure? 12. A mixture of hydrogen and chlorine occupying 36 cm was exploded. On shaking it with water, 4 cm of hydrogen was left behind. Find the composition of the mixture. 13. What volume of air (containing 20% O by volume) will be required to burn completely 10 cm each of methane and acetylene. CH4 + 2O2 -> CO2 + 2H2O 2C2H2 + 5O2 -> 4CO2 + 2H2O 14. LPG has 60% propane and 40% butane: 10 litres of this mixture is burnt. Calculate the volume of carbon dioxide added to atmosphere. C3H8 + 5O2 -> 3CO2 + 4H2O 2C4H10 + 13O2 -> 8CO2 + 10H2O 15. 200 cm of CO2 is collected at STP when a mixture of acetylene and oxygen is ignited. Calculate the volume of acetylene and oxygen at STP in original mixture 2C2H2(g) + 5O2(g) -> 4CO2(g) + 2H2O(g) 16. You have collected (a) 2 litres of CO2 (b) 3 litres of chlorine (c) 5 litres of hydrogen (d) 4 litres of nitrogen and (e) 1 litre of SO2 under similar conditions of temperature and pressure. Which gas sample will have: (a) the greatest number of molecules, and (b) the least number of molecules? Justify your answers. 17. The gases chlorine, nitrogen, ammonia and sulphur dioxide are collected under the same conditions of temperature and pressure. The following table gives the volumes of gases collected and the number of molecules (x) in 20 litres of nitrogen. You are to complete the table giving the number of molecules in the other gases in terms of x. RELATIVE ATOMIC MASS (ATOMIC WEIGHT) The relative atomic mass or atomic weight of an element is the number of times one atom of the element is heavier than 1/12 times of the mass of an atom of carbon-12. Thus, Relative Molecular Mass = Mass of 1 atom of the element / 1/12th mass of one C-12 atom Atomic mass is expressed in atomic mass units [a.m.u.]. Atomic mass unit is defined as 1/12 the mass of carbon atom C-12. Fractional atomic masses It is observed that most atomic masses are not whole numbers. The reason is that most natural elements are a mixture of constant composition containing two or more isotopes. For example, chlorine consists of a mixture of two isotopes of masses 35 and 37 in the ratio of 3:1. RELATIVE MOLECULAR MASS (MOLECULAR WEIGHT) The relative molecular mass (or molecular weight) of an element or a compound is the number that represents how many times one molecule of the substance is heavier than 1/12 of the mass of an atom of carbon-12. GRAM ATOMIC MASS The atomic mass of an element expressed in grams is called gram atomic mass. The quantity of the element which weighs equal to its gram atomic mass is called one gram atom of that element. For example 16 g of oxygen is equal to one gram atom of oxygen. Similarly, 1 g atom of sodium = 23 g of sodium 1 g atom of He = 4 g of helium GRAM MOLECULAR MASS The molecular mass of a substance expressed in grams is called gram molecular mass or molar mass. A sample of a substance with its mass equal to its gram molecular mass is called one gram molecule of the substance. MOLE CONCEPT It is not practically possible to find the mass of a minute particle like an atom, a molecule, or an ion etc. Therefore, a collection of 6.022 10^23 elementary particles called Mole is taken for all practical purposes. A mole is the amount of pure substance containing the same number of chemical units as there are atoms in exactly 12 grams of carbon -12. Avogadro's number is defined as the number of atoms present in 12 g (gram atomic mass) of C-12 isotope, i.e., 6.022 x 1023 atoms. OR Avogadro's number is the number of elementary units, i.e., atoms, ions or molecules present in one mole of a substance. It is denoted by NA. VAPOUR DENSITY AND ITS RELATION TO RELATIVE MOLECULAR MASS The Relative Vapour Density of a gas (or a vapour) is: The ratio between the masses of equal volumes of gas (or vapour) and hydrogen under the same conditions of temperature and pressure. Relative V.D.