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JEE Main Mole Concept Previous Year Questions With Solutions 1. Number of atoms in the following samples of substances is the largest in : (1) 127.0g of iodine (2) 48.0g of magnesium (3) 71.0g of chlorine (4) 4.0g of hydrogen Solution: 1 mole represents 6.023×1023 particles. 1 mole of iodine atom= 6.023×1023 Given 127.0g of iodine. no. of iodine atom = 1 mole of iodine 1mole of magnesium = 24g of Mg = 6.023×1023no.of Mg Given 48g of Mg = 2×6.023×1023 no. of Mg = 2 moles of Mg 1 mole of chlorine atom= 6.023× 1023 no. of chlorine atom = 35.5g of chlorine atom Given 71g of chlorine atom=2× 6.023× 1023 no. of chlorine atom = 6.023×1023 2 moles of chlorine atom. Given that 4g of hydrogen atom. will be equal to 4 × 6.023 × 1023 no. of atoms of hydrogen= 4 moles of hydrogen atom. Hence option(4) is the answer. 2. The ratio of mass percent of C and H of an organic compound (CxHyOz) is 6 : 1. If one molecule of the above compound ( CxHyOZ ) contains half as much oxygen as required to burn one molecule of compound CXHY completely to CO2 and H2O. The empirical formula of compound CxHyOz is : (1) C2H4O (2) C3H4O2 (3) C2H4O3 (4) C3H6O3 Solution: Given the ratio of mass percent of C and H of an organic compound (CxHyOz) is 6 : 1. Atomic mass of carbon = 12 Atomic mass of Hydrogen = 1 If we have x atoms of Carbon and y atoms of Hydrogen, 12*x = 6(1*y) 12x = 6y So y = 2x Given one molecule of compound ( CxHyOZ ) contains half as much oxygen as required to burn one molecule of compound CXHY completely to CO2 and H2O CXHy + O2 → xCO2+ (y/2)H2O Put y = 2x in above equation CXH2x + O2 → xCO2+ xH2O Oxygen needed = 2x+x = 3x z is half of oxygen required to burn. So z = 3x/2 = 1.5 x Check the given options which satisfies z = 1.5x. So the empirical formula is C2H4O3. Hence option (3) is the answer. 3. The concentrated sulphuric acid that is peddled commercially is 95% H2SO4 by weight. If the density of this commercial acid is 1.834 g cm-3, the molarity of this solution is :- (1) 17.8 M (2) 15.7 M (3) 10.5 M (4) 12.0 M Solution: Given Density = 1.834 1 ml solution contains 1.834 g 1000 ml solution will contain 1834 g 95% H2SO4 means 100 gm contain 95 gm H2SO4 Mass of solute = (95/100)×1834 Molecular weight of H2SO4 = 98 Molarity = No. of moles/ volume = mass of solute/98 = (95/100)×(1834/98) = 17.8 M Hence option (1) is the answer. 4. The ratio of masses of oxygen and nitrogen in a particular gaseous mixture is 1 : 4. The ratio of number of their molecule is : (1) 1 : 8 (2) 3 : 16 (3) 1 : 4 (4) 7 : 32 Solution: Given ratio of masses of oxygen and nitrogen = 1:4 Let mass of O2 = w Mass of N2 = 4w Molecules of O2 = w/(32×NA) Molecules of N2 = 4w/(28×NA) Ratio of number of molecules = w/(32×NA)÷4w/(28×NA) = w/(32×NA)×(28×NA)/4w = 7/32 So the ratio is 7:32. Hence option (4) is the answer. 5. 3g of activated charcoal was added to 50 mL of acetic acid solution (0.06N) in a flask. After an hour it was filtered and the strength of the filtrate was found to be 0.042 N. The amount of acetic acid adsorbed (per gram of charcoal) is : (1) 42 mg (2) 54 mg (3) 18 mg (4) 36 mg Solution: Molarity of CH3COOH solution = mass of acetic acid/molar mass)/volume of solution in litre Acetic acid is monobasic. 0.042 = W/(60×0.05) W = 0.042×60×0.05 = 0.126 g Amount of acetic acid actually adsorbed = 0.180-0.126 = 54mg Amount of charcoal available = 3 g So amount of acetic acid adsorbed per gram of charcoal = 54mg×1g/3.0g = 18 mg Hence option (3) is the answer. 6. The density of a solution prepared by dissolving 120 g of urea (mol. mass = 60 u) in 1000 g of water is 1.15 g/mL. The molarity of this solution is (1) 2.05 M (2) 0.50 M (3) 1.78 M (4) 1.02 M Solution: Given density of solution = 1.15g/mL mass of solution = 1000+120 = 1120 gm Molar mass = 60 Volume = mass /density of solution = 1120/1.15 No. of moles = 120/60 = 2 Molarity = No. of moles/ volume = 2÷ (1120×10-3/1.15) = 2×1.15×1000 /1120) = 2.05 M Hence option (1) is the answer. 