Problem: You need to prepare 100.0mL of a pH = 4.00 buffer solution using 0.100 M benzoic acid (pKa = 4.20) and 0.240 M sodium benzoate. How much of each solution should be mixed to prepare the buffer?

FREE Expert Solution

We are asked to determine the volume of each solution needed to prepare the buffer.

Since we are dealing with a buffer solution, we are going to use the Henderson-Hasselbalch equation:


pH=pKa + logconjugate baseweak acid


We need to know the volume of the acid (VA)and the conjugate base (VB)required to prepare the buffer. 


But first we have to calculate the ratio of the conjugate base and the weak acid:

• benzoic acid is a weak acid

• benzoate in sodium benzoate is the conjugate base

            - concentration of benzoate = concentration of sodium benzoate


pH=pKa + logconjugate baseweak acid


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Problem Details

You need to prepare 100.0mL of a pH = 4.00 buffer solution using 0.100 M benzoic acid (pKa = 4.20) and 0.240 M sodium benzoate. How much of each solution should be mixed to prepare the buffer?