Sodium benzoate, the sodium salt of benzoic acid, is used as a food preservative

Question

Sodium benzoate, the sodium salt of benzoic acid, is used as a food preservative. A sample containing solid sodium benzoate mixed with sodium chloride is dissolved in 50.0 mL of .5 M HCl, giving an acidic solution (benzoic acid mixed with HCl ). This mixture is then titrated with .393 M NaOH. After the addition of 46.50 mL of the solution, the pH is found to be 8.2. At this point, the addition of one more drop ( .02 mL) of NaOH raises the pH to 9.3 . Calculate the mass of sodium benzoate (NaC6H5COO) in the original sample. (Hint: At the equivalence point, the total number of moles of acid [here HCl] equals the total number of moles of base [here, both NaOH and NaC6H5COO]

Explanation / Answer

moles of HCl added = 0.5 M x 50 ml = 25 mmol

moles of NaOH to titrate excess HCl = 0.393 M x 46.50 ml = 18.2745 mmol

moles of sodium benzoate present = 6.7255 mmol

Mass of sodium benzoate in original sample = 6.7255 mmol x 144.11 g/mol/1000 = 0.97 g

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