**Chp 1 Solutions board exam questions**

**HSC Exam 10 Years Que Ans**

**Multiple Choice Questions**

- Among the following equimolar aqueous solutions, identify the one having highest boiling point.

[Mar 08]

(A) Urea

(B) Sucrose

(C) Sodium chloride

(D) Sodium sulphate

2. The addition of non-volatile solute into the pure solvent

[Oct 08]

(A) increases the vapour pressure of solvent

(B) decreases the boiling point of solvent

(C) decreases the freezing point of solvent

(D) increases the freezing point of solvent

3. Which of the following solutions shows maximum depression in freezing point?

(A)

(B)

(C)

(D)

4. The temperature at which vapour pressure of a liquid becomes equal to the atmospheric pressure is

(A) melting point

(C)

(B) boiling point

(D)

[Oct 13]

5. Which of the following is a colligative property?

[Mar 14]

(A) Vapour pressure

(B) Depression in freezing point

(C) Elevation in boiling point

(D) Osmotic pressure

6. Colligative property depends only on in a solution.

[Mar 15]

(A) number of solute particles

(B) number of solvent particles

(C) nature of solute particles

(D) nature of solvent particles

7. The substance ‘ ‘, when dissolved in solvent water gave molar mass corresponding to the molecular formula ‘ ‘. The van’t Hoff factor (i) is

[Oct 15]

(A) 3

(B) 0.33

(C) 1.3

(D) 1

8. The determination of molar mass from elevation in boiling point is called as

[Mar 16]

(A) cryoscopy

(B) colorimetry

(C) ebullioscopy

(D) spectroscopy

9. Which of the following aqueous solutions will exert the highest osmotic pressure?

(A)

(B)

(C)

(D)

[Mar 18]

10. In calculating osmotic pressure, the concentration of solute is expressed in

(A) molarity

(C) mole fraction

(B) molality

(D) percentage mass

[Mar 22]

**Answers:**

1.

(D) 2 .

(C) 3 .

(C) 4

4.

(B)

9. (A) 10 . (A)

(B) (C)

- 5 . (A) 6

Solutions:

- The solution having highest number of particles will have highest boling point. gives 3 ions on complete dissociation. Hence, among the given equimolar aqueous solutions, solution has highest boiling point.
- shows maximum depression in freezing point since 0.5 mole of will give the highest number of particles, i.e., 1 mole of and 1.5 mole of ions as compared to other solutions. 0.5 mole of gives: 1.5 mole of ions, 1 mole of gives 2 mole of ions and 0.5 mole of gives 1.5 mole of ions.
- van’t Hoff factor (i)

- Colligative properties are proportional to the number of solute particles present in the solution.

Osmotic pressure is a colligative property; hence, exerts the highest osmotic pressure.

Calculation: For bcc unit cell, .

Using formula (i),

Density

.

Using formula (ii),

Ans: Radius of niobium atom ( ) is .

- Calculate the number of atoms and unit cells present in of Niobium if it forms body centred cubic structure. The density of Niobium is and edge length of unit cell is .

[July 22]

**Solution:**

Given: Type of unit cell is bcc.

Density ( ) of niobium is ,

Edge length (a) ,

Mass of niobium

To find: i. Number of atoms in of niobium

ii. Number of unit cells in of niobium

Formulae: i. Number of atoms in of metal

ii. Number of unit cells in of metal

Calculation: i. For bcc unit cell, .

Using formula (i),

Number of atoms in of niobium

ii. Using formula (ii),

Number of unit cells in of niobium

Ans: i. Number of atoms in of niobium is

ii. Number of unit cells in of niobium is .

Theory Orestions:

**2.4 Solubility**

- What is the effect of temperature on solubility of a gas in a liquid?

[Oct 15]

Ans:

i. When gases are dissolved in water, the gas molecules in liquid phase are condensed.

ii. The condensation is an exothermic process.

Hence, the solubility of gases in water decreases with increase in temperature.

- State Henry’s law. How does solubility of a gas in water varies with the temperature?

[Oct 13; July 17]

Ans:

i. Statement: Henry’s law states that the solubility of a gas in a liquid is directly proportional to the pressure of the gas over the solution.

ii. ‘ Refer Subtopic 2.4: Q. No. 1.

- State Henry’s law.

Ans: Refer Subtopic 2.4: Q. No. 2.(i).

