Solution : 22.4 dm 3 of a gas at STP 1 mol of gas molar mass of the gas Now, 4.48 dm 3 of NH 3 at STP 3.49 g dm 3 of NH 3 at STP.

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1 Formulae 1. One mole of atoms Mass of elements Atomic mass Atomic mass 2. Mass of one atom Number of moles (n) Mass of substance Molar mass of substance 4. Number of molecules n Avogadro number 5. Volume of gas at STP n L 6. Percentage (By weight) Mass of the element in 1 mole of the compound 100% Gram molecular weight of the compound 7. Molecular formula r empirical formula where r is ratio of molecular mass to empirical mass. Molecular mass 8. Mass of one molecule Solved Problems *1. Calculate the volume of hydrogen required for complete hydrogenation of 0.25 dm 3 of ethyne at STP. The hydrogenation of ethyne (C 2 H 2 ) is represented as, C 2 H 2(g) + 2H 2(g) C 2 H 6(g) ethyne ethane 1 vol 2 vol 1 vol 1 volume C 2 H 6 1 dm 3 C 2 H 2 requires 2 dm 3 H 2 gas 0.25 dm 3 C 2 H 2 will require dm 3 H 2 gas Volume of H 2(g) required 0.5 dm 3. * g of ammonia at STP occupies volume of 4.48 dm 3.Calculate molar mass of ammonia dm 3 of a gas at STP 1 mol of gas molar mass of the gas 4.48 dm 3 of NH 3 at STP 3.49 g dm 3 of NH 3 at STP Molar mass of NH g mol 1. *3. Calculate the volume of oxygen required for complete combustion of 0.25 mole of methane at STP. 1 mol of methane (CH 4 ) at STP dm mol methane at STP dm 3 The combustion of CH 4 is represent as, CH 4(g) + 2O 2(g) CO 2(g) + 2H 2 O (l) 1 vol 2 vol 1 vol CH 4 1 dm 3 CH 4 2 dm 3 O dm 3 CH dm 3 Volume of O 2 required 11.2 dm 3 at STP *4. Calculate the number of moles and molecules of ammonia present in 5.6 dm 3 of its volume at STP dm 3 of NH 3 1 mol NH dm of NH mol NH mol NH 3 contains molecules of NH mol NH 3 contains molecules of NH 3 Number of moles of NH Number of molecules of NH *5. Calculate the mass of potassium chlorate required to liberate 6.72 dm 3 of oxygen at STP, molar mass of potassium chlorate is g/mol. Molar mass of potassium chlorate (KClO 3 ) g mol dm 3 of O 2 at STP 1 mol O dm 3 of O 2 at STP 1.16

2 mol O 2 KClO 3 KCl O2 1 mol 3 mol 2 3 mol O2 requires 1 mol KClO mol KClO g KClO mol O 2 requires 3/ g KClO 3 Mass of potassium chlorate required 24.5 g *6. Calculate number of atoms of hydrogen present in 5.6 g of urea (molar mass of urea 60 gmol 1 ). Also calculate number of atoms of N, C and O. Molecular formula of mass CO(NH 2 ) 2 CON 2 H 4 Molar mass of urea 60 g mol 1 W Number of moles of urea n M mol 1 mol urea contains 4 gram atoms of hydrogen (H) mol urea contains gram atom of H 1 gm atom of H contains atoms of H gm atom of H contains atoms of H 1 mol of urea contains 2 gram atoms of nitrogen (N) mol of urea contains gram atom of N Number of N atoms N atoms 1 mol of urea contains 1 gram atom of carbon (C) mol urea contains gram atom of C HORIZON Publications Number of C atoms mol of urea contains 1 gram atom of oxygen (O) mol urea contains gram atom of O Number of oxygen atoms (O) Number of H atoms Number of N atoms Number of C atoms Number of O atoms *7. Calculate the number of atoms of C, H and O in 72.5 g isopropanol, C 3 H 7 OH (molar mass 60). Molar mass of isopropanol, C 3 H 7 OH 60 g mol 1 Mass of isopropanol 72.5 g Number of moles of isopropanol mass of isopropanol molar mass W n M mol 1 mol of C 3 H 7 OH 3 gram atom of C 8 gram atom of H 1 gram atom of O. Gram atom of C Number of atoms of C Gram atom of H Number of atoms of H Gram atom of O Number of atoms of O Number of atoms of C Number of atoms of H Number of atoms of O

3 *8. Phosphoric acid is widely used in carbonated beverages, detergents, toothpastes and fertilizers. Calculate the mass percentage of H, P and O in phosphoric acid if atomic masses are H 1, P 31 and O 16. Phosphoric acid has molecular formula H 3 PO 4. Molecular (molar) mass of H 3 PO g mol 1 Percentage composition of H in H 3 PO 4 Atomicmass of H Molecular mass of H PO % Percentage composition of P in H 3 PO 4 Atomicmass of P Molecular mass of H PO % 98 Percentage of O 100 ( ) % Percentage of H in H 3 PO % Percentage of P in H 3 PO % Percentage of O in H 3 PO % *9. Calculate the mass percentage composition of the elements in nitric acid. (H 1, N 14, O 16). The molecular formula of nitric acid is NHO 3. Molecular mass of HNO g mol 1 Percentage composition of H in HNO 3 Atomicmass of H 100 Molecular mass of HNO % 63 Percentage composition of N in HNO 3 Atomicmass of N 100 Molecular mass of HNO % 63 Percentage composition of O in HNO ( ) % Percentage composition of H in NHO % Percentage composition of N in HNO % Percentage composition of O in HNO % *10. Analysis of vitamin C shows that it contains % carbon by mass 4.58 % hydrogen and % oxygen. Determine the empirical formula of vitamin C. Element % by mass Gram atom C H O Atomic ratio Whole No. ratio (Atoms) Hence the empirical formula of vitamin C is C 3 H 4 O 3. Empirical formula of vitamin C is C 3 H 4 O An inorganic compound contained 24.75% (w/w) potassium and 34.75% (w/w) manganese and some other common elements. Give the empirical formula of the compound. (K 39u, Mn 59u, O 16u). [Intext Question text book page no. 17] Given : Atomic mass of K 39 u, Mn 59 u and O 16 u Percent mass of potassium % Percent mass of manganese % Total percent mass % Remaining mass must be that of oxygen Percent mass of oxygen % To find : The empirical formula of the given inorganic compound? Moles of potassium % of potassium Atomic mass of potassium Moles of manganese % of manganese Atomic mass of manganese 1.18

