Chemistry Class 12 NCERT Solutions: Chapter 2 Solutions Part 11

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

Answer:

Here,

Molar mass of sugar

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

Now, number of moles of cane sugar

Therefore, molality of the solution,

Applying the relation,

Molar of glucose

glucose in water means of glucose is present in of water

Number of moles of glucose

Therefore, molality of the solution,

Applying the relation,

(Approximately)

Hence, the freezing point of glucose solution is K.

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

Answer:

We know that,

Then,

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

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

Now, we can write:

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

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

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

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

Answer:

Here,

Applying the relation,

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