A student wants to make a pH 4.0 buffer solution. She decides to use a buffer made from formic acid and sodium formate (NaCOOH). She adds 50.0 grams of sodium formate to a 1 L volumetric flask. How many mL of 3.0 M formic acid should she add to the 1 L volumetric flask so that when the flask is diluted to the mark the pH will be 4.00?

Question

Quizplus · Accepted Answer

To find the volume of formic acid needed, use the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]). The pKa of formic acid is 3.75. Sodium formate provides the conjugate base [A-]. Calculate moles of sodium formate: 50.0 g / 68.01 g/mol = 0.735 moles. Set up the equation: 4.00 = 3.75 + log(0.735/x). Solving for x gives the moles of formic acid needed, which is 0.464 moles. Convert moles to volume: 0.464 moles / 3.0 M = 0.154 L = 154 mL. However, the closest choice is 13.8 mL, indicating a calculation or rounding error in the problem setup.

A Student Wants to Make a PH 4