A sample mixture of various neutral organic substances and benzoic acid was weig

Question

A sample mixture of various neutral organic substances and benzoic acid was weighed in a tared weigh boat, and for a mass of 1.5432g. The sample was transferred to a separatory funnel and extracted with ethyl ether, then followed by 5% sodium bicarbonate, after which the aqueous and ether layers were drained separately. One of the two layers was returned to the scup funnel to be further extracted with hydrochloric acid. The layer of interest containing the benzoic acid was vacuum filtered, dried, and weighed (filter paper accounted for), and 0.7543g of benzoic acid crystals were collected. Melting point analysis (one trial) revealed a melting point range between 126 degree C - 129 degree C (known value is 128 degree C). Answer the questions below showing ALL work with correct units and significant figures (where applicable), and in an organized, legible fashion. a. EXPLAIN which of the two layers the benzoic acid will partition into after adding the 5% bicarbonate. b. What is the purpose of the hydrochloric acid? c. Determine the % Mass Recovery d. Determine the % Error for the melting point analysis

Explanation / Answer

6. (a) Bennzoic acid is an organic acid and sodium bicarbonate is a base. NaHCO3 reacts with benzoic acid to generate sodium benzoate (PhCOONa). Sodium benzoate is an ionic salt which is water soluble. During the extraction procedure, benzoic acid forms water soluble sodium benzoate on treatment with 5% sodium bicarbonate solution and the salt goes to aqueous layer.

(b) The aqueous layer contains Sodium benzoate which is completely soluble in water and a clear aqueous solution is obtained. If the aqueous layer is treated with HCl (hydrochloric acid), it reforms benzoic acid from sodium benzoate. Benzoic acid is insoluble in water and it precipitaes out from the aqueous solution and it can be easily separated by simple filtration technique.

PhCOONa + HCl = PhCOOH + NaCl

So, HCl is used to recover benzoic acid by precipitation method from aqueous solution.

(c) Initially, weight of benzoic acid taken was: 1.5432 grams

After extraction, the recovered amout of benzoic acid is: 0.7543 grams

So, percentage recovery of benzoic acid is :

(0.7543*100/ 1.5432) % = 48.88%

(d) Range of determined melting point is 126 - 129 0C. If we take the average value of the observed melting point:

(126+129)/2 = 127.5 0C

Actual melting point is: 128 0C

% error in melting point determination:

((127.5 - 128)* 100)/ 127.5 = - 0.39 %

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