Use the Henderson-Hasselbalch equation, eq. (3), to calculate the pH expected for a buffer solution prepared from this acid and its conjugate base, assuming: the mass of the weak acid is 0.2 g and the mass of the conjugate base is 0.2 g will be added to 25 mL of water. Also assume the molar mass of the weak acid is 234 and its conjugate base is the sodium salt.

In this case, given the Henderson-Hasselbach equation, it is possible for us to compute the pH by firstly computing the concentration of the acid and the conjugate base; for this purpose we assume that the volume of the total solution is 0.025 L and the molar mass of the sodium base is 234 - 1 + 23 = 256 g/mol as one H is replaced by the Na:

[tex]n_{acid}=\frac{0.2g}{234g/mol}=0.000855mol\\\\n_{base}= \frac{0.2g}{256g/mol}=0.000781mol[/tex]

And the concentrations are:

[tex][acid]=0.000855mol/0.025L=0.0342M[/tex]

[tex][base]=0.000781mol/0.025L=0.0312M[/tex]

Then, considering that the Ka of this acid is 2.5x10⁻⁵, we obtain for the pH:

[tex]pH=-log(2.5x10^{-5})+log(\frac{0.0312M}{0.0342M} )\\\\pH=4.60-0.04\\\\pH=4.56[/tex]

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