pKb to Kb

What is pKb?

pKb is the criterion used to estimate the alkalinity of the molecule. It is used to measure basic strength. The lesser the pKb is, the more potent the base will be. It is equivalent to the negative logarithm of base dissociation constant, Kb.

pKb = – log Kb

What is Kb?

Kb or base dissociation constant is the criterion used to estimate a base’s strength. It tells us about how much a base dissociates in an aqueous solution. Kb is used in distinguishing a strong base from a weak base. More the value of Kb more would be its dissociation.

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Understanding Kb and pKb in detail

pKb is the criterion used to estimate the alkalinity of the molecule. It is used to measure basic strength. The lesser the pKb is, the more potent the base will be. It is equivalent to the negative logarithm of base dissociation constant, Kb.

pKb = – log Kb

  • The larger the value of pKb weaker would be the base.

In contrast, Kb or base dissociation constant is the criterion used to estimate a base’s strength. It tells us about how much a base dissociates in an aqueous solution. Kb is used in distinguishing a strong base from a weak base. More the value of Kb more would be its dissociation.

Consider dissociation of base BOH:

BOH ⥦ B+ + OH

For the above reaction, the base dissociation constant Kb would be,

\(\begin{array}{l}K_{b} = \frac{[OH^-][B^+]}{[BOH]}\end{array} \)

Whereas

pKb = – log Kb

\(\begin{array}{l}pKb = -log\frac{[OH^-][B^+]}{[BOH]}\end{array} \)
\(\begin{array}{l}pKb = log\frac{[BOH]}{[OH^-][B^+]}\end{array} \)

Here, quantities in the square brackets are the equilibrium concentration.

A strong base will dissociate into its constituent ions quickly. Thus, we can say that a strong base has a larger Kb. If the concentration of B+ and OH is more, the Kb value would increase.

pKb to Kb

We can convert pKb to Kb as

pKb = – logKb

– pKb = log Kb

e – (pKb) = Kb

Kb = e-(pKb).

The dissociation constant of a strong base is as high as 102, while a weak base has as low as 10-10, which is quite challenging to remember. So to ease that, pKb came into existence.

We know that

pKb = -log Kb.

Putting the base dissociation constant value in the equation, we get -2 pKb for a strong base and 10 pKb for a weak base.

pKb value of few bases

S. No. Name of the Base pKb value
1 Lithium Hydroxide -0.36
2 Potassium Hydroxide 0.5
3 Sodium Hydroxide 0.2
4 Ammonia 4.76

Frequently Asked Questions on pKb to Kb

Q1

What is pKb?

pKb is the criterion used to estimate the alkalinity of the molecule. It is used to measure basic strength. The lesser the pKb is, the more potent the base will be. It is equivalent to the negative logarithm of base dissociation constant, Kb.

pKb = – log Kb

Q2

What is Kb?

Kb or base dissociation constant is the criterion used to estimate a base’s strength. It tells us about how much a base dissociates in an aqueous solution. Kb is used in distinguishing a strong base from a weak base. More the value of Kb more would be its dissociation.

Q3

How will you convert pKb to Kb?

We can convert pKb to Kb as,

pKb = – logKb

– pKb = log Kb

e – (pKb) = Kb

Kb = e-(pKb).

Q4

What is the chemical importance of pKb?

The dissociation constant of a strong base is as high as 102, while a weak base has as low as 10-10, which is quite challenging to remember. So to ease that, pKb came into existence.

We know that

pKb = -log Kb.

Putting the base dissociation constant value in the equation, we get -2 pKb for a strong base and 10 pKb for a weak base, which is easy to remember.

Q5

Are Kb and pKb related?

Yes, Kb and pKb are related.

pKb = – log Kb

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