= Relative V.D. = (Molecule of hydrogen contains 2 atoms) By multiplying both sides by 2 2 x Rel. V.D. = 2 x Rel. V.D. = Rel. molecular mass of a gas or vapour. The relative molecular mass of a gas or vapour is twice its vapour density. The molar volume of a gas is the volume occupied by one grammolecular mass or simply, by one mole of the gas at S.T.P. It is equal to 22.4 dm . NUMERICALS: 1. Calculate the relative molecular masses of: (a) Ammonium chloroplatinate (NH4)2 PtCl6, (b) Potassium chlorate, (c) CuSO4.5H O, (d) (NH4)2SO4 (e) CH3COONa, (f) CHCl3, (g) (NH4)2Cr2O7 2. Find the: (a) number of molecules in 73 g of HCl, (b) weight of 0.5 mole of O2 (c) number of molecules in 1.8 g of H2O (d) number of moles in 10 g of CaCO3, (e) weight of 0.2 mole of H2 gas, (f) number of molecules in 3.2 g of SO2 3. Calculate the number of: (a) particles in 0.1 mole of any substance, (b) hydrogen atoms in 0.1 mole of H2SO4 (c) molecules in one kg of calcium chloride. 4. How many grams of: (a) Al are present in 0.2 mole of it, (b) HCl are present in 0.1 mole of it? (c) H2O are present in 0.2 mole of it, (d) CO2 is present in 0.1 mole of it? 5. (a) The mass of 5.6 litres of a certain gas at STP is 12 g. What is the relative molecular mass or molar mass of the gas? (b) Calculate the volume occupied at S.T.P. by 2 moles of SO2. 6. Calculate the number of moles of: (a) CO2 which contain 8.00 g of O2 (b) methane in 0.80 g of methane. 7. Calculate the weight/mass of: (a) an atom of oxygen, (b) an atom of hydrogen, (c) a molecule of NH3 (d) 1022 atoms of carbon, (e) the molecule of oxygen, (f) 0.25 gram atom of calcium. 8. Calculate the mass of 0.1 mole of each of the following: (Ca = 40, Cl = 35.5, Na = 23, O = 16, Mg = 24, H = 1, S = 32, C = 12) (a) CaCO3 (b) Na SO 10H2O (c) CaCl2 (d) Mg 9. Calculate the number of (a) oxygen atoms in 0.10 mole of Na2CO310H2O (b) gram atoms in 4.6 gram of sodium. (c) moles in 12 g of oxygen gas. 10. What mass of Ca will contain the same number of atoms as are present in 3.2 g of S? 11. Calculate the number of atoms in each of the following: (a) 52 moles of He, (b) 52 amu of He, (c) 52 g of He. 12. Calculate the number of atoms of each kind in 5.3 grams of sodium carbonate. 13. (a) Calculate the mass of nitrogen supplied to soil by 5 kg of urea [CO(NH2)2] [O = 16, N = 14, C = 12, H = 1] (b) Calculate the volume occupied by 320 g of sulphur dioxide at STP [S = 32; O = 16] 14. (a) What do you understand by the statement that 'vapour density of carbon dioxide is 22'? (b) Atomic mass of chlorine is 35.5. What is its vapour density? 15. What is the mass of 56 cm of carbon monoxide at STP? [C = 12 O = 16] 16. Determine the no. of molecules in a drop of water which weighs .09 g. 17. The molecular formula for elemental sulphur is Sg. In a sample of 5.12 g of sulphur: (a) How many moles of sulphur are present? (b) How many molecules and atoms are present? 18. If phosphorus is considered to contain P4 molecules, then calculate the number of moles in 100 g of phosphorus. 19. Calculate: (a) the gram molecular mass of chlorine if 308 cm of it at STP weighs 0.979 g, (b) the volume of 4 g of H at 4 atmosphere, (c) the mass of oxygen in 2.2 litres of CO2 at STP. 20. A student puts his signature with graphite pencil. If the mass of carbon in the signature is 10-12 g. Calculate the number of carbon atoms in the signature. 21. An unknown gas shows a density of 3 g per litre at 273 C and 1140 mm Hg pressure. What is the gram molecular mass of this gas? 22. Cost of Sugar (C12H22O11) is 40 per kg; calculate its cost per mole. PERCENTAGE COMPOSITION: Percentage composition of a compound, is the percentage by weight of each element present in it. Percentage of an element in a compound . = 100 EMPIRICAL FORMULA OF A COMPOUND The empirical formula of a compound is the simplest formula, which gives the simplest ratio in whole numbers of atoms of different elements present in one molecule of the compound. The empirical formula mass is the sum of atomic masses of various elements present in the empirical formula. DETERMINATION OF MOLECULAR FORMULA The molecular formula of a compound denotes the actual number of atoms of different elements present in one molecule of the compound. Molecular formula of Blue Vitriol is CuSO4.5H2O It gives the information that a molecule of blue vitriol is made of1. One atom of copper, 2. One atom of sulphur, 3. Four atoms of oxygen, 4. Five molecules of water of crystallization. Steps to find the Molecular formula of a compound: (1) Calculate the empirical weight of the compound from its empirical formula. (2) Divide its molecular weight by empirical weight which gives the number (n). (3) Multiply the empirical formula by this