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

Amount of pyridine required = 0.0316 M

Step-by-step explanation:

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

`pH=pK_a+log([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([Py])/(PyH^+])`

pH = 4.7

`pK_a=5.2`

`PyH^+` (Pyridinium)=0.100 M

Substitute the values in the formula

`pH=pK_a+log([Py])/(PyH^+])\\4.7=5.2 log([Py])/(0.100)`

`4.7-5.2=log([Py])/(0.100) \\-0.5=log([Py])/(0.100)\\([Py])/(0.100)=antilog -0.5\\([Py])/(0.100)=0.316`

`([Py])/(0.100) =0.316`

[Py]=0.0316\ M

Amount of pyridine required = 0.0316 M