57. Sodium benzoate, the sodium salt of benzoic acid, is used as a food preserva

Question

57. Sodium benzoate, the sodium salt of benzoic acid, is used as a food preservative. A sample containing solid sodium benzo- ate mixed with sodium chloride is dissolved in 50.0 mL of 0.500 M HCI, giving an acidic solution (benzoic acid mixed with HCl). This mixture is then titrated with 0.393 M NaOH. After the addition of 46.50 mL of the NaOH solution, the pH is found to be 8.2. At this point, the addition of one more drop (0.02 mL) of NaOH raises the pH to 9.3. Calculate the mass of sodium benzoate (NaC,H,COO) in the original sample. (Hint: At the equivalence point, the total number of moles of acid Ihere HCll equals the total number of moles of base here, both NaOH and NaC&H;,COOj.)

Explanation / Answer

The no. of mmol of HCl = 50 mL * 0.5 mmol/mL = 25 mmol

The no. of mmol of NaOH = 0.393 mmol/mL * 46.5 mL = 18.2745 mmol

i.e. The no. of mmol of benzoic acid at the first equivalence point = 18.2745 mmol

The no. of mmol of benzoic acid in the solution = 0.02 mL * 0.393 mmol/mL = 0.00786 mmol

i.e. The no. of mmol of benzoic acid at the second equivalence point = 0.00786 mmol

Therefore, the total no. of mmol of sodium benzoate = 18.2745 + 0.00786 = 18.28236 mmol

The molar mass of sodium benzoate = 144 g/mol

Therefore, the mass of sodium benzoate present in the sample = 18.28236 mmol * 144 g/mol = 2.633 g

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