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.#
#----------------#
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).