2.6 Colligative properties of nonelectrolyte solutions

- Define colligative properties.

[Oct 08]

Ans: The physical properties of solutions that depend on the number of solute particles in solutions and not on their nature are called colligative properties.

2.7 Vapour pressure lowering

- Derive the relationship between relative lowering of vapour pressure and molar mass of non-volatile solute.

[Mar 13, 17]

Ans:

i. The relative lowering of vapour pressure is equal to the mole fraction of solute in the solution. That is,

ii. The mole fraction of a component of solution is equal to its moles divided by the total moles in the solution. Thus,

where, and are the moles of solvent and solute respectively, in the solution

iii. In dilute solutions, and . Thus, the mole fraction is then given by,

iv. From equations (1) and (2), .

v. Suppose that we prepare a solution by dissolving of a solute in of solvent. The moles of solute and solvent in the solution are then,

and

where, and are molar masses of solvent and solute, respectively.

vi. Substitution of equation (4) into equation (3) yields

2.8 Boiling point elevation

- What is ‘boiling point’?

[Mar 14]

Ans: The boiling point of liquid is the temperature at which its vapour pressure equals the applied pressure.

- Define ebullioscopic constant.

Write its unit.

[Oct 15]

Ans: Ebullioscopic constant ) is the boiling point elevation produced by 1 molal solution.

Units of :

- Derive the relation between elevation of boiling point and molar mass of solute.

[Mar 18] OR

Derive the mathematical expression between molar mass of a non-volatile solute and elevation of boiling point.

[Mar 20]

OR

How will you determine molar mass of non volatile solute by elevation of boiling point?

[Mar 23]

Ans:

i. The boiling point elevation is directly proportional to the molality of the solution.

Thus,

ii. Suppose we prepare a solution by dissolving of solute in of solvent.

Moles of solute in of solvent

where, is the molar mass of solute.

Mass of solvent

iii. The molality is expressed as,

Std. XII Sci: Board Questions with Solutions (Chemistry)

iv. Substituting equation (2)

Hence,

- Define ebullioscopic constant.

[Oct 09; July 22]

Ans: Refer Subtopic 2.8: Q. No. 2. (Definition)

**2.9 Depression in freezing point**

- Define boiling point. Write the formula to determine molar mass of a solute using freezing point depression method. [Mar 16]

Ans:

i. Refer Subtopic 2.8: Q. No. 1.

ii. The formula to determine molar mass of a solute using freezing point depression method is

- Define: Freezing point. OR

[July 16]

What is freezing point of a liquid? [July 18]

Ans: The freezing point of a liquid is the temperature at which liquid and solid are in equilibrium and the two phases have the same vapour pressure.

- Define :Cryoscopic constant. [Oct 14; July 18]

Ans: Cryoscopic constant is the depression in freezing point produced by 1 molal solution of a non-volatile solute.

- Write mathematical expression between cryoscopic constant and molar mass of solute.

[July 19]

Ans: Refer Subtopic 2.9: Q. No. 1.(ii).

- Write the SI unit of cryoscopic constant.

[Mar 23]

Ans: The SI unit of cryoscopic constant is .

**2.10 Osmotic pressure**

- Define semipermeable membrane. [Mar 18]

Ans: Semipermeable membrane is a film such as cellophane which has pores large enough to allow the solvent molecules to pass through them.

- Define osmotic pressure.

[Mar 20; July 17, 22]

Ans: The hydrostatic pressure (on the side of solution) that stops osmosis is called an osmotic pressure of the solution.

OR

The excess of pressure on the side of the solution that stops the net flow of solvent into the solution through a semipermeable membrane is called osmotic pressure.

- Define the following terms:

i. Isotonic solution [Oct 13; Mar 09, 19, 22]

ii. Hypertonic solution [Mar 19]

iii. Hypotonic solution [Mar 19]

Ans:

i. Two or more solutions having the same osmotic pressure are said to be isotonic solutions.

e.g. For example, urea solution and sucrose solution are isotonic because their osmotic pressures are equal.

ii. If two solutions have unequal osmotic pressures, the more concentrated solution with higher osmotic pressure is said to be hypertonic solution.

