How to find Ka

What is Ka?

Ka or dissociation constant is a standard used to measure the acidic strength. It determines the dissociation of acid in an aqueous solution. Ka is generally used in distinguishing strong acid from a weak acid. More the value of Ka higher would be acids dissociation.

Table of Content

• How to calculate Ka
• Ka from Molarity
• Ka from pH
• Ka from pKa
• Ka from Titration Curve
• Frequently Asked Questions – FAQs

How to calculate Ka

Ka or dissociation constant is a standard used to measure the acidic strength. It determines the dissociation of acid in an aqueous solution. Ka is generally used in distinguishing strong acid from a weak acid. More the value of Ka higher would be its dissociation.

We can use numerous parameters to determine the Ka value. Few of them are enlisted below.

• Ka from Molarity
• Ka from pH
• Ka from pKa
• Ka from Titration Curve

Ka from Molarity

We can use molarity to determine the Ka value.

Consider dissociation of acid HX,

HX ⥦ H⁺ + X^–

By definition, we can quantify the Ka formula as a product divided by the reactant of the reaction.

Ka = [Product] / [Reactant]

We can fill the concentrations to write the Ka equation based on the above reaction.

So,

Ka = [H⁺] [X^–] / [HX]

For the reaction,

MgCl₂ ⥦ Mg²⁺ + 2Cl^–

So for the above reaction, the Ka value would be

Ka = [Mg²⁺] [Cl^–]² / [MgCl₂ ]

Thus, we can quickly determine the Ka value if the molarity is known.

Ka from pH

We can use pH to determine the Ka value. pH is a standard used to measure the hydrogen ion concentration.

pH = – log [H⁺]

We can rewrite it as,

If the pH of acid is known, we can easily calculate the relative concentration of acid and thus the dissociation constant Ka.

Example:

• Calculate the Ka value of 0.2 M Hydrofluoric Acid with a pH of 4.88.

HF will dissociate as

HF ⥦ H⁺ + F^–

We are constructing an ICE table.

We know that,

pH = – log [H⁺] [H⁺] = 10 ^-pH.

Further,

Ka = [H⁺] [F^–] / [HF]

Ka = x² / 0.2 – x

Ka = (1.32 X 10^-5)² / 0.2 – 1.32 X 10^-5

Ka = 8.69 X 10^-10

Thus, we can quickly determine the Ka value if the pH is known.

Ka from pKa

We can use pKa to determine the Ka value. We know that pKa is equivalent to the negative logarithm of Ka.

pKa = – logarithm [Ka]

– pKa = logarithm [Ka]

We can rewrite it as

10 ^{– (pKa)} = Ka

Or, Ka = 10 ^{– (pKa).}

Thus if the pKa is known, we can quickly determine the Ka value.

Ka from Titration Curve

We can use the titration curve to determine the Ka value. To find out the Ka of the solution, firstly, we will determine the pKa of the solution. At the equivalence point, the pH of the solution is equivalent to the pKa of the solution. Thus using Ka = – log pKa equation, we can quickly determine the value of Ka using a titration curve.