35 mL of 10% aqueous hydrochloric acid was dispensed into two conical flask, respectively. The desired pH value (pH<2) of the solutions was confirmed by using pH indicator paper. Red color was expected to be appeared.
Filtration apparatus was set up and the water aspirator was then turned on. The filter paper was placed on the Büchner funnel and was wet by de-ionized water. The solutions was filtered and the conical flask was rinsed with small portion of de-ionized water. The solid was dried for 10 minutes in the Büchner funnel and was then transferred onto a filter paper. Another filter paper was placed on the solid and two pieces of filter paper was pressed slightly for further drying.
The solid was weighed and was transferred into a clean and dry plastic bottle.
Melting point determination
The dry solid was grounded into a fine powder on a watch glass by using the flat portion of a spatula.
The melting point capillary was forced down into the powder and was reversed. The powder was arrived down to the closed end of the capillary by dropping down a 2-ft length of glass tubing onto a hard surface.
The melting point of powder was determined by using the electrical melting point apparatus.
Recrystllization
2.21(g) powder from experiment 1 was transferred to a 125 mL conical flask. 33 mL of water and few boiling stones was then added into the conical flask. The conical flask was placed on a steam bath and additional 5 mL of water was added into the conical flask.
The conical flask was cooled down to room temperature and was then placed in an ice-bath.
The recrystallized solid was filtered by suction filtration. The filter paper was placed on the Büchner funnel and was wet by de-ionized water. The conical flask was rinsed with small portion of de-ionized water. The crystals was dried for 10 minutes by the suction drawing air through the funnel and was then transferred onto a filter paper. Another filter paper was placed on the solid and two pieces of filter paper was pressed slightly for further drying.
The crystals were weighed and was transferred into a clean and dry plastic b
Result and Calculation:
Table 1. The informations and data in experiment 1.
Table 2. The informations and data in experiment 2.
Table 3. The data of melting point range.
Answer for questions
The maximum weight of benzoic acid that could be isolated from a saturated solution of benzoic acid in 30mL of toluene [The solubility of benzoic acid in toluene: 11g/100 mL at 25°C]
(11 g / 100 mL) x (30 mL)
= 3.3 g
2. Mass of NaOH / 1000 mL x 100% = 10%
Mass of NaOH = 100 g
No. of moles of NaOH in 1 L of 10% aqueous NaOH = 100 g / 39.9971 gmol-1
= 2.500 mol
No. of moles of benzoic acid = 3.3 g / 122.12 gmol -1
= 0.027 mol
∵ By moles ratio, NaOH: benzoic acid = 1 : 1
∴ No. of moles of NaOH required = 0.027 mol
∴ Minimum of 10% aqueous NaOH = 0.027 mol / 2.5 mol x 1000 mL
= 10.8 mL
3. HCl + NaOH ➔ NaCl + H2O
No. of moles of NaOH: 2.5 x 0.03 = 0.075 mol
∵ By moles ratio, HCl:NaOH = 1 : 1
∴ No. of moles of HCl required: 0.075 mol
∴ Volume of HCl required: 0.075 mol / (12/10 M) = 0.0625 L
= 62.5 mL
Using water can distinguish two layers. The layer which can mix with water is aqueous layer. It is because water is polar compound and therefore it is able to dissolve into polar aqueous layer. In contrast. the layer which cannot dissolve water is organic layer.
Discussion:
Due to the fact that the solubility of benzoic acid in 100 mL water is 0.4 g and 11 g per 100 mL toluene. Therefore, water is not a good solvent to extract benzoic acid in toluene-benzoic acid mixture. Hence, sodium hydroxide is used to convert benzoic acid to sodium benzoate because the solubility of this sodium salt of benzoic acid is 1g per 2 mL and thus it will go to the aqueous layer from organic layer. However, a small portion of sodium benzoate maybe still distributed into the organic layer because of the distribution coefficient, KD. To avoid this situation, another 15 mL of 10% sodium hydroxide solution was used to further extract sodium benzoate. Sodium benzoate was then converted to the benzoic acid by the reaction with 10% hydrochloric acid solution. There is no any observable benzoic acid in second conical flask which means the majority of benzoic acid was extracted in first conical flask.
After extraction, the percentage yield of benzoic acid was 130.8%. The main reason may be water was not completely removed or other impurities were present. This reason can be further proved by the melting point of raw product. The melting point of raw product was 120-121 °C, which is lower than the benzoic acid standard.
After crystallization, the percentage yield of crystals was 62.17%. Part of the benzoic acid may be lost during placing the conical on the steam bath because the temperature was too high and thus the benzoic acid was evaporated out.
A narrow range of melting point of mixed product was obtained, which confirms the crystals are benzoic acid and relatively high purity of benzoic acid was obtained because of the same melting temperature to the standard.
In order to obtain higher percentage yield of crystal products, at least 63.5mL of 10% aqueous hydrochloric is used instead of 35 mL. The temperature of steaming process should be lower to avoid evaporation of benzoic acid. Also, care should be taken to collect all the benzoic acid on the surface of Büchner funnel or on the filter paper.
Conclusion:
To conclude, the percentage yield of benzoic acid from the toluene-benzoic mixture was 76.3% and the melting point was 117.9-121°C.
References:
Acid-Base Extraction: Separation of a Four-Component Mixture; Thin Layer Chromatography. Retrieved 30 September 2012 from
Emerald Performance Materials. Retrieved 30 September 2012 from
Recrystallization Technique. Retrieved 30 September 2012 from
Wired Chemist. Recrystallization. Retrieved 30 September 2012 from
Wired Chemist. Determination of Melting Point. Retrieved 30 September 2012 from
Principle of Organic Chemistry Report (Expt. 1 & 2)
by
freemanmak (student)