- e.g. For ‘example, if osmotic pressure of sucrose solution is higher than that of urea solution, the sucrose solution is hypertonic to urea solution.

iii. If two solutions have unequal osmotic pressures, the more dilute solution exhibiting lower osmotic pressure is said to be hypotonic solution.

e.g. For example, if osmotic pressure of sucrose solution is higher than that of urea solution, the urea solution is hypotonic to sucrose solution.

- Define osmosis. [Mar 13, 22, 23; July 16, 19]

Ans: The net spontaneous flow of solvent molecules into the solution or from more dilute solution to more concentrated solution through a semipermeable membrane is called osmosis.

- Derive an expression to calculate molar mass of non volatile solute by osmotic pressure measurement.

[Mar 22]

Ans:

i. For very dilute solutions, the osmotic pressure follows the equation,

ii. If the mass of solute in litres of solution is and its molar mass is , then .

Substituting the value of in equation (1), we get

This formula can be used for the calculation of molar mass of a nonionic solute (i.e., nonelectrolyte), by osmotic pressure ‘ measurement.

**2.11 Colligative properties of electrolytes**

- Define van’t Hoff factor. How is it related to the degree of dissociation?

[Oct 14]

Ans:

i. van’t Hoff factor (i) is defined as the ratio of colligative property of a solution of electrolyte divided by the colligative property of nonelectrolyte solution of the same concentration.

ii. The van’t Hoff factor is related to degree of ionization as follows:

or

where, Degree of ionization/dissociation

van’t Hoff factor

Moles of ions obtained from ionization of 1 mole of electrolyte.

**Numerical**

2.4 Solubility

- What is the concentration of dissolved oxygen at under pressure of one atmosphere if partial pressure of oxygen at is ? (Henry’s law constant for oxygen )

Solution: Henry’s law constant ,

[Mar 20]

Partial pressure of the gas

Ans: Concentration of dissolved oxygen is .

water at same temperature and partial pressure of ?

Solution: Henry’s law constant ,

Pressure of the gas

Solubility

Ans: SÃ³lubility of gas in water is .

**2.7 Vapour pressure lowering**

- The vapour pressure of pure benzene is of non-volatile solute is added to of benzene, the vapour pressure of solution is of . Calculate molar mass of solute .

[Mar 16]

**Solution:**

Given: Vapour pressure of pure benzene

Molar mass of benzene

Mass of non-volatile solute

Mass of benzene

To find: Molar mass of non-volatile solute

Formula:

Calculation: Using the formula,

Ans: The molar mass of the non-volatile solute is .

- Calculate the mole fraction of solute, if the vapour pressure of pure benezene at certain temperature is and vapour pressure of solution of a solute in benzene is .

**Solution:**

Given: Vapour pressure of pure benzene

Vapour pressure of solution

To find: , Mole fraction of solute

Formula:

Calculation: Using the formula,

Ans: The mole fraction of solute is .

**2.8 – Boiling point elevation**

- A solution containing of camphor (molar mass ) in of acetone (boiling point ) boils at . A solution of of unknown compound in the same weight of acetone boils at . Calculate the molar mass of the unknown compound.

[Oct 14]

**Solution:**

Given: Mass of camphor , Molar mass of camphor

MassÌƒ of acetone , Mass of unknown compound

To find: Molar mass of unknown compound

Formula:

CÃ¡lculation: For solution of camphor in acetone,

For solution of unknown compound in acetone,

Using the formula,

Ans:- The molar mass of the unknown compound is .

- of sulphur is dissolved in of . This solution boils at . What is molecular formula of sulphur in solution? The boiling point of is .

Solution:

[Given that for and atomic mass of .]

Given: Mass of sulphur , Mass of solvent

Boiling point of solution , Boiling point of pure solvent

Molal elevation constant

Atomic mass of sulphur

To find: Molecular formula of sulphur in solution

Formula:

Calculation: For solution of sulphur in (solvent),

From formula,

Now, atomic mass of

Number of Sulphur atoms in a molecule

Molecular formula of sulphur in

Ans: Molecular formula of sulphur in ‘solution is .

- The boiling point of benzene is . When 1.80 gram of non-volatile solute was dissolved in 90 gram of benzene, the boiling point is raised to . Calculate the molar mass of solute. for benzene

[Mar 17]

**Solution:**

Given:

Boiling point of benzene

Boiling point of solution

Mass of non-volatile solute

Mass of benzene

To find: Molar mass of solute

Formula:

Calculation: For solution of non-volatile solute and benzene,

Now, using formula and substituting values,

Ans: The molar mass of non-volatile solute is .

