Question

An acid `X' is prepared by acidifying an aqueous solution of borax. Which of the following statements are true for `X'?
I. It is white liquid with soapy touch
II. It is sparingly soluble in water
III. It is strong monobasic acid
IV. It acts as Lewis acid

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

Hint:

Boric acid is not a protonic acid (doesn't release $H^+$ directly). It is a weak Lewis acid.

Answer 1

The Correct Option is A

Step 1: Identify Acid 'X':
When an aqueous solution of Borax ($Na_2B_4O_7 \cdot 10H_2O$) is acidified (usually with HCl or $H_2SO_4$), it yields Orthoboric acid ($H_3BO_3$). Reaction: \[ Na_2B_4O_7 + 2HCl + 5H_2O \rightarrow 4H_3BO_3 \downarrow + 2NaCl \] So, X is Boric Acid ($H_3BO_3$).
Step 2: Analyze the Statements about Boric Acid:

• Statement I: It is a white crystalline solid (not liquid) with a soapy touch. So, "liquid" makes this statement False.
• Statement II: Boric acid is sparingly soluble in cold water but highly soluble in hot water. True.
• Statement III: It is a very weak monobasic acid (not strong). It does not donate $H^+$ directly; rather, it accepts $OH^-$ from water. False.
• Statement IV: It acts as a Lewis acid because the central boron atom has an incomplete octet (electron deficient) and accepts an electron pair from $OH^-$ ion of water ($B(OH)_3 + 2H_2O \rightleftharpoons [B(OH)_4]^- + H_3O^+$). True.

Step 3: Conclusion:
Statements II and IV are true.