Question

$p{K}_a$ values of four acids are given below at ${25}^{o}C$. Indicate the strongest acid.

• A. 2
• B. 3
• C. 2.5
• D. 4

Answer 1

The correct option is A 2

$Ka$ represents the ionisation constant of an acid. It is a measure of the strength of the acid, provided the temperature remains constant.

$pKa=-log\left(Ka\right)$

When $pKa=2$

$2=-log\left(Ka\right)$

$Ka=0.01$

When $pKa3$

$3=-log\left(Ka\right)$

$Ka=0.001$

Similarly, as $pKa$ values increase, $Ka$ value decrease and the acid becomes less strong. Hence smaller the $pKa$ value larger the $Ka$ value and hence stronger the acid.

So, the correct option is $A$