Vidaara.orgClass 12 · Chemistry
CodeVID-C12-02-CH-01
Solutions — Full Chapter Assignment
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- This is a full-length test covering the whole chapter — every topic is included.
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks. Show all working for Sections B, C and D.
Section A — Multiple Choice Questions
6 × 1 = 6 marks
1.
Molality is independent of:
- A.amount of solute
- B.temperature
- C.mass of solvent
- D.identity of solute
2.
Henry's law constant for a gas is large when the gas is:
- A.highly soluble
- B.poorly soluble
- C.a solid
- D.non-polar only
3.
Benzene + toluene forms:
- A.a positive-deviation solution
- B.a negative-deviation solution
- C.an ideal solution
- D.a maximum-boiling azeotrope
4.
The relative lowering of vapour pressure equals the mole fraction of the:
- A.solvent
- B.solute
- C.azeotrope
- D.gas
5.
Osmotic pressure is given by:
- A.$K_b m$
- B.$CRT$
- C.$K_f m$
- D.$p^0 x$
6.
For complete dissociation of $\text{Al}_2(\text{SO}_4)_3$, the van't Hoff factor is:
- A.2
- B.3
- C.4
- D.5
Section B — Short Answer (2 marks)
4 × 2 = 8 marks
7.
State Raoult's law for a non-volatile solute and write the expression for the solution's vapour pressure.
8.
Define isotonic solutions and give an everyday example.
9.
Why is the freezing point of a solution lower than that of the pure solvent?
10.
Explain why aquatic life is more comfortable in cold water than warm water.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
11.
A 5% (w/w) aqueous solution of a non-volatile solute (M = 90) is prepared. Find its molality and mole fraction of solute.
12.
At 298 K the osmotic pressure of a glucose solution is 1.52 bar. Calculate its molar concentration. $R=0.083$.
Section D — Long Answer (5 marks)
2 × 5 = 10 marks
13.
A 1.0 m solution of $\text{MgCl}_2$ shows a boiling-point elevation of 1.30 K. Given $K_b=0.52$, calculate (i) the van't Hoff factor, (ii) the degree of dissociation, and state the expected limiting value of $i$.
14.
Define colligative properties and describe all four, giving the equation for each and explaining how the van't Hoff factor modifies them for an electrolyte.
Answer Key
Section A — Multiple Choice Questions
- (B) temperature
- (B) poorly soluble
- (C) an ideal solution
- (B) solute
- (B) $CRT$
- (D) 5
Section B — Short Answer (2 marks)
- The vapour pressure of the solution equals the pure-solvent vapour pressure times the solvent mole fraction: $p_{solution}=p_{solvent}^0 x_{solvent}$.
- Solutions with the same osmotic pressure are isotonic; e.g. 0.9% NaCl saline is isotonic with blood cells.
- The dissolved solute lowers the solvent's vapour pressure, so the solid-liquid equilibrium is reached at a lower temperature, depressing the freezing point by $\Delta T_f=K_f m$.
- Gas solubility decreases with temperature, so cold water holds more dissolved oxygen, supporting aquatic life better.
Section C — Short Answer (3 marks)
- In 100 g solution: 5 g solute, 95 g water. $n_{solute}=5/90=0.0556$ mol; molality $=0.0556/0.095=0.585$ m. $n_{water}=95/18=5.28$; $x_{solute}=0.0556/(0.0556+5.28)=0.0104$.
- $C=\Pi/(RT)=1.52/(0.083\times298)=1.52/24.73=0.0615$ M.
Section D — Long Answer (5 marks)
- Calculated $\Delta T_b=0.52\times1.0=0.52$ K. $i=1.30/0.52=2.50$. For MgCl2, $n=3$; $\alpha=(i-1)/(n-1)=1.50/2=0.75$ (75%). The limiting (complete dissociation) value of $i$ is 3.
- Colligative properties depend only on the number of solute particles: (1) relative lowering of vapour pressure $(p^0-p)/p^0=x_2$; (2) elevation of boiling point $\Delta T_b=K_b m$; (3) depression of freezing point $\Delta T_f=K_f m$; (4) osmotic pressure $\Pi=CRT$. For electrolytes the van't Hoff factor $i$ accounts for dissociation/association, giving $\Delta T_b=iK_b m$, $\Delta T_f=iK_f m$ and $\Pi=iCRT$.
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