JEE Chemistry Solutions MCQs Set 34

Multiple Choice Questions (MCQs) for JEE Chemistry: Solutions

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Practice Solutions MCQs for JEE Chemistry

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Multiple Choice Questions

Question 1: In an aqueous solution, ethylene glycol has a mass percentage (w/w) of 30%. The mole fraction of ethylene glycol will be:
(a) 0.428
(b) 0.124
(c) 0.11
(d) 0.889
Show Answer & Explanation

Answer: (c) 0.11

Explanation:
1. In 100 g: glycol = 30/62 = 0.484 mol; water = 70/18 = 3.89 mol.
2. X = 0.484/(0.484 + 3.89) = 0.11.
3. Answer: (c)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 2: What will be the approximate mass percentage of the solution made by mixing 500 g of 15% (w/w) aqueous urea solution with 400 g of 25% (w/w) aqueous solution of it?
(a) 18%
(b) 20%
(c) 25%
(d) 15%
Show Answer & Explanation

Answer: (b) 20%

Explanation:
1. Urea = 0.15 × 500 + 0.25 × 400 = 75 + 100 = 175 g in 900 g.
2. Percentage = 175/900 × 100 = 19.4%, closest to 20%.
3. Answer: (b)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 3: What will be the molality of a solution made by dissolving 3.7 g of propanoic acid in 80 g of benzene?
(a) 0.77 m
(b) 0.625 m
(c) 0.045 m
(d) 46.25 m
Show Answer & Explanation

Answer: (b) 0.625 m

Explanation:
1. Moles of CH3CH2COOH = 3.7/74 = 0.05.
2. Molality = 0.05/0.080 kg = 0.625 m.
3. Answer: (b)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 4: If sea water is a 3.5% (w/w) aqueous solution of NaCl, its molality will be:
(a) 0.598
(b) 36.27 m
(c) 0.62 m
(d) 0.578 m
Show Answer & Explanation

Answer: (c) 0.62 m

Explanation:
1. 3.5 g NaCl = 0.0598 mol in 96.5 g of water.
2. Molality = 0.0598/0.0965 = 0.62 m.
3. Answer: (c)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 5: A 10% (w/v) solution of glucose is isotonic with a 4% (w/v) solution of a non-volatile solute. The molar mass of the solute will be:
(a) 36
(b) 72
(c) 54
(d) 63
Show Answer & Explanation

Answer: (b) 72

Explanation:
1. Isotonic solutions have equal molar concentration: 10/180 = 4/M.
2. M = 72 g mol−1.
3. Answer: (b)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 6: The vapour pressure of a mixture of toluene and xylene at 90°C is 0.5 atm. If at this temperature the vapour pressures of pure toluene and xylene are 400 mm and 150 mm respectively, what is the mole fraction of toluene in the mixture?
(a) 0.08
(b) 0.92
(c) 0.88
(d) 0.46
Show Answer & Explanation

Answer: (b) 0.92

Explanation:
1. Convert: 0.5 atm = 380 mm Hg.
2. 380 = 150 + x(400 − 150), so x = 230/250 = 0.92.
3. Answer: (b)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 7: What will be the freezing point of a 0.2 molal aqueous solution of MgBr2, if the salt dissociates 40% in solution and Kf for water is 1.86 K kg mol−1?
(a) −3.35°C
(b) −0.67°C
(c) −0.6°C
(d) −0.45°C
Show Answer & Explanation

Answer: (b) −0.67°C

Explanation:
1. i = 1 + 2α = 1 + 2(0.4) = 1.8.
2. ΔTf = 1.8 × 1.86 × 0.2 = 0.67, so Tf = −0.67°C.
3. Answer: (b)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 8: The density of a 12.25% (w/w) H2SO4 solution is 1.052 g/mL. Its molarity is:
(a) 1.315 M
(b) 2.63 M
(c) 0.657 M
(d) 1 M
Show Answer & Explanation

Answer: (a) 1.315 M

Explanation:
1. M = (% × 10 × d)/molar mass = 12.25 × 10 × 1.052/98.
2. = 1.315 M.
3. Answer: (a)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 9: A 0.1 molal solution of Hg(NO3)2 freezes at −0.558°C. The cryoscopic constant of water is 1.86 K kg mol−1. What is the percentage ionisation of the salt?
(a) 33.33%
(b) 50%
(c) 75%
(d) 100%
Show Answer & Explanation