- The normal boiling point of ethyl acetate is . A solution of of non-volatile solute in of ethyl acetate boils at . Evaluate the molar mass of solute if for ethyl acetate is .

[July 22]

**Solution:**

Formula:

Calculation: For solution of non-volatile solute and ethyl acetate,

Now, using formula and substituting values,

Ans: The molar mass of non-volatile solute is .

**2.9 Depression in freezing point**

- of urea when dissolved in of a solvent, decreases freezing point of the solvent by of another non-electrolyte solute when dissolved in of the same solvent depresses the freezing point by . Calculate the molar mass of the another solute.

[Given: molar mass of urea ]

**Solution:**

For solution containing urea:

Mass of urea , Molar mass of urea

Mass of solvent

Depression in freezing point of solution containing urea

For solution containing unknown solute:

Mass of unknown solute

Mass of solvent

Depression in freezing point of solution containing unknown solute

To find: Molar mass of unknown solute

Formula:

Calculation: Using the formula and rearranging, we get,

From equations (1) and (2),

( Since the solvent is same in both solutions, the value of will be same for both the solutions.)

Ans: The molar mass of the unknown solute is .

- The freezing point of pure benzene is . Calculate the freezing point of the solution when of a solute having molecular weight is added to of benzene.

of benzene

**Solution:**

Freezing point of pure solvent

Mass of solute

Molar mass of solute

Mass of solvent

Molal depression constant

To find: Freezing point of solution

Formula:

Calculation: Rearranging the formula,

Ans: Freezing point of the solution is .

- aqueous solution of cane sugar has freezing point of . Calculate freezing point of glucose solution. [Molar mass of cane sugar ]

[July 22]

**Solution:**

Given: Percentage by mass of cane sugar solution

Percentage by mass of glucose solution ,

Freezing point of cane sugar solution

Molar mass of cane sugar

To find: . Freezing point of glucose solution

Formula:

Calculation: solution (by mass) of cane sugar means that mass of cane sugar , and mass of solvent .

glucose solution means that mass of glucose , and mass of solvent .

Molar mass of glucose

for cane sugar solution

Now, using the formula,

Rearranging the formula, we get

From equations (1) and (2),

Freezing point of glucose solution

Ans: Freezing point of glucose solution is .

**Alternate method:**

Formula:

Calculation: solution (by mass) of cane sugar means that mass of cane sugar , and mass of solvent .

for cane sugar solution

Now, using formula,

Now, glucose solution means that mass of glucose , and mass of solvent .

Molar mass of glucose

Using formula,

Freezing point of glucose solution

Ans: Freezing point of glucose solution is .

[Note: In the above problem, the solvent is water and therefore the value should be and based on this value, (glucose) will be and will be . However; we find that on using the alternate method, we get the value of as .]

**2.10 Osmotic pressure**

- of a substance dissolved in of water gave an osmotic pressure of 1.331 atmospheres at . Calculate the molecular weight of the substance.

(Given: ).

[Mar 09]

**Solution:**

Given:

Volume

Mass of substance

Osmotic pressure

Temperature

To find: Molecular weight

Formula:

Calculation: From formula,

Ans: The molecular weight of the solute is .

- A solution of a substance having mass has the osmotic pressure of at .

Calculate the molar mass of the substance used. [Volume ] [Mar 17]

**Solution:**

Given: Volume

Mass of substance

Osmotic pressure

or

Temperature

To find: Molar mass

Formula:

Calculation: From formula,

Ans: The molar mass of a solute is .

- An organic substance is dissolved in of water. Its osmotic pressure at was found to be . If , calculate the mass of the solute.

[July 17]

**Solution:**

Given: Volume

Molar mass of the substance

Osmotic pressure

Temperature

To find:

Mass of the solute

Formula:

Calculation: From formula,

Ans: The mass of the solute is .

**2.11 Colligative properties of electrolytes**

- Calculate the amount of (van’t Hoff factor ) dissolved in solution so that its osmotic pressure at is 0.75 atmosphere.

**Solution:**

[Given: Molar mass of is ]

Given: . Osmotic pressure of solution

Volume of solution , van’t Hoff factor

Temperature , Molar mass of solute,

To find:

Formula:

Calculation: Using formula,

Ans: Amount of in the solution is .