Question

A 0.10 M pyridine solution has a pH of 5.87. To the nearest hundredth of a unit, what is the ratio of pyridinium ions (pKa = 5.3) to neutral pyridine molecules at this pH?

A 0.10 M pyridine solution has a pH of 5.99. To the nearest hundredth of a unit, what fraction of the molecules are in the neutral form? (The pKa of the pyridinium ion is 5.3.)

Answer 1

1.We can use Henderson Hasselbalch equation to find the required ratio

Given ,

pH = 5.87

pKa =5.3

We have to find the ratio  [pyridinium ions ] / [pyridine] i.e [Acid] / [Base]

Using Henderson Hasselbalch equation,

pH = pKa - log{ [Acid] / [Base]}

Plugging the values in the above equation

5.87 = 5.3 - log{  [pyridinium ions ] / [pyridine]}

log{  [pyridinium ions ] / [pyridine]} = 5.3- 5.87 = -0.57

{  [pyridinium ions ] / [pyridine]} = 10^(-0.57) =0.2692

Therefore, the ratio, [pyridinium ions ] / [pyridine] = 0.2692

2. Given ,

pH = 5.99

pKa =5.3

Using Henderson Hasselbalch equation,

pH = pKa - log{ [Acid] / [Base]}

Plugging the values in the above equation,

5.99 = 5.3 - log{  [pyridinium ions ] / [pyridine]}

log{  [pyridinium ions ] / [pyridine]} = 5.3- 5.99 = -0.69

[pyridinium ions ] / [pyridine] = 10^(-0.69) = 0.20418

[pyridinium ions ] = 0.20418 [pyridine]

Let  [pyridine] be x,

then  [pyridinium ions ] = 0.20418 x

Now, concentration of pyridine solution = 0.10 M

Therefore,

[pyridine] +   [pyridinium ions ] = 0.10 M

x + 0.20418 x = 0.10 M

1.20418 x =  0.10 M

x =0.083 M

Therefore, concentration of neutral pyridine molecules = 0.083 M

Fraction of neutral pyridine molecules =  concentration of neutral pyridine molecules /  concentration of pyridine solution

= 0.083 M / 0.1 M

= 0.83