Question

A, B and C are weak acids. Their dissociation constants (\(K_a(A)\), \(K_a(B)\), \(K_a(C)\)) are \[ 3.5 \times 10^{-4}, 1 \times 10^{-5} \text{ and } 5 \times 10^{-10} \text{ respectively. The pH of 1L of 0.01 M each of these solutions follow the order.} \]

• A. \( C>B>A \)
• B. \( B>A>C \)
• C. \( A>C>B \)
• D. \( A>C>B \)

Hint:

For weak acids, a higher \(K_a\) value corresponds to a stronger acid and a lower pH.

Answer 1

The Correct Option is A

The dissociation constant \(K_a\) indicates the strength of an acid. A higher dissociation constant means a stronger acid, which will dissociate more and produce more hydrogen ions (\(H^+\)). For weak acids, the pH is inversely related to the dissociation constant. Given that \(K_a(C) = 5 \times 10^{-10}\), \(K_a(B) = 1 \times 10^{-5}\), and \(K_a(A) = 3.5 \times 10^{-4}\), we can conclude that the pH will follow the order: \[ C>B>A \] because acid \(C\) has the lowest \(K_a\) (weakest acid) and will have the highest pH, followed by \(B\) and then \(A\), which has the strongest acid.