Answer: (d) 100%

Explanation:
1. i = ΔTf/(Kfm) = 0.558/(1.86 × 0.1) = 3.
2. i = 1 + 2α = 3 gives α = 1, i.e. 100%.
3. Answer: (d)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 10: Which solution has the maximum vapour pressure?
(a) 1 N KNO3
(b) 1 N Ba(NO3)2
(c) 1 N Al2(SO4)3
(d) 1 N Ti(NO3)4
Show Answer & Explanation

Answer: (c) 1 N Al2(SO4)3

Explanation:
1. Particle concentration = molarity × ions: KNO3 1 × 2 = 2; Ba(NO3)2 0.5 × 3 = 1.5; Ti(NO3)4 0.25 × 5 = 1.25.
2. Al2(SO4)3 is (1/6) × 5 = 0.83, the least, so it has the highest vapour pressure.
3. Answer: (c)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 11: When HgI2 is added to a KI solution, the freezing point of the solution:
(a) increases
(b) decreases
(c) remains unchanged
(d) cannot be predicted
Show Answer & Explanation

Answer: (a) increases

Explanation:
1. HgI2 + 2KI → K2[HgI4].
2. Fewer particles (2K+ + 1 complex ion in place of 2K+ + 2I−) are present, so the depression falls and the freezing point rises.
3. Answer: (a)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 12: A solute dissociates in solution according to the reaction 2A → 5B. If the solute shows 30% dissociation, the van't Hoff factor will be:
(a) 2.2
(b) 1.45
(c) 2.9
(d) 1.9
Show Answer & Explanation

Answer: (b) 1.45

Explanation:
1. Per mole of A, 2.5 mol of B form, so i = 1 + α(2.5 − 1).
2. i = 1 + 0.3 × 1.5 = 1.45.
3. Answer: (b)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 13: An aqueous solution of urea (6% w/v) is isotonic with an NaCl solution. The mass-volume percentage (% w/v) of the NaCl solution will be:
(a) 1.46%
(b) 5.85%
(c) 2.92%
(d) 11.7%
Show Answer & Explanation

Answer: (c) 2.92%

Explanation:
1. Urea: 6 g/100 mL = 0.1 mol/100 mL (1 M).
2. NaCl (i = 2) needs 0.5 M = 0.05 mol/100 mL = 2.925 g, i.e. 2.92%.
3. Answer: (c)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 14: The vapour pressures of pure liquids A and B at 350 K are 440 and 720 mm Hg. If the total vapour pressure of the solution is 580 mm Hg, the mole fraction of liquid A in the vapour phase will be:
(a) 0.31
(b) 0.38
(c) 0.62
(d) 0.76
Show Answer & Explanation

Answer: (b) 0.38

Explanation:
1. 580 = 720 − 280 xA, so xA = 0.5 in the liquid.
2. yA = (0.5 × 440)/580 = 0.38.
3. Answer: (b)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 15: The composition of the azeotropic mixture of HCl and H2O is:
(a) 85% H2O + 15% HCl (by mass)
(b) 50% H2O + 50% HCl
(c) 70.8% H2O + 29.2% HCl
(d) 79.8% H2O + 20.2% HCl
Show Answer & Explanation

Answer: (d) 79.8% H2O + 20.2% HCl

Explanation:
1. HCl and water form a maximum boiling azeotrope at about 108.6°C.
2. It contains 20.2% HCl and 79.8% water by mass.
3. Answer: (d)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 16: If T1, T2 and T3 are the boiling points of component A, component B and the azeotropic mixture, which relation is correct for a minimum boiling azeotrope?
(a) T1 < T3
(b) T2 < T3
(c) T3 < T1
(d) T3 = T2
Show Answer & Explanation

Answer: (c) T3 < T1

Explanation:
1. A minimum boiling azeotrope boils below both pure components.
2. So T3 is lower than T1 (and T2).
3. Answer: (c)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 17: Which of the following relationships is correct? (L = latent heat per gram, R in J)
(a) Kb = RTb2/(1000 × ΔVv)
(b) Kb = RTb2/(1000 Lv)
(c) Kf = RTf2/(1000 ΔHf)
(d) Kf = RTf2 × M/(1000 × Lf)
Show Answer & Explanation

