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).