Question

Pyridinium is a weak acid having a pKa of 5.2. How much pyridine (the conjugate base of pyridinium) must be added to an aqueous solution of 0.100M pyridinium (assume the solution volume is constant) to make a buffer that is designed to keep the pH at 4.7?

Answer 1

Answer:

Amount of pyridine required = 0.0316 M

Explanation:

pH of a buffer solution is calculated by using Henderson - Hasselbalch equation.

$pH=pK_a+log\frac{[Conjugate\ base]}{[weak\ acid]}$

Pyridinium is a weak acid and in the presence of its conjugate base, it acts as buffer.

Henderson - Hasselbalch equation for pyridine/pyridinium buffer is as follows:

$pH=pK_a+log\frac{[Py]}{PyH^+]}$

pH = 4.7

$pK_a=5.2$

$PyH^+$ (Pyridinium)=0.100 M

Substitute the values in the formula

$pH=pK_a+log\frac{[Py]}{PyH^+]}\\4.7=5.2 log\frac{[Py]}{0.100}$

$4.7-5.2=log\frac{[Py]}{0.100} \\-0.5=log\frac{[Py]}{0.100}\\\frac{[Py]}{0.100}=antilog -0.5\\\frac{[Py]}{0.100}=0.316$

$\frac{[Py]}{0.100} =0.316$

[Py]=0.0316\ M

Amount of pyridine required = 0.0316 M