Question

The vapor pressure of benzene at 25°C is 95.15 torr. A solution of 6.00 g of a non-volatile solution dissolved in 250.00 g of benzene has a vapor pressure of 92.8 torr at 25°C. What is the molecular weight of the solute?

Answer 1

The vapor pressure of the pure solvent will be equal to 55.7 mmHg.

You're dealing with a solution of sucrose, which is your solute, dissolved in ethanol, which is your solvent.

Since sucrose is a non-volatile solute, the vapor pressure of the solution can be expressed using the mole fraction of the solvent - this is known as Raoult's Law.

P

solution

=

χ

solvent

⋅

P

∘

solvent

, where

P

solution

- the vapor pressure of the solution;

χ

solvent

- the mole fraction of the solvent;

P

∘

solvent

- the vapor pressure of the pure solvent.

The mole fraction of the solvent is defined as the number of moles of ethanol divided by the total number of moles present in the solution.

Use sucrose and ethanol's respective molar masses to determine how many moles of each you have

10.0

g

⋅

1 mole sucrose

342.3

g

=

0.0292 moles sucrose

and

100.0

g

⋅

1 mole ethanol

46.07

g

=

2.171 moles ethanol

The total number of moles will be

n

total

=

0.0292

+

2.171

=

2.20 moles

The vapor pressure of the pure solvent will thus be

P

∘

solvent

=

P

solution

χ

solvent

P

∘

solvent

=

55.0 mmHg

2.171

moles

2.20

moles

=

55.7 mmHg

Step-by-step explanation: