The benzoic acid was discovered in the sixteenth century and received this name

Question

The benzoic acid was discovered in the sixteenth century and received this name to have been extracted from the styrax benzoin. In 1875, Salkowski found that this acid acts as a fungicide and since then, benzoic acid has been used as germicide in food preservation and as adjunctive anti-fungal drug, typically in concentrations ranging from 0.05 to 0.15%. It is a compound of formula molecular C7H6O2 (Molecular weight = 122 g mol -1 ) Sparingly soluble in Water. In an aqueous solution, it behaves as a weak acid, pKa = 4. An aqueous solution of benzoic acid in concentration of 0,12% (W / v) exhibits approximately the same pH

(A) 8. (B) 6. (C) 4. (D) 3. (E) 2.

Explanation / Answer

HA -----> H+ + A- Pka = 4

concentration is 12g of HA per 100ml of solution so 0.12g/ml M = n/V wher n are the mol m/MW and V is volume in liter

0.12g/ml x mol/122g x 1000ml/l = 0.98M

Ka = [H][A]/]HA] as in the concetration molar of H and A are the same we can rewrite as

Ka = [H]2/[HA] =) apply -log to two sides of the equation get

-logKa = -2log[H] + log[HA] as -logH = pH and -logKa = pKa replacing

pKa = 2pH + log[HA] =) pH = (-log[HA] + pKa)/2 now replacing for your value and get

pH =  (-log[0.98] + 4)/2 = 4.01 answer C

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