You'd need #"41.7 mL"# of citric acid for this particular buffer.
You need to find two equations that you can use to determine the volume of the citric acid solution. The first one will be
#V_("buffer") = V_1 + V_2 = "125 mL" = "0.125 L"# (1)
The total volume of the buffer solution will be equal to the sum of the two solutions mixed together - #V_1# is the volume of the citric acid solution, while #V_2# is the volume of the sodium citrate solution.
Next, use the Henderson-Hasselbalch equation
#pH_("solution") = pKa + log(([C_6H_5O_7^(3-)])/([C_6H_8O_7]))#
#3.45 = 3.15 + log(([C_6H_5O_7^(3-)])/([C_6H_8O_7])) => ([C_6H_5O_7^(3-)])/([C_6H_8O_7]) = 2.0# (2)
Now, the concentration of the citric acid in the buffer is equal to
#C_("citric") = n_("citric")/(V_1+V_2)#
The number of moles of citric acid can be determined from the initial concentration
#n_("citric") = C * V_1 = "0.150 M" * V_1 = 0.150 * V_1#
Likewise, the concentration of the citrate is
#C_("citrate") = n_("citrate")/(V_1 + V_2)#, and
#n_("citrate") = C * V_2 = "0.150 M" * V_2 = 0.150 * V_2#
Plug all of this into equation (2) and you'll get
#(0.150 * V_2)/(V_1 + V_2) * (V_1 + V_2)/(0.150 * V_1) = 2.0#, or
#V_2/V_1 = 2 => V_2 = 2 * V_1#. Plug this into equation (1)
#V_1 + 2 * V_1 = 0.125 => V_1 = 0.125/3 = 0.0417#, which means that
#V_2 = 0.125 - 0.0417 = 0.0833#
Therefore, the volume for the citric acid solution will need to be
#V_1 = "0.0417 L" = "41.7 mL"#
