Question

How many grams of C6H5COOH are needed to make 3.9 liters of aqueous solution of pH 3.4?Be sure to back correct from equilibrium molecular acid molarity to starting molecular acid molarity

Answer 1

grams of C₆H₅COOH = 1.4

Explanation

pH = 3.4

[H⁺] = 10^-pH

[H⁺] = 10^-3.4

[H⁺] = 4.0 x 10^-4 M

initial concentration of benzoic acid = ([H⁺]² / K_a) + [H⁺]

initial concentration of benzoic acid = [(4.0 x 10^-4 M)² / (6.3 x 10^-5)] + (4.0 x 10^-4 M)

initial concentration of benzoic acid = 2.91 x 10^-3 M

moles benzoic acid = (initial concentration of benzoic acid) * (volume of solution in Liter)

moles benzoic acid = (2.91 x 10^-3 M) * (3.9 L)

moles benzoic acid = 0.0114 mol

mass benzoic acid = (moles benzoic acid) * (molar mass benzoic acid)

mass benzoic acid = (0.0114 mol) * (122.12 g/mol)

mass benzoic acid = 1.4 g