A Study of Azeotrope and Acetone/Chloroform Liquid-Vapor Phase Diagram

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  • Topic: Distillation, Azeotrope, Vapor pressure
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A Study of Azeotrope and Acetone/Chloroform Liquid-Vapor Phase Diagram

Liquid-vapor phase of acetone/chloroform was studied through distilling a series of mixtures with different mole fraction. When the mixtures were boiling, their vapor was condensed through a water column and collected in a receiving container. Refractive index was collected for starting mixture, distillate and residue for each sample. A boiling temperature versus acetone’s mole fraction was constructed to show the liquid-vapor phase diagram. The boiling temperature of azeotrope was determined to be 62.2oC with the composition of 23% acetone and 77% chloroform.

Keyword: liquid-vapor phase, acetone/chloroform mixture, azeotrope

In an ideal binary mixture, the interactions between two components A and B are equal to each other. The interaction between A-A, A-B, and B-B are the same. Raoult’s law is hold in ideal liquid mixture giving that at a specific temperature, the vapor pressure of a component is proportional to its mole fraction in the mixture. In reality, many binary mixture do not follow Raoult’s law. The actual vapor pressure can be higher or less than what predicted by Raoult’s law and causes positive or negative deviation. For a positive deviation system, A-B interaction are favorable and the boiling temperature curve at different mole fraction gives a maximum. For a negative deviation system, A-B interaction are unfavorable and the boiling temperature curve shows a minimum. At maximum or minimum point on the curve, the composition of the liquid and of the vapor are the same. Such a mixture is called azeotrope. This study focuses on the mixture of acetone and chloroform. To obtain a boiling temperature vs. mole fraction diagram, a series of simple distillation will be performed. Distillation basically means boiling the mixture and the vapor is obtained in a receiving container. The liquid samples will be analyzed by refractive index. In Figure 1, the structure of acetone and chloroform shows that hydrogen bonds will likely be formed in the mixture. Therefore, favorable interactions between two components a positive deviation boiling temperature diagram are expected. EXPERIMENTAL

A series of experimental mixture of acetone and chloroform were made as shown in Table 1. Each sample was distilled until obtaining a constant boiling temperature (no or very slow change in temperature when heating). As the mixture was boiled, the vapor was condense through a water column and collected. Refractive Index (RI) was obtained for starting mixture, distillate in receiver container, and residue left over for each sample. RESULT

Mole fraction of acetone, boiling temperature and refractive index of starting mixture, distillate and residue are presented in Table 2. Mole fraction of acetone in starting solution was calculated by using density of acetone (0.791 g/mole) and density of chloroform (1.492 g/mole) (Sigma-Aldrich). The process of calculation was shown below mole acetone/chloroform = nA,B = Volume×density× 1Molecular Weight mole fraction = nA,BnA+nB

The refractive index of starting solution at different mole fraction of acetone was put together in a graph as Figure 2. It was used as a calibration curve to determine the mole fraction of acetone corresponded to each boiling temperature. The equation of calibration curve is y = -0.0857x + 1.4433 so at an obtained boiling temperature T, the mole fraction of acetone in either distillate or residue was calculated as Mole fraction=(T-1.4433)/(-0.0857)

A graph of mole fraction of distillate and residue versus boiling temperature was shown together in Figure 3. DISCUSSION
The graph of boiling point versus mole fraction of acetone of a liquid-vapor pressure in Figure 3 showed a convex down with a maximum. The shaped of the graph is similar to a positive deviation system where the interaction between components is more favorable due to hydrogen bonding between acetone and...
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