Answer: (b) Kb = RTb2/(1000 Lv)

Explanation:
1. The elevation constant in terms of specific latent heat of vaporisation is Kb = RTb2/(1000 Lv).
2. In terms of molar enthalpy it would be RTb2M/(1000 ΔHv), so the other options mix the forms.
3. Answer: (b)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 18: Which of the following 0.2 m aqueous solutions will show the minimum freezing point?
(a) CaCl2
(b) K2SO4
(c) Al(NO3)3
(d) KBr
Show Answer & Explanation

Answer: (c) Al(NO3)3

Explanation:
1. The lowest freezing point needs the largest van't Hoff factor.
2. Al(NO3)3 gives 4 ions, against 3, 3 and 2 for the others.
3. Answer: (c)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 19: For which of the following solutions is the observed boiling point greater than the theoretical boiling point?
(a) CHCl3 + CCl4
(b) CCl4 + SiCl4
(c) C2H5OH + C6H6
(d) CHCl3 + C6H6
Show Answer & Explanation

Answer: (d) CHCl3 + C6H6

Explanation:
1. A higher boiling point than expected means lower vapour pressure, i.e. negative deviation from Raoult's law.
2. CHCl3 and C6H6 attract each other by weak hydrogen bonding, so they show negative deviation.
3. Answer: (d)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 20: The boiling point of a solution obtained by dissolving 0.51 g of anthracene in 35 g of chloroform increases by 0.32°C. What will be the molar mass of anthracene for chloroform Kb = 3.9 K kg mol−1?
(a) 175.2
(b) 177.6
(c) 178.6
(d) 182.3
Show Answer & Explanation

Answer: (b) 177.6

Explanation:
1. M = Kb × w × 1000/(ΔTb × W) = 3.9 × 0.51 × 1000/(0.32 × 35).
2. = 177.6 g mol−1.
3. Answer: (b)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 21: Which of the following relationships is correct? (A is solvent, B is solute)
(a) (P° − Ps)/P° = XA
(b) YB = (P°B − XB)/P°A
(c) PA = P°AXA
(d) Ps = P°AXA + P°BXA
Show Answer & Explanation

Answer: (c) PA = P°AXA

Explanation:
1. Raoult's law: partial pressure of a component = P° × its mole fraction, so PA = P°AXA.
2. The relative lowering equals XB, not XA, and Ps = P°AXA + P°BXB.
3. Answer: (c)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 22: What will be the osmotic pressure of a 5% (w/v) aqueous solution of urea at 17°C?
(a) 19.81 atm
(b) 1.16 atm
(c) 1.981 atm
(d) 6.61 atm
Show Answer & Explanation

Answer: (a) 19.81 atm

Explanation:
1. C = 5/60 per 100 mL = 0.833 mol L−1.
2. π = CRT = 0.833 × 0.0821 × 290 = 19.8 atm.
3. Answer: (a)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 23: The van't Hoff factor is 1.92 for a MgI2 solution of concentration 0.2 M. The degree of dissociation of the salt at this concentration is:
(a) 46%
(b) 96%
(c) 30.67%
(d) 64%
Show Answer & Explanation

Answer: (a) 46%

Explanation:
1. MgI2 gives 3 ions, so i = 1 + 2α.
2. 1.92 = 1 + 2α gives α = 0.46, i.e. 46%.
3. Answer: (a)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 24: The vapour pressure of any liquid shows the following change with temperature:
(a) exponential increase
(b) exponential decrease
(c) linear increase
(d) linear decrease
Show Answer & Explanation

Answer: (a) exponential increase

Explanation:
1. By the Clausius–Clapeyron relation, ln P varies linearly with 1/T.
2. So the vapour pressure rises exponentially with temperature.
3. Answer: (a)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

Question 25: During dissolution of a gas in a liquid solvent:
(a) heat is released
(b) heat is absorbed
(c) no heat transfer takes place
(d) the colour of the liquid solvent always changes
Show Answer & Explanation

Answer: (a) heat is released

Explanation:
1. Dissolving a gas reduces its freedom, so the process is exothermic.
2. That is why gas solubility falls as temperature rises.
3. Answer: (a)

Teacher's Note:
1. Write the colligative-property relation first and decide the van't Hoff factor. Check the answer against the options.

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