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Vidaara.orgClass 12 · Chemistry
CodeVID-C12-02-T1-01
Concentration & Solubility — Topic Assignment
Chapter: Solutions
Topic: Concentration & Solubility
Maximum Marks: 30
Time: 60 minutes
Name: ____________________ Roll No.: __________ Date: ____________

General Instructions

  • 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. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions 5 × 1 = 5 marks
1.
Molarity is expressed in:
  • A.mol kg$^{-1}$
  • B.mol L$^{-1}$
  • C.g L$^{-1}$
  • D.dimensionless
2.
Which is a solid-in-solid solution?
  • A.Soda water
  • B.Brass
  • C.Air
  • D.Salt water
3.
Henry's law constant $K_H$ has units of:
  • A.mol L$^{-1}$
  • B.pressure (bar/Pa)
  • C.dimensionless
  • D.kg mol$^{-1}$
4.
Mole fraction of solvent + mole fraction of solute equals:
  • A.0
  • B.0.5
  • C.1
  • D.depends on amounts
5.
ppm is most appropriate for expressing:
  • A.concentrated acids
  • B.trace pollutants
  • C.molten alloys
  • D.pure solvents
Section B — Short Answer (2 marks) 3 × 2 = 6 marks
6.
Define molality and explain why it is preferred over molarity in colligative-property work.
7.
State Henry's law and write its mathematical form.
8.
Why does the solubility of gases in water decrease with rising temperature?
Section C — Short Answer (3 marks) 2 × 3 = 6 marks
9.
A solution contains 12 g of urea (M = 60) in 250 g of water. Calculate (i) molality and (ii) mole fraction of urea.
10.
The Henry's constant for $\text{O}_2$ in water at 298 K is $34.86\ \text{kbar}$. Find the mole fraction of $\text{O}_2$ at a partial pressure of 0.21 bar.
Section D — Long Answer (5 marks) 1 × 5 = 5 marks
11.
A 6.90 M solution of KOH (M = 56) has a density of $1.288\ \text{g mL}^{-1}$. Calculate (i) mass percentage and (ii) molality of the solution.

Answer Key

Section A — Multiple Choice Questions
  1. (B) mol L$^{-1}$
  2. (B) Brass
  3. (B) pressure (bar/Pa)
  4. (C) 1
  5. (B) trace pollutants
Section B — Short Answer (2 marks)
  1. Molality $m=\frac{n_{solute}}{w_{solvent}(\text{kg})}$. Being mass-based it is independent of temperature, whereas molarity changes as the solution volume changes with temperature.
  2. The partial pressure of a gas over a solution is proportional to its mole fraction in solution: $p=K_H x$.
  3. Dissolution of a gas is exothermic; by Le Chatelier's principle, raising temperature shifts the equilibrium to release dissolved gas, lowering solubility.
Section C — Short Answer (3 marks)
  1. $n_{urea}=12/60=0.2$ mol; molality $=0.2/0.250=0.8$ m. $n_{water}=250/18=13.89$; $x_{urea}=0.2/(0.2+13.89)=0.0142$.
  2. $x_{O_2}=p/K_H=0.21/34860=6.02\times10^{-6}$.
Section D — Long Answer (5 marks)
  1. Mass of KOH in 1 L $=6.90\times56=386.4$ g; mass of solution $=1000\times1.288=1288$ g. Mass % $=386.4/1288\times100=30.0\%$. Mass of water $=1288-386.4=901.6\ \text{g}=0.9016$ kg; molality $=6.90/0.9016=7.65$ m.
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