Chemistry Class 12 NCERT Solutions: Chapter 2 Solutions Part 11

Q: 20. A Equation solution (by mass) of cane sugar in water has freezing point of Equation . Calculate the freezing point of Equation glucose in water if freezing point of pure water is Equation .

Answer:

Here, Equation

Equation

Molar mass of sugar Equation

Equation

Equation Solution (by mass) of cane sugar in water means 5 g of cane sugar is present in Equation g of water.

Now, number of moles of cane sugar Equation

Equation

Therefore, molality of the solution, Equation

Equation

Applying the relation,

Equation

Equation

Equation

Equation

Molar of glucose Equation

Equation

Equation glucose in water means Equation of glucose is present in Equation of water

Equation Number of moles of glucose Equation

Equation

Therefore, molality of the solution, Equation

Equation

Applying the relation,

Equation

Equation

Equation (Approximately)

Hence, the freezing point of Equation glucose solution is Equation K.

Q: 21. Two elements A and B form compounds having formula Equation and Equation . When dissolved in Equation of benzene Equation 1 g of Equation lowers the freezing point by 2.3 Kwhereas Equation of Equation lowers it by Equation . The molar depression constant for benzene is Equation Calculate atomic masses of A and B.

Answer:

We know that,

Equation

Then, Equation

Equation

Equation

Equation

Now, we have the molar masses of Equation and Equation as Equation and Equation respectively.

Let the atomic masses of A and B be x and y respectively.

Now, we can write:

Equation

Equation

Subtracting equation (i) from (ii), we have

Equation

Equation

Putting the value of 'y' in equation (1), we have

Equation

Equation

Hence, the atomic masses of A and B are Equation and Equation respectively.

Q: 22. At Equation of glucose present in a litre of its solution has an osmotic pressure of Equation bar. If the osmotic pressure of the solution is Equation bars at the same temperature, what would be its concentration?

Answer:

Here,

Equation

Equation

Equation

Applying the relation,

Equation

Equation

Equation

Equation

Since the volume of the solution is Equation , the concentration of the solution would be Equation .

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