Question

Identify the correct statements from the following I) \[ \text{Conjugate base of nitrous acid is } NO_2^{-} \] II) \[ \text{The concentration of } OH^{-} \text{ ions in } 0.1\,M \text{ aqueous pyridine solution is } \sqrt{2}\times10^{-5}\,M \] \[ (K_b \text{ of pyridine }=2\times10^{-9}) \] III) The acid strength of aqueous hydrogen halides follows \[ HF \gt HCl \gt HBr \gt HI \]

• A. I, II, III
• B. I, II only
• C. I, III only
• D. II, III only

Hint:

For a weak base: \[ [OH^-]=\sqrt{K_bC} \] and for hydrogen halides in water: \[ HF \lt HCl \lt HBr \lt HI. \] HI is the strongest hydrohalic acid, while HF is the weakest.

Answer 1

The Correct Option is B

Statement I: Nitrous acid is \[ HNO_2. \] On losing one proton, \[ HNO_2 \rightleftharpoons H^+ + NO_2^-. \] Hence, the conjugate base is \[ \boxed{NO_2^-} \] Therefore, \[ \boxed{\text{Statement I is correct}} \]

Statement II: For pyridine (\(C_5H_5N\)), \[ K_b = \frac{[OH^-]^2}{C} \] for a weak base. Given, \[ K_b=2\times10^{-9} \] and \[ C=0.1. \] Thus, \[ [OH^-] = \sqrt{K_bC}. \] \[ = \sqrt{(2\times10^{-9})(0.1)}. \] \[ = \sqrt{2\times10^{-10}}. \] \[ = \sqrt{2}\times10^{-5}\ M. \] Therefore, \[ \boxed{\text{Statement II is correct}} \]

Statement III: The acid strength of hydrogen halides in aqueous solution increases down the group: \[ HF \lt HCl \lt HBr \lt HI. \] This is because the H--X bond strength decreases from HF to HI. Hence the given order \[ HF \gt HCl \gt HBr \gt HI \] is incorrect. \[ \boxed{\text{Statement III is incorrect}} \]

Final Result: Correct statements are I and II only. \[ \boxed{\text{I, II only}} \] \[ \boxed{\text{Answer = (B)}} \]