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1) 4-Nitrophenol has a pKa of 7.16. 3-Chlorobenzoic acid has a pKa of 3.82. p-Cr

ID: 488992 • Letter: 1

Question

1) 4-Nitrophenol has a pKa of 7.16. 3-Chlorobenzoic acid has a pKa of 3.82. p-Cresol has a pKa of 10.26. You will need the following information: The pKa of HCl is –7; pKa of carbonic acid is 6.52 and the pKa of water is 15. Mixture A is a mixture of p-nitrophenol and m-chlorobenzoic acid. Mixture B is a mixture of p-nitrophenol and p-cresol. Mixture C is a mixture of m-chlorobenzoic acid and p-cresol. All mixtures are dissolved in methylene chloride.

a) Why would reacting Mixture A with an aqueous solution of NaOH NOT separate one substance from the other?

b)) Why would reacting Mixture B with an aqueous solution of NaHCO3 NOT separate one substance from the other?

c) What substance (among HCl, NaOH and NaHCO3) could you use to separate the components of Mixture C? What substance (you must consider the exact species) would exist in the aqueous layer AFTER the extraction?

Explanation / Answer

a)

p-nitrophenol is a weak organic acid (pKa=7.16) and m-chlorobenzoic acid(pKa = 3.82) is a strong organic acid

NaOH is a strong base it can deprotonates both the weak and strong acids and form salts and make ions soluble in aqueous solution.

Thus NaOH cant seperate the mixture A.

b) P-Nitrophenol has a pKa of 7.16 (weak organic)

and p-Cresol has a pKa of 10.26 (weak organic acid)

Both the above acids are weak organic acids and NaHCO3 is a weak base so it cannot deprotonate above acids and cannot make them salts.

Thus NaHCO3 cant differentiate the mixture and cant seperate them.

C)

m-chlorobenzoic acid, pKa = 3.82 and p-cresol, pKa =10.26

Mixture C has a strong organic acid and a weak acid

If we use HCl no use because all are acids

If we use NaOH no use because it protonates all acids

It is better to use a weak base, NaHCO3, which deprotonates only strong acid, m-chlorobenzoic acid and make salt soluble in aqueous solution and leaves weak acid, p-cresol in methylene chloride.

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