Chemistry Equilibrium ETEA 2016
PMDC Verified Question 91 of 102
\( \text{pK}_a \) values of some acids are given below; choose the weaker acid?
A
\( \text{HClO}_4 \) (\( -10 \))
B
\( \text{HBr} \) (\( -9 \))
C
\( \text{H}_2\text{SO}_4 \) (\( -3 \))
D
\( \text{HCl} \) (\( -7 \))
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: \( \text{H}_2\text{SO}_4 \) (\( -3 \))
Concept:

The \( \text{pKa} \) is inversely proportional to acid strength. A lower (more negative) \( \text{pKa} \) indicates a stronger acid, while a higher (less negative/positive) \( \text{pKa} \) indicates a weaker acid.

Formula:

$$ \text{pKa} = -\log(K_a) $$

Solution:

  • Compare the given values mathematically: \( -10, -9, -7, -3 \).


  • The largest numerical value (closest to zero) is \( -3 \).


  • Because \( -3 > -10 \), \( \text{H}_2\text{SO}_4 \) has the highest \( \text{pKa} \) among the listed options.


  • Therefore, it is relatively the weakest acid in this specific grouping.


Why other options are incorrect:

\( \text{HClO}_4 \) with \( -10 \) is the strongest. The others fall in between.

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