Kb Formula

What is the Kb Formula?

The basic dissociation constant is Kb. In water, the base dissociation constant is a measurement of how thoroughly a base dissociates into its component ions.

When predicting whether a species would donate or accept protons at a certain pH value, Ka, pKa, Kb, and pKb are the most useful. They describe an acid or base’s degree of ionisation and are accurate indicators of acid or base strength because adding water to a solution has no effect on the equilibrium constant. Kb is the base dissociation constant, while pKb is the -log of the constant.

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How to Find the Kb of a Base?

The hydroxide ion is formed when weak bases react with water, as stated in the general equation below, where B is the parent base and BH+ is its conjugate acid.

B(aq) + H2O(l) ⇋ BH+(aq) + OH(aq)

The base ionisation constant (Kb), also known as the base dissociation constant, is the equilibrium constant for this reaction.

Kb = [BH+][OH]/[B]

Bronsted is a town in Bronsted, Denmark. The difference between an acid and a base, according to Lowry, is that an acid contributes hydrogen ions while a base receives them. The base dissociation constant (Kb) describes how the ions that make up the base split into positive and negative components. The acid dissociation constant is Ka. The stronger the base, the greater the value of Kb, and the stronger the acid, the larger the value of Ka.

A base that ionises only slightly in an area solution; a base accepts a hydrogen ion from another substance; for example a weak base of ammonia dissolves in water to give ammonium ion by accepting H+ ion.

Understanding Kb and pKb

Kb is the base dissociation constant, while pKb is the – log of the constant.

Kb = [B+][OH]/[BOH]

pKb = – log Kb

A significant Kb value implies a strong base’s high amount of dissociation. A stronger base is indicated by a lower pKb value.

pKa and pKb are related by the simple relation:

pKa + pKb = 14

Ka and Kb Relationship

The magnitude of Ka for an acid and Kb for its conjugate base have a straightforward connection. Consider the ionisation of hydrocyanic acid (HCN) in water, which results in an acidic solution, and the reaction of CN with water, which results in a basic solution.

HCN(aq) ⇋ H+(aq) + CN(aq)

CN(aq) + H2O(l) ⇋ OH(aq) + HCN(aq)

The following is the equilibrium constant expression for HCN ionisation.

Ka = [H+][CN]/[HCN]

The following is the equivalent formulation for the reaction of cyanide with water.

Kb = [OH][HCN]/[CN]

Remember that the equilibrium constant for the sum of two reactions is the product of the equilibrium constants for the individual reactions when we combine equations.

In this scenario, the equation for water autoionization is the total of the processes given by Ka and Kb, and the product of the two equilibrium constants is

Ka.Kb = Kw

We can compute the other equilibrium constant for any conjugate acid–base pair if we know either Ka for an acid or Kb for its corresponding base.

Frequently Asked Questions on Kb Formula


What is acid base theory?

According to the Arrhenius theory, a substance which produces hydrogen ions in water, is called acid. A substance which produces hydroxide ions in water, is called base. According to Bronsted-Lowry theory, an acid is a proton donor and a base is a proton acceptor.


How do you identify acids and bases?

To determine whether a substance is an acid or a base, count the hydrogens on each substance before and after the reaction. If the number of hydrogens has decreased, that substance is the acid (donated hydrogen ions). If the number of hydrogens has increased, that substance is the base (accepts hydrogen ions).


What is the value of Kb in chemistry?

Kb is the base dissociation constant. The base dissociation constant is a measure of how completely a base dissociates into its component ions in water. pKb is the negative base-10 logarithm of the base dissociation constant (Kb) of a solution. It is used to determine the strength of a base or alkaline solution.


What is Ka value?

The acid dissociation constant (Ka) is used to distinguish strong acids from weak acids. Strong acids have exceptionally high Ka values. The Ka value is found by looking at the equilibrium constant for the dissociation of the acid. The higher the Ka, the more the acid dissociates.


What is the autoionization of water?

The autoionization of water refers to the reaction of water molecules to form two ions, the hydronium ion, which is H3O+, and the hydroxide ion, which is OH. In the reaction, the base takes an H+ ion from the acid and these two electrons are left behind on this oxygen.

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