number to get the molecular formula. Molecular formula = empirical formula x n Where n = Molecular Weight / Empirical formula weight Note: The value of n must be rounded off to its nearest whole number. VAPOUR DENSITY Vapour density is the ratio of the mass of a volume of a gas, to the mass of an equal volume of hydrogen, measured under the same conditions of temperature and pressure. A chemical equation is a balanced account of a chemical transaction. A reaction having an equal number of atoms of different elements in the reactants and products is known as a balanced chemical equation. In spite of having several limitations, chemical equations have various implications. Molecular mass = V.D. 2 NUMERICALS: 1. Give the empirical formula of: (a) C6H6 (b) C6H18O3 (C6H18O3) (c) C2H2 (d) CH3COOH. 4. Find the percentage of water of crystallisation in CuSO4-5H2O. (At. mass Cu = 64 H = 1 O = 16 S = 32). 5. Calculate the percentage of phosphorus in: (a) Calcium hydrogen phosphate Ca(H2PO4)2 , (b) Calcium phosphate Ca3(PO4)2 6. Calculate the percentage composition of: Potassium chlorate, KCIO3. 7. Find the empirical formula of the compounds with the following percentage composition: Pb = 62.5% N = 8.5% O = 29% 8. Calculate the mass of iron in 10 kg of iron ore which contains 80% of pure ferric oxide. 9. If the empirical formula of two compounds is CH and their vapour densities are 13 and 39 respectively, find their molecular formula. 10. Find the empirical formula of a compound containing 17-64% hydrogen and 82-35% nitrogen. 11. On analysis, a substance was found to contain: C= 54 * 54 \% H = 9.09% O = 36.36% The vapour density of the substance is 44, calculate: (a) its empirical formula, (b) its molecular formula. 12. An organic compound, whose vapour density is 45, has the following percentage composition, H= 2 * 22 \% O= 71 * 19 \% and remaining carbon. Calculate: (a) its empirical formula, (b) its molecular formula. 13. An organic compound contains 4.07% hydrogen, 71.65% chlorine and remaining carbon. Its molar mass is 98.96. Find its, (a) Empirical formula (b) Molecular formula 14. A hydrocarbon contains 4.8 g of carbon per gram of hydrogen. Calculate: (a) the gram atom of each, (b) find the empirical formula, (c) find molecular formula, if its vapour density is 29. 15. 0.2 g atom of silicon combine with 21.3 g of chlorine. Find the empirical formula of the compound formed. 16. A gaseous hydrocarbon contains 82.76% of carbon. Given that its vapour density is 29, find its molecular formula. 17. In a compound of magnesium (Mg = 24) and nitrogen (N = 14), 18 g of magnesium combines with 7g of nitrogen. Deduce the simplest formula by answering the following questions: (a) How many gram-atoms of magnesium are equal to 18g? (b) How many gram-atoms of nitrogen are equal to 7g of nitrogen? (c) Calculate simple ratio of gram-atoms of magnesium to gramatoms of nitrogen and hence the simplest formula of the compound formed. 18. Barium chloride crystals contain 14.8% water of crystallisation. Find the number of molecules of water of crystallisation per molecule 19 Urea is a very important nitrogenous fertilizer. Its formula is CON H . Calculate the percentage of nitrogen in urea (C = 12, O = 16 N = 14 and H = 1) . 20. Determine the formula of the organic compound if its molecule contains 12 atoms of carbon. The percentage compositions of hydrogen and oxygen are 6.48 and 51.42 respectively. 21. (a) A compound with empirical formula A*B_{2} has the vapour density equal to its empirical formula weight. Find its molecular formula. (2017) (b) A compound with empirical formula AB has vapour density three times its empirical formula weight. Find the molecular formula. (c) 10-47 g of a compound contained 6-25 g of metal A and rest non metal B. Calculate the empirical formula of the compound [At. wt of A = 207 B = 35.5 ] .100 22. A hydride of nitrogen contains 87-5 per cent by mass of nitrogen. Determine the empirical formula of this compound. 23. A compound has O = 61.32% S= 11 * 15 \%, H= 4 * 88 \% and Zn = 22.65% . The relative molecular mass of the compound is 287 a.m.u. Find the molecular formula of the compound, assuming that all the hydrogen is present as water of crystallisation.

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