7. The ratio of number of oxygen atoms (O) in 16.0g oxygen (O), 28.0 g carbon monoxide (CO) and 32.0g oxygen (O2) is : (Atomic mass :C =12, O =16 and Avogadro’s constant NA = 6.0 * 1023 mol-1) (1) 3 : 1 : 1 (2) 1 : 1 : 2 (3) 3 : 1 : 2 (4) 1 : 1 : 1 Solution: Molar mass of O3 = 48 Given 16 g O3 . So no. of moles of O3 = 16/48 = ⅓ 1 mole = 3 ×NA oxygen atoms So 1/3 mole = NA×3×1/3 no of atoms = NA oxygen atoms Molar mass of CO = 28 Given 28 g CO. So no of moles = 28/28 = 1 No. of atoms = 1×NA = NA Molar mass of O2 = 32 Given 32g O2 No. of moles = 32/32 = 1 No.of atoms = 1×NA = NA So the ratio is 1:1:1 Hence option (4) is the answer. 8. When CO2 (g) is passed over red hot coke it partially gets reduced to CO(g). Upon passing 0.5 litre of CO2 (g) over red hot coke, the total volume of the gases increased to 700 mL. The composition of the gaseous mixture at STP is : (1) CO2 = 200 mL: CO = 500mL (2) CO2 = 350 mL: CO = 350mL (3) CO2 = 0.0 mL: CO = 700mL (4) CO2 = 300 mL: CO = 400mL Solution: CO2 (g) + C (s) → 2CO (g) Total volume = 700 ml = 0.7 L 0.5+x = 0.7 x = .2L = 200 mL CO2 (g) = 0.5-0.2 = 300ml CO (g) = 2x = 400 mL Hence option (4) is the answer. 9. An open vessel at 300 K is heated till ⅖ th of the air in it is expelled. Assuming that the volume of the vessel remains constant, the temperature to which the vessel is heated is : (1) 750 K (2) 400 K (3) 500 K (4) 1500K Solution: At constant V and P, n1T1 = n2T2 n1 = n n2 = n-2n/5 = 3n/5 T1 = 300 K 300 n = (3n/5) T2 T2 = 300×5/3 = 500 K Hence option (3) is the answer. 10. The density of 3M solution of sodium chloride is 1.252 g mL-1. The molality of the solution will be (molar mass, NaCl = 58.5 g mol-1) (1) 2.18 m (2) 3.00 m (3) 2.60 m (4) 2.79 m Solution: Given Molar mass of NaCl = 58.5 g M = 3 mol L-1 Mass of weight W2 of NaCl in 1L solution W2 = 3×58.5 = 175.5g Mass of L solution = V × d = 1000 ×1.25 = 1250g Mass of H2O in solution (W1) = 1250-175.5 = 1074g m = W2×1000/Mw2 ×W1 = (175.5×1000)/58.5×1074.5 = 2.79m Hence option (4) is the answer. 11. 0.6 g of urea on strong heating with NaOH evolves NH3. Liberated NH3 will combine with which of the following HCl solution? (1) 100 mL of 0.2 N HCl (2) 400 mL of 0.2 N HCl (3) 100 mL of 0.1 N HCl (4) 200 mL of 0.2 N HCl Solution: NH2CONH2 + 2NaOH → Na2CO3 + 2NH3 2 mole of urea ≡ one mole of NH3 one mole of NH3 = one mole of HCl So one mole of HCl = 2 mole of urea = 2×0.6/60 = 0.02 mol. Hence option (1) is the answer. 12. Calculate the mass of FeSO4.7H2O which must be added in 100 kg of wheat to get 10 PPM of Fe. Solution: Ppm = (Mass of Fe/total mass)×106 Total mass = 100 kg = 100 × 1000 g Mass of Fe = (ppm × total mass )/106 = 10× 100 × 1000/106 = 1 g Molecular mass of FeSO4.7H2O = 278 Mass of one Fe = 56 g 56 g of Fe → 278 g of FeSO4.7H2O So 1 g of Fe → 278/56 = 4.96 g Hence 4.96 g is the answer. 13. Given a solution of HNO3 of density 1.4 g/mL and 63% w/w. Determine the molarity of HNO3 solution. Solution: Density = mass/volume of solution Volume = mass / density = 100g/1.4 g/ml = (100/1.4)ml Molarity = no. of moles of solute /Volume of solution( l) = 1.4×1000/100 = 14 M Hence 14 M is the answer. 14. A transition metal M forms a volatile chloride which has a vapour density of 94.8. If it contains 74.75% of chlorine the formula of the metal chloride will be (1) MCl2 (2) MCl4 (3) MCl5 (4) MCl3 Solution: Given vapour density = 94.8 Vapour density = molecular mass/2 Molecular mass = 94.8×2 = 189.6 Given 74.75% chlorine. So 74.75/100 * 189.6 = 141.72 g of chloride is there. Then the number of atoms of chloride will be 141.72/35.5 =3.97 which is approximately 4. So the formula of metal chloride will be MCl4. Hence option (2) is the answer. 15. 10 mL of 2(M) NaOH solution is added to 200 mL of 0.5 (M) of NaOH solution. What is the final concentration? (1) 0.57 M (2) 5.7 M (3) 11.4 M (4) 1.14 M Solution: No. of moles of NaOH in 10 mL of 2 M solution = (10/1000)×2 = 0.02 mol Number of moles of NaOH in 200 mL of 0.5M solution = (200/1000)×0.5 = 0.1 mol Total number of moles of NaOH = 0.02+0.1 = 0.12 mol Total volume = 10+200 = 210 mL = 0.210 L Final concentration = 0.12/0.210 = 0.57 M Hence option (1) is the answer. 16. A 5.2 molal aqueous solution of methyl alcohol, CH3OH, is supplied. What is the mole fraction of methyl alcohol in the solution? (1) 0.086 (2) 0.050 (3) 0.100 (4) 0.190 Solution: We know mole fraction = moles of solute/(moles of solute + moles of solvent) Let mass of water is 1 kg . Moles of CH3OH is 5.2 Xsolute = 5.2/(5.2+1000/18) = 5.2/(5.2+55.556) = 5.2/60.756 = 0.086 Hence option (1) is the answer