Question

The conjugate base of phosphorus acid is $ x $. The conjugate base of oleum is $ y $. What are $ x $ and $ y $, respectively?

• A. \(\mathrm{H_2PO_4^-},\ \mathrm{HS_2O_7^-}\)
• B. \(\mathrm{H_2PO_4^-},\ \mathrm{HSO_5^-}\)
• C. \(\mathrm{H_2PO_3^-},\ \mathrm{HS_2O_7^-}\)
• D. \(\mathrm{H_2PO_3^-},\ \mathrm{HSO_4^-}\)

Hint:

The conjugate base of an acid is formed by removing one \(\mathrm{H^+}\) ion. For polyprotic acids like \(\mathrm{H_3PO_3}\), only one hydrogen from the acidic OH groups is usually removed per step.

Answer 1

The Correct Option is C

Step 1: Phosphorus acid \(\mathrm{H_3PO_3}\) is a diprotic acid. When it donates one proton, the conjugate base is: \[ \mathrm{H_3PO_3\ (aq)} \rightarrow \mathrm{H_2PO_3^- (aq)} + \mathrm{H^+} \]
Step 2: Oleum is a solution of \(\mathrm{SO_3}\) in \(\mathrm{H_2SO_4}\). It can be considered a source of pyrosulfuric acid \((\mathrm{H_2S_2O_7})\). The conjugate base of pyrosulfuric acid is: \[ \mathrm{H_2S_2O_7} \rightarrow \mathrm{HS_2O_7^-} + \mathrm{H^+} \] Thus, the conjugate bases are: \[ x = \mathrm{H_2PO_3^-},\quad y = \mathrm{HS_2O_7^-} \]