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In this experiment, sublimation was used to purify solid mixtures. The compound used to undergo purification is impure benzoic acid. The product obtained after the heating process is called the sublimate in the form of crystals. The weight of the impure benzoic acid gave a lot of difference from the sublimate obtained from 5.0014 g to 0.3213 g, respectively. Thus, the measured sublimate showed the low percentage recovery of the said compound. The sublimate was then subjected to melting point determination together with the pure benzoic acid through the oil bath. The oil bath was used since it is able to measure temperatures even those beyond 100 °C and are highly recommendable for compounds which have higher boiling points. As it was seen in the results for the melting point determination, the sublimate started to melt at 116 °C and stopped melting at 120 °C while the pure benzoic acid started to melt at 120 °C and ended at 121 °C. The temperatures measured indicated that the pure benzoic acid has a higher vapour pressure than the sublimate.

Sublimation is the direct vaporization of a solid by heating without passing through the liquid state. [1] The process of sublimation in this experiment is used to purify the impure benzoic acid. The impurities in the benzoic acid have amino compounds that are present in a larger extent and must be removed because of phenyl and benzyl compounds that seriously affect the product.[3] This process occurs if the vapour pressure of the substances greater than the atmospheric pressure at the melting point.[2] The process of purification involved in sublimation is applicable for impurities which are non-volatile or with lower vapor pressure than the pure compound or substance. The objectives of this experiment are: (1) to purify the impure benzoic acid through sublimation, (2) to calculate the percentage recovery of the benzoic acid after sublimation, and (3) to identify the melting points of the sublimate and the pure benzoic acid through the use of an oil bath.

extent because of the presence of hydrogenous compounds. [3]

B. Procedure 1. Sublimation
Sublimation set-up was used to purify the impure benzoic acid. Hot plate, evaporating dish, 5 g of impure benzoic acid, clean sheet of bond paper to cover and masking tape for sealing, watch glass, and moistened tissue paper were needed as materials. 5 g of impure benzoic acid was put into the evaporating dish and sealed with a clean sheet of bond paper with holes on it using a masking tape. Before putting the evaporating dish on the hot plate, a pre-weighed watch glass was set on the dish invertedly. While heating, the tissue paper was put above the watch glass and continuously moistened for 10-15 minutes until most of the sample has vaporized. The set-up was cooled and then the watch glass was carefully inverted to collect the sublimate in a transparent, crystalline, needle-like appearance. The sublimate was weighed for percentage recovery.

2. Melting Point Determination EXPERIMENTAL A. Sample Used
The samples used in the experiment were impure benzoic acid, its sublimate and the pure benzoic acid. The impure benzoic acid is said to be non-volatile meaning it has a lower vapor pressure than the pure compound. The impurities found in formed benzene affect it to quite a large The 0.3213 g of sublimate collected was ground with the mortar and pestle. The sublimate’s powdered form was well-packed into 3-5 mm capillary tube. The pure benzoic acid followed the same procedure separately with the sublimate. The capillary tubes were dropped into the glass tubing to get rid of air spaces when if present could affect the melting point of the compound. The closed ends of the capillary tubes

were attached to a thermometer with a thread, rubber band or copper wire. The tubes are aligned with the mercury bulb of the thermometer. After attaching, the tubes were immersed in an oil bath while being heated. The oil bath is stirred constantly. The temperatures at which the sublimate and pure benzoic acid started melting as well as the temperature at which they were completely melted were both noted.

has generally far higher boiling points than water and can be used to provide temperature greater than 100 °C. [6]

Stirring Rod Thermometer

Sublimation was the process used in the experiment. The impure benzoic acid in the evaporating dish has to be heated to be able to form needle-like crystals which will eventually be called the sublimate. The sublimate that passed through the holes of the paper used as its covering is to be collected to know its percentage recovery. It was noticed that during the process of sublimation the impure benzoic acid did not pass through the liquid phase. The reason for is that because the benzoic acid has non-volatile impurities. The impure benzoic acid is also said to have lower vapour pressure than the pure ones.

Oil Bath

Capillary Tube with the Sublimate

Bunsen burner

Capillary Tube with Pure Benzoic Acid

Figure 2. Melting Point Determination set-up The melting point determination is the temperature at which the material changes from a solid to its liquid state. [7] Determining the melting point is a simple and fast method used in many diverse areas of chemistry to obtain a first impression of the purity of a substance. This is because even small quantities of impurities change the melting point, or at least clearly enlarge its melting range. Table 1. Data Weight of the impure benzoic acid Weight of the watch glass + sublimate Weight of the watch glass (empty)

5.0014 g 3.7262 g 3.4049 g 0.3213 g 6.42 % 120 °C – 121 °C 116 °C – 120 °C

Evaporating dish Hot plate

Watch Glass with Moistened Tissue Paper

Weight of the sublimate Percentage Recovery Melting Point of the pure benzoic acid Melting point of the sublimate

Figure 1. Sublimation set-up The sublimate collected was ground to be able to determine its melting point through the capillary tube method with the use of oil bath. The melting point determination in this experiment was done through the oil bath. The oil bath is a laboratory heating device which uses boiling oil as the temperature regulator.[5] It operates the same principle as water bath but oil

Through the data obtained from the experiment, it showed how sublimation affected the weight of the impure benzoic acid until it became a sublimate. The change in weight (from 5.0014 g to 0.3213 g) showed the turning point of the where in the impure benzoic acid is to its

gaseous state. The pure benzoic acid has a high vapour pressure which made it melt in a higher temperature rather than the sublimate. It has a smaller dipole moment meaning the higher vapour pressure due to lower electrostatic attractive forces in the form of crystals.[1]

REFERENCES Bayguen, A.V., Cruz, C.T., de Guia, R.M, Lampa, F.F., Peña, G.T., Sarile, A.S. and Torres, P.C. (2009),Laboratory Manual in Organic Chemistry,C&E Publishing, Inc Bruice, P. Y. (2006). The Essentials of Organic Chemistry. 1st Edition. Pearson Education, Inc. pp 74-82 Impure Benzoic Acid. 2010.



Determination. March 2009.

Oil Oil Bath as a Laboratory

Bath. Technique. Feb 3 2011 2008.

Sublimation. .html. June 14 2007.

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