Question

Which of the following molecules has highest \(pK_{a}\) value?

• A. \(\text{HCl}\)
• B. \(\text{HF}\)
• C. \(\text{HBr}\)
• D. \(\text{HI}\)

Hint:

The weakest acid has the smallest Ka and so the highest pKa; among HX the weakest acid is HF.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept
\(pK_a=-\log K_a\), so a weaker acid has a larger \(pK_a\).

Step 2: Acid strength trend
Acidic strength of hydrogen halides increases down the group: \(\text{HF}<\text{HCl}<\text{HBr}<\text{HI}\), because the H-X bond becomes weaker.

Step 3: Conclude
HF has the strongest H-X bond, so it is the weakest acid and has the smallest \(K_a\).

Step 4: Compare the options
HF has the highest \(pK_a\), option (B).

Final Answer:
Hydrogen fluoride has the highest pKa value. \[ \boxed{\text{(B) }\text{HF}} \]