a) The pH of the benzoic acid solution before adding the salt can be calculated using the Henderson-Hasselbalch equation. The pKa of benzoic acid is 4.20, so the pH of the solution is 4.20.
b) The Henderson-Hasselbalch equation is pH = pKa + log([salt]/[acid]). To calculate the amount of salt needed, we can rearrange the equation to [salt]/[acid] = 10^(pH-pKa). Plugging in the values, we get [salt]/[acid] = 10^(4.00-4.20) = 0.7943. Since the initial concentration of benzoic acid is 0.0200 M, the amount of sodium benzoate needed is 0.0200 x 0.7943 = 0.0159 M.
c) To calculate the amount of sodium benzoate in grams, we can use the molar mass of sodium benzoate, which is 144.11 g/mol. The amount of sodium benzoate needed is 0.0159 mol, so the amount of sodium benzoate in grams is 0.0159 x 144.11 = 2.27 g.
You are asked to prepare a pH = 4.00 buffer starting from 1.50 L of 0.0200 M solution of benzoic acid (C6H5COOH) and some added sodium benzoate salt (C6H5COONa).
a) Find the pH of the benzoic acid solution BEFORE adding the salt.
b) Use the Henderson-Hasselbalck equation to derive how many moles of the salt you should add.
c) How many grams of the salt should you add?
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