4 Moles of oxygen % of oxygen Atomic mass of oxygen Ratio of K : Mn : O : : 1.08 :1: :1: The empirical formula of given inorganic compound is KMnO Phosphoric acid used in carbonated beverages contain 3.086% (w/w) hydrogen, 31.61% (w/w) phosphorous and remaining oxygen. If atomic masses of hydrogen, phosphorous and oxygen are 1.01 u, 31.o u and 16 u respectively and if molar mass of phosphoric acid is g mol -1, what is the molecular formula of phosphoric acid? [Intext Question text book page no. 17] Given : Atomic mass of H 1.01 u, P 31.0 u and O 16 u Percent mass of hydrogen % Percent mass of phosphorous % Total percent mass % Remaining amount is oxygen Percent mass of oxygen % To find : The molecular formula of phosphoric acid? Moles of hydrogen Moles of phosphorous Moles of Oxygen % mass of hydrogen Atomic mass of hydrogen % mass of phosphorous Atomic mass of phosphorous % mass of oxygen Atomic mass of oxygen Ratio of H : P : O : : 3:1: Hence, the empirical formula of phosphoric acid H 3 PO 4 HORIZON Publications Empirical formula mass 3 (1.01) + 1 (31) + 4 (16) g mol 1 Molar mass r 1 Empirical formula mass Molecular formula r Empirical formula l H 3 PO 4 H 3 PO 4 The molecular formula of phosphoric acid is H 3 PO 4. Multiple Choice Questions 1. The gas law was formulated by (a) Avogadro (b) Gay-Lussac (c) Aristotle (d) Joseph Priestley 2. The branch of chemistry which deals with carbon compound is called chemistry. (a) organic (b) inorganic (c) carbon (d) bio 3. can not be carried out in a lab. (a) Photosynthesis (b) Reduction (c) Oxidation (d) Hydration 4. How may moles of electron weigh one kilogram? (a) (b) (c) (d) What is the mass of 0.5 mole of ozone molecules? (a) 8 g (b) 16 g (c) 24 g (d) 48 g 6. Percentage of nitrogen in urea is about (a) 46 % (b) 85 % (c) 18 % (d) 28 % 7. Which of the following has same moleculear formula and empirical formula? (a) CO 2 (b) C 6 H 12 O 6 (c) C 2 H 4 (d) all of these 8. The empirical formula of C 2 H 2 is. (a) C 2 H 4 (b) CH (c) CH 4 (d) all of these 1.19

5 9. is the quantitative relationship between the reactants and products in a balanced chemical equation. (a) Stiochiometry (b) Complexometry (c) Chemistry (d) Reactions 10. If 0.5 mol of BaCl 2 is mixed with 0.2 mole of Na 3 PO 4, the maximum number of moles of Ba 3 (PO 4 ) 2 that can be formed is (a) 0.7 (b) 0.5 (c) 0.3 (d) When oxygen forms ozone, the atomicity changes from (a) 1 to 2 (b) 1 to 3 (c) 2 to 3 (d) 2 to If dm 3 of a gas at STP weigths 0.06g, the moleuclar mass of the gas will be (a) 6 (b) 600 (c) 30 (d) mg of sulphur on comustion will give number of molecules of SO 2 equal to (a) (b) (c) 0.2 (d) mol of NH 4 NO 3 will weigh (Atomic mass, H 1m N 14, O 16) (a) kg (b) kg (c) kg (d) kg 15. The number of atoms of He present in 104 u of He is (a) 13 (b) 26 (c) kg (d) kg 16. Mole is the SI unit of (a) Volume (b) Pressure (c) Amount of substance (d) Density 17. Mixture of solids constitute system. (a) Homogeneous (b) Heterogenous (c) Either (a) or (b) (d) Neither (a) not (b) 18. Two elements, A and B, combine to form a compound in which a g of A combines with b 1 and b 2 g of B respectively. According to law of multiple proportion.. (a) b 1 b 2 (b) b 1 and b 2 bear a simple whole number ratio (c) a 1 and b 1 bear a whole number ratio (d) no relation exists between b 1 and b Molecule is the smallest particle of.. (a) compound (b) substance (c) mixture (d) element 20. One mole of oxygen weighs (a) 8 g (b) 32 g (c) 16 g (d) g 21. The number of atoms present in a molecule of a substance is called. (a) Atomicity (b) Volume (c) Density (d) Mass 22. The number of molecules present in 8 g of oxygen gas are (a) (b) (c) (d) Atomicity of mercury vapour is.. (a) 1 (b) 2 (c) 3 (d) 4 Answers 1. b 2. a 3. a 4. d 5. c 6. a 7. a 8. b 9. a 10. d 11. c 12. d 13. a 14. d 15. b 16. c 17. b 18. b 19. a 20. b 21. a 22. d 23. a 1.20

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