Figure 4.6 (the image provided) shows a temperature (oC) vs. composition diagram

Question

Figure 4.6 (the image provided) shows a temperature (oC) vs. composition diagram for a mixture of acetone and ethyl acetate at 760 torr. You can calculate the mole fraction of ethyl acetate at any given point of the plot by substracting the mole fraction of acetone from one because of the relationship, mole fraction of acetone + mole fraction ethyl acetate = 1. Answer the following questions using the image below.

a. Specify the normal boiling point of acetone and of ethyl acetate.

b. For a mixxture comprised of 50 mol % acetone and 50 mol % ehtyl acetate:

i. At what temp will the mix begin to boil?

ii. When it begins to boil, what is the mol % acetone in the vapor?

iii. what is the b.p. of the liquid formed by the condensation of the vapor from (ii) above?

iv. How many theoretical plates (distillation stages) will be necessary to isolate acetone of at least 90% purity?

c. A mixture of acetone and ethyl acetate of unknown molar ration is heated to boil. Immediately after boiling starts a sample of the vapor is found to contain 0.8 mole fraction of ehtyl acetate. What is the composition in mol % of the original binary mixture and what is its boiling point?

d Would you expect a solution comprised of 30 mol % acetone and 70 mol % ethyl acetate to boil above or below ca. 67 degree C in Denver, CO, which is at an elevation of one mile above sea level? Explain your answer

Explanation / Answer

a. The nbp of acetone: 56 C

The nbp of ethyl acetate: 77 C

b. i. For 50% Acetone and 50% Ethyl Acetate, we draw a vertical line from x=0.5 towards the curve. Temperature at which it intersects with the liquid line is the required temperature

Temperature at which boiling begins: 64 C

ii. To find this we draw a horizontal line from liquid line at x=0.5 to vapor line and draw a vertical line down to mole fraction axis. This gives us vapor composition of acetone as: 0.67

iii. The intersection of the vertical line drawn in above question ii. and the liquid line gives the boiling point of this vapor as

bp: 61 C

iv. For this we draw a vertical line to vapor line from x=0.9 and then a horizontal tie line to liquid line followed by another vertical tie line to vapor line and so on till we reach x=0. The no. of such steps that we can draw gives us the no of theoretical plates as 9.

c. We draw a horizontal tie line from vapor line at x=0.8 to liquid line giving us

Composition of b=mixture: 65% acetone, 35% ethyl acetate.

Boiling point: 62 C

d. Boiling point is lower at higher altitudes due to lower air pressure, hence bp is reduced from 67.

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