Question

Question 1 Suppose two liquids (A and B) are mixed together and a phase diagram constructed for this real solution (shown in green in the graph on the next page). Vapour pressure lines are also drawn in blue for the ideal version of this solution. a) Calculate the mole fraction of B in a solution where 4.6 moles of A is mixed with 3.6 moles of В. b) What has the higher vapour pressu re, the real solution or the ideal solution? c) What would be the partial vapour pressure of B over this solution if it was an ideal solution? d) What is pa over the real solution? e) What is the total pressure over the real solution? ) Is B more volatile than A? Explain how you found your answer g) Which interaction is stronger, the interaction of B with B, or the interaction between B and A? Provide a one-sentence explanation of your answer h) Assuming that this is an ideal real solution, estimate Henry's constant for B from the graph.

Answer 1

(a)

Mole fraction is defined as the moles of the substance divided by the total moles present in the system.

Thus, let us consider a system having two species A and B.

So, according to definition of mole fraction:-

ng Mole fraction of B=- natno

Given that:-

$n_A$ = Moles of A = 4.6 moles

$n_B$ = Moles of B = 3.6 moles

Applying the values in the above equation as:-

3.6 3.6 Mole fraction of B=7 = 0.44 4.6 +3,6 =

Thus, the mole fraction of B = 0.44

(b)

From the graph, it is clearly stated that the real solution has the higher vapor pressure.

This state is known as non-ideal solutions having positive deviation. This occurs because the vapor pressure by each component is greater than what is being calculated by the Raoult’s law (ideal solutions).

Thus, the total vapor pressure curve of the system appears higher than the vapor pressure of the pure components.

This arises because the solute-solvent interactions in the system are weaker when compared to the solute-solute and solvent-solvent interactions in the system.

(c)

Considering, for a binary system involving two components A and B,

Mole fraction of B = 1 - Mole fraction of A

So, graph can be translated to:-

pB (Pa) / 1 0 0.2 0.4 0.6 0.8 0.8 0.8 0.6 0.4 0.2 0.2 0

As calculated above,

x_B = 0.44

So, from the graph:-

200 pB (Pa) 120 Pa 100 0.8 0.6 0.4 0.2

The partial pressure of B if the system is an ideal solution is 120 Pa.

(d)

As calculated above,

x_B = 0.44

So, from the graph:-

300 210 Pa 200 pB (P.) 1 0.8 0.6 0.40.4 0. 2 0

Since, it is between 220 and 200 Pa, the middle should be 210 Pa.

The partial pressure of B if the system is a real solution is 210 Pa.