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"
