Question

5 grams of substance X is dissolved in 100 grams of water.
Given: vapour pressure of water = 760 mm of Hg
vapour pressure of solution = 700 mm of Hg.

Calculate the molecular weight of X​

Answer 1

Answer:

Raoult's law states that relative lowering of vapour pressure and is equal to

the mole fraction of the solute.

p

1

p

1

0

−p

1

=x

2

p

1

0

=3000 Nm

−2

= vapour pressure of solution.

p

1

=2985 Nm

−2

= vapour pressure of sollvent.

x

2

= mole-fraction of solute.

Let molecular wt. of X is x.

No of moles of X is

x

5

No of moles of solvent is

18

100

.

Mole-fraction of X is

(100/18)

(5/x)

=9/10x

So,

3000

3000−2985

=

10

9

x

⟹x=180

The molecular weight of glucose is 180 g/mol.

Therefore, X is glucose

Explanation:

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