Question

What is the molality of a solution that contains 74.2 g of 1,4-dichlorobenzene (C 6H 4Cl 2) in 492 mL of carbon tetrachloride (CCl 4)? The density of CCl 4 is 1.60 g/mL.

Answer 1

Answer:

Given,

W

B

= mass of naphthalene =51.2g

M

B

= molar mass of naphthalene (C

10

H

8

)=12×10+8=128g

W

A

= density (d)× volume (V)

(∵d=

V

W

)

W

A

=1.60gmL

−1

×500mL=800g

∵m=

M

B

W

B

×(

W

A

(kg)

1000

)

m=

128×800

51.2×1000

=0.5m

Answer 2

Molality= (no. of moles of solute)/ kilograms of solvent

The solute is: C6H4Cl2, it has a molar mass of 6*12 + 4*1 + 2*35.5

=>72+4+71  => 147 g mol-1

So, 74.2 g of the solute is 74.2/147 => 0.504 mol

As, 492 ml of CCl4 are used, ρ=1.6 g/ml

ρ=m/v

1.6=m/492

m=787.2 g

m=0.7872 kg

Molality= 0.504/0.7872

Molality=0.64 moles per kilogram of CCl4.