Question

9. The vapour pressure of water at 20 0
C is
17 mm Hg. What is the vapour pressure
of solution containing 2.8 g urea in 50 g of
water?

Answer 1

ANSWER

Vapour pressure of water= P

1

o

=17mmHg

Weight of water= W

1

=50g

Weight of urea= W

2

=2.8g

Molar mass of water= M

1

=18g/mol⇒n

1

=

18

50

=2.78

Molar mass of water= M

2

=60g/mol⇒n

2

=

60

2.8

=0.047

∴ Vapour pressure of solution P

1

P

1

o

P

1

o

−P

1

=

n

2

+n

1

n

2

⇒

17

17−P

1

=

2.78+0.047

0.047

=0.0166

⇒P

1

=16.71mmHg