What mass of sodium benzoate should be added to 140.0 mL of a 0.15 M benzoic acid solution to obtain a buffer with a pH of 4.25?

1 Answer
Jul 12, 2016

3.3 \ g of sodium benzoate must be dissolved in the benzoic acid solution to obtain the solution with the desired pH (4.25).

Explanation:

The buffer solution is prepared by mixing benzoic acid and sodium benzoate.

Use the Handerson-Hasselbalch equation to determine the number of moles of the salt needed to produce the buffer solution with the desired pH.

The pK_a values for the common weak acids is provided in tables at the end of most chemistry text books.

The pK_a value for benzoic acid is 4.20.

n_"Salt" = ??

n_"Acid"= C_M xx V

n_"Acid"= 0.15 \ (mol.)/Lxx 0.1400 L

n_"Acid"= 0.0210 \ mol.

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Solve for the unknown (n_"Salt")

pH = pK_a + log ((n_"Salt")/(n_"Acid"))

4.25 = 4.20 + log ((n_"Salt")/(n_"Acid"))

log ((n_"Salt")/(n_"Acid")) = 4.25 -4.20

log ((n_"Salt")/(n_"Acid")) = 0.05

(n_"Salt")/(n_"Acid") = 10^0.05

(n_"Salt")/(n_"Acid") = 1.1

n_"Salt" = 0.0210 xx 1.1

n_"Salt" = 0.0231 \ mol.

m_"Salt" = 0.0231 \ mol. xx 144.11 g/(mol.)

m_"Salt" ~= 3.3 \ g

3.3 \ g of sodium benzoate must be dissolved in the benzoic acid solution to obtain the solution with the desired pH (4.25).