Distillation and Purity Assessment of an Ester
A student synthesizes ethyl acetate by Fischer esterification and purifies it by simple distillation. The student collects a fraction boiling at 74–78 °C. The literature boiling point of ethyl acetate is 77.1 °C. The student measures the refractive index of the distillate and obtains 1.3705 at 20 °C; pure ethyl acetate has n20D = 1.3723. The student also records the mass of the distillate as 12.45 g. The student suspects the product may contain ethanol (n20D = 1.3611) or water (n20D = 1.3330). The student has a 25.0 mL graduated cylinder and a analytical balance. The density of ethyl acetate is 0.902 g/mL; ethanol 0.789 g/mL; water 1.000 g/mL. The student must assess the purity and decide whether to redistill or use another method.
1. Estimate the volume of the distillate and calculate its density. Use the density and refractive index to evaluate the likely purity of the ethyl acetate.
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Volume of distillate: the student likely measured volume in the graduated cylinder. Assuming the mass is 12.45 g, if pure ethyl acetate, volume = 12.45 g / 0.902 g/mL = 13.80 mL. The student should have measured the volume; if measured volume is, say, 13.8 mL, density = 12.45/13.8 = 0.902 g/mL, consistent with pure ethyl acetate. However, the refractive index is 1.3705, which is lower than pure ethyl acetate (1.3723). The difference is 0.0018. Using a linear mixing rule for refractive index, if the impurity is ethanol (n=1.3611), the mole fraction of ethanol x can be estimated: 1.3705 = x(1.3611) + (1-x)(1.3723) → 1.3705 = 1.3723 - 0.0112x → 0.0018 = 0.0112x → x = 0.161. So about 16 mol% ethanol. If the impurity is water (n=1.3330), 1.3705 = 1.3723 - 0.0393x → x = 0.0458, about 4.6 mol% water. The density can help distinguish: a mixture with 16 mol% ethanol would have a lower density than pure ethyl acetate. Calculate mole fraction to mass fraction: molar masses: ethyl acetate 88.11 g/mol, ethanol 46.07 g/mol. For x_ethanol = 0.161, mass fraction ethanol = (0.161×46.07)/(0.161×46.07 + 0.839×88.11) = 7.42/(7.42+73.93)=0.0912. Density of mixture ≈ 1/(0.0912/0.789 + 0.9088/0.902) = 1/(0.1156+1.0075)=0.891 g/mL. If water impurity 4.6 mol%, mass fraction water = (0.046×18.02)/(0.046×18.02+0.954×88.11)=0.829/(0.829+84.06)=0.00977; density ≈ 1/(0.00977/1.000 + 0.9902/0.902)=1/(0.00977+1.0978)=0.903 g/mL. The measured density would be closer to pure ethyl acetate if water is the impurity. The student should measure the density accurately to distinguish. Given the refractive index alone, the product is not pure; likely contains ethanol or water. The student should redistill with a fractionating column to improve separation.
2. Propose a purification strategy to obtain purer ethyl acetate, and explain how you would monitor the purity. Include a decision about whether simple distillation is sufficient.
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Simple distillation is not sufficient because the boiling point difference between ethyl acetate (77.1 °C) and ethanol (78.4 °C) is very small (about 1.3 °C), so they co-distill. Water has a higher boiling point (100 °C) but may form an azeotrope with ethyl acetate (minimum boiling azeotrope with water, about 70.4 °C, 8.2% water). To obtain purer ethyl acetate, the student should use fractional distillation with a fractionating column to increase the number of theoretical plates. Alternatively, the student could wash the distillate with saturated sodium carbonate solution to remove acetic acid and ethanol (ethanol is somewhat soluble in water), then dry with anhydrous sodium sulfate or magnesium sulfate, and redistill. The purity can be monitored by refractive index, density, and possibly boiling point. The student should compare the refractive index to the literature value and use the density to check for water. If the refractive index remains low after drying and redistillation, the student may need to use a different method, such as preparative gas chromatography or treatment with a drying agent followed by distillation. The student should also record the boiling point range; a narrow range near 77 °C indicates purity.