Question

The correct statements about the compounds of boron are
I. In borax bead test, the colour of cobalt metaborate is blue
II. Diborane is prepared by the oxidation of sodium borohydride with iodine
III. In diborane oxidation state of hydrogen is +1
IV. Boric acid is a tribasic acid

• A. I & II
• B. III & IV
• C. I & III
• D. II & IV

Hint:

Remember: Boric acid is a proton donor only in the sense that it abstracts \(\text{OH}^-\) from water, releasing \(\text{H}^+\). It is a monobasic Lewis acid.

Answer 1

The Correct Option is A

Step 1: Understanding the Statement I: Borax bead test: When borax is heated with cobalt salts, it forms cobalt metaborate \(\text{Co(BO}_2)_2\), which is indeed blue in colour. \(\rightarrow\) Statement I is Correct.
Step 2: Understanding the Statement II: Preparation of Diborane: Diborane (\(\text{B}_2\text{H}_6\)) is prepared in the lab by the oxidation of sodium borohydride (\(\text{NaBH}_4\)) with iodine (\(\text{I}_2\)). Reaction: \(2\text{NaBH}_4 + \text{I}_2 \rightarrow \text{B}_2\text{H}_6 + 2\text{NaI} + \text{H}_2\). \(\rightarrow\) Statement II is Correct.
Step 3: Understanding the Statement III: Oxidation state in Diborane: Boron (Electronegativity \(\approx 2.0\)) is less electronegative than Hydrogen (Electronegativity \(\approx 2.1\)). Therefore, hydrogen is assigned an oxidation state of \(-1\) (hydride), and Boron is \(+3\). The statement says \(+1\). \(\rightarrow\) Statement III is Incorrect.
Step 4: Understanding the Statement IV: Boric Acid: Orthoboric acid (\(\text{H}_3\text{BO}_3\)) is a weak monobasic Lewis acid, not tribasic. It accepts an \(\text{OH}^-\) ion from water rather than donating 3 protons. Reaction: \(\text{B(OH)}_3 + 2\text{H}_2\text{O} \rightleftharpoons [\text{B(OH)}_4]^- + \text{H}_3\text{O}^+\). \(\rightarrow\) Statement IV is Incorrect. Conclusion: Only statements I and II are correct.
Required Answer: Option (A).