Describe and explain the possible effects on your results of the following exper

Question

Describe and explain the possible effects on your results of the following experimental errors or variations. In each case, specify the component(s) whose percentage(s) would be too high or too low.

a. After adding 10% NaHCO3 to your mixture, you did not stir the mixture long enough. (mixed with benzoic acid, precipitated from aq HCl)

b. During the 10% NaOH extraction, you failed to mix the aqueous and organic layers thoroughly. (mixed with 2-naphthol, precipitated from aq HCl)

c. You only neutralized the NaHCO3 solution to a pH of 7 instead of pH of 2.

Explanation / Answer

a) The compound of interest here is benzoic acid which has been precipitated out by aqueous HCl as the acid as in its salt form is soluble in the aqueous layer itself making it impossible to separate it into the organic layer. If a 10% solution of sodium bicarbonate is added to the mixture, it will result in the neutralization of HCl to give NaCl and carbon dioxide gas often observed as an efferverscence. This reaction however is very slow and stirring improperly or for insufficient times results in improper neutralization of the aqueous HCl added. Considering the concentration of benzoic acid and HCl in the medium, the bicarbonate should be added as excess will start neutralizing the benzoic acid also rendering the whole procedure useless giving a lower yield of benzoic acid than the actual.

Thus if the procedure is carried out without long duration of stirring, a higher percentage of sodium bicarbonate is required to effect complete neutralization of the HCl in the medium. The precise amount of base to be added can be monitored by pH change.

b) Similar to the previous answer, naphthol dissolves in water in its salt form calling for a precise acidic pH to ensure that 2-naphthol stays as a precipitate. If however the organic and aqueous layers are not mixed thoroughly, a greater concentration of 2-naphthol tends to remain in the aqueous layer as a suspension.

To avoid this, a lower concentration of NaOH should be used to prevent some of the 2-naphthol in the aqueous layer from forming a salt.

c) When isolating organic compounds that can form a salt when treated with a strong enough base, their salt forms are highly soluble in water and thus do not dissolve in the organic layer in which it should go to ensure complete absence of all the other products that have risen from the reaction as they all are water-soluble. Thus the only way to bring the salt back to the water-insoluble form is to protonate the conjugate base giving back the original organic compound. To achieve this, the basic solution is neutralized till it is considerably acidic as at such low pH all of the organic compound will definitely be protonated ensuring complete recovery without significant loss in the yield.

Thus, a greater quantity of neutralizing acid should be used and neutralized all the way till pH of 2 instead of stopping at the neutral pH as the compound being a weak acid tends to be in an equilibrium with its salt form at neutral pH with the latter being water-soluble and insoluble in organic solvents.

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