Answer :
Using the pKa of the pyridinium ion and the final concentration of the remaining reactants, the calculated pH value is 5.23. The pH of the solution, with HCl and pyridine, can be determined by calculating the moles of each reactant and applying the Henderson-Hasselbalch equation. All the HCl reacts with pyridine, forming pyridinium ion.
This problem involves a strong acid (HCl) reacting with a weak base (pyridine, C5H5N).
Using the initial concentrations and volumes, we can determine the number of moles of each reactant.
The moles of HCl are (38.2 mL x 0.197 mol/L) = 0.00753 mol, while the moles of pyridine are (150.0 mL x 0.242 mol/L) = 0.036 mol.
Since HCl is a strong acid, it will react completely with pyridine, the base, until it runs out.
After the reaction, no HCl will left and pyridinium ion (C5H5NH+), the conjugate acid of pyridine, will be produced.
The pH can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([base]/[acid]).
You would need the pKa of the pyridinium ion, which is 5.23, and the remaining concentrations of the base (which is 0 because all pyridine reacts) and the acid (which is (0.036-0.00753) mol / 0.1882 L = 0.151 M) after the reaction.
Plugging these values into the equation gives a final pH value of 5.23.
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