Ka to pH

What is Ka?

Ka or acid dissociation constant is a standard used to measure the strength of an acid. It determines the extent of acid dissociation in an aqueous solution. It is used to distinguish strong acid from a weak acid. More the Ka, more would be dissociation and hence stronger would be the acid.

What is pH?

The pH or potential of hydrogen is a criterion for measuring the acidity or alkalinity of the solution. It determines the hydrogen ion concentration in a solution. The pH of the solution is equal to the negative logarithm of hydrogen ion (H⁺) concentration.

pH = – log [H⁺]

Table of Content

• Understanding Ka and pH in detail
• Ka to pH
• Ka and pH value of a few Acid
• Difference between Ka and pH
• Frequently Asked Questions – FAQs

Understanding Ka and pH in detail

Ka or acid dissociation constant is a standard used to measure the strength of an acid. It determines the extent of acid dissociation in an aqueous solution. It is used to distinguish strong acid from a weak acid. More the Ka, more would be dissociation and hence stronger would be the acid.

In contrast, the pH or potential of hydrogen is a criterion for measuring the acidity or alkalinity of the solution. It determines the hydrogen ion concentration in a solution. The pH of the solution is equal to the negative logarithm of hydrogen ion (H⁺) concentration.

pH = – log [H⁺]

At pH value 7, the solution is neutral. In contrast, if the pH value is greater than 7, the solution will be alkaline, or if the ph value is less than 7, the solution will be acidic.

Both Ka and pH are associated with each other. More the Ka, more would be its dissociation and thus stronger would be the acid. A strong acid has less pH value. Therefore, a larger Ka corresponds to a lesser pH value.

Ka to pH

Henderson-Hasselbalch has given a relation between pH and pKa. He said that pH is equal to sum of pKa and log {[salt] / [Acid]}.

Thus,

pH = pKa + log([salt]/[Acid])

At equivalence point, ([salt]/[Acid]) is equal to 1.

So,

pH = pKa . . . 1

We know that,

pKa = – log Ka

Putting value in equation1.

pH = – log Ka

Or, log Ka = – pH

Ka = e ^{– pH}

Thus, by using the Henderson-Hasselbalch equation, we can quickly convert Ka to pH.

Ka and pH value of a few Acid

Difference between Ka and pH