Question

A solution contains 3.95 g of carbon disulfide (CS2, molar mass = 76.13 g/mol) and 2.43 g of acetone ((CH3)2CO, molar mass = 58.08 g/mol). The vapor pressure of pure carbon disulfide and acetone at 35 oC are 515 torr and 332 torr, respectively. Assuming ideal solution behavior, calculate the vapor pressure of each of the components and the total vapor pressure above the solution. The total vapor pressure above the solution at 35 oC was experimentally determined to be 645 torr. Is the solution ideal? If not, indicate whether the solution deviates from Raoult’s law in a positive or negative manner.

Answer 1

Psol = 433 torr

The solution is not ideal and the solution deviates from Raoult's law in a positive manner

Step-by-step explanation:

The vapout pressure of a solution can be obtained using Raoult's law:

Psol = P°A* XA + P°B*XB

Where Psol is vapour pressure of solution

P° is vapour pressure of pure substance

X is mole fraction

As we have vapour pressure of pure carbon disulfide and acetone we need to find mole fraction of each compound to solve the vapour pressure of the solution:

Moles carbon disulfide:

3.95g CS₂ * (1mol / 76.13g) = 0.05188 moles

Moles acetone:

2.43g acetone * (1mol / 58.08g) = 0.04184 moles

Mole fraction of CS₂:

0.05188 moles / (0.05188mol + 0.04184mol) =

0.5536

Mole fraction acetone:

1 = X(CS₂) + X(Acetone)

X(Acetone) = 1 - 0.5536 = 0.4464

Solving for vapour pressure of the solution:

Psol = 515torr*0.5536 + 332torr*0.4464

Psol = 433 torr

As the experimental vapor pressure was 645torr > 433 torr (Theoretical vapor pressure), the solution is not ideal and the solution deviates from Raoult's law in a positive manner