Acids And Bases

Let's evaluate each statement about orthoboric acid (H₃BO₃):

• It can be prepared by the hydrolysis of boron trihalide: This statement is correct. Orthoboric acid can be prepared by hydrolyzing boron trihalide (such as BCl₃) in water.
• It is not a protonic acid but acts as a Lewis acid: This statement is correct. While orthoboric acid doesn't behave as a typical protonic acid, it acts as a Lewis acid by accepting electron pairs.
• It has a layer structure: This statement is correct. Orthoboric acid has a layered structure in its solid state, where each boron atom is connected to three hydroxyl groups.
• It is freely soluble in cold water: This statement is incorrect. Orthoboric acid is only sparingly soluble in cold water, not freely soluble.
• On heating above 370K it forms first metaboric acid which on further heating yields B₂O₃: This statement is correct. Orthoboric acid undergoes dehydration upon heating to form metaboric acid, which can further dehydrate to form boron oxide (B₂O₃).

Correct Answer: (B) It is freely soluble in cold water

When zinc metal is reacted with aqueous sodium hydroxide, the products formed are:

• zinc hydroxide and oxygen only
• sodium zincate and oxygen only
• sodium zincate, hydrogen and oxygen
• sodium zincate and hydrogen only
• sodium zincate and hydrogen oxide only

Correct Answer: sodium zincate and hydrogen only

Explanation:

The reaction of zinc metal with aqueous sodium hydroxide is as follows:

Zn(s) + 2NaOH(aq) + 2H₂O(l) → Na₂ZnO₂(aq) + 3H₂(g)

• Na₂ZnO₂ is sodium zincate (soluble complex).
• H₂ is evolved as hydrogen gas.

This is an example of an amphoteric metal (Zn) reacting with a strong base (NaOH).

Step 1: The total concentration of ions is calculated as: Step 2: Total concentration: Step 3: pH calculation: Step 4: Therefore, the correct answer is (B).

Step 1: Ammonium acetate ( ) is a salt derived from a weak acid (acetic acid) and a weak base (ammonia).
Step 2: When dissolved in water, it completely dissociates into and .
Step 3: Both the anion and the cation can react with water, but their effects on pH largely cancel each other out, resulting in a solution that acts as a buffer.
Step 4: Therefore, the pH of ammonium acetate solutions is relatively independent of its concentration compared to salts that yield ions from strong acids or bases.

Step 1: The process of converting alkanes into aromatics or cyclic hydrocarbons by dehydrogenation and isomerization under high temperature and pressure is known as catalytic reforming.
Step 2: serves as a catalyst in this reaction, facilitating the rearrangement and dehydrogenation necessary to transform n-hexane into benzene.
Step 3: The conditions described (773K and 10-20 atmospheres) are typical for a reforming process, which aims to increase the octane number of gasoline.
Step 4: Therefore, the conversion of n-hexane to benzene under these conditions is best described as reforming.

A Lewis acid is a substance that can accept an electron pair. This definition contrasts with the Brønsted-Lowry definition of an acid as a proton donor. Lewis acids are often metal cations or molecules with an empty orbital that can accept an electron pair.
- is a Brønsted-Lowry acid, not a Lewis acid.
- is a Lewis base because it can donate an electron pair.
- , cobalt(III), is a Lewis acid because it is a metal cation with an empty orbital that can accept electron pairs.
Thus, the correct answer is .