LaPugh Discount Perfumes has accidentally leaked one of its raw ingredients, met

Question

LaPugh Discount Perfumes has accidentally leaked one of its raw ingredients, methylphenyl ester (MPE, also known as phenyl acetate, CH_3-COO-C_6H_5) into a river. The following hydrolysis data are available for MPE (from Mabey and Mill, 1978). k_a = 7.7 x 10^-5 M^-s^-1 k_b= 1.4 M^-1s^-1 k'_n = 6.7 x 10^-8 s^-1 The MPE has an observed half-life of 52 days. What is the river pH? Show all calculations. Circle One Correct Answer (within significant digits of your answer) a. 6.5 b. 6.8 c. 7.1 d. 7.4 e. 7.7 f. 8.0

Explanation / Answer

First we write the equation for the hydrolysis of phenyl-acetate:

CH3-COO-C6H5 + H2O -----> CH3-COOH + C6H5-OH, we will have a H+ ions contribution from the ethanoic acid.

kh = Kw/ka = 10^-14 M/7.7x10^-5 M s-1 = 1.3x10^-10 s-1 = [OH-]^2/C

C = 10^-7 (standard concentration of OH-] in water

[OH-] = sqrt(1.3x10^-17) = 3.61x10^-9 M

pH = -log[3.61x10^-9] approximately equals 8

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