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

Answer:

#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.#

#----------------#

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).