Chemistry - Complete Pblock- Question with Solution | TestHub

A. Red phosphorus is insoluble in water and CS₂.

Red phosphorus is an amorphous allotrope of phosphorus. Unlike white phosphorus, it is polymeric and much less reactive. It is indeed insoluble in water and also in organic solvents like carbon disulfide (CS₂). This statement is CORRECT.

B. Number of C-C linkages and $\pi$-bonds present in C₉₀ form of fullerene are respectively 270 and 60.

Fullerenes are allotropes of carbon with a cage-like structure. For any fullerene C${}_n$, the number of carbon atoms is $n$.

The general formula for the number of C-C bonds (linkages) in a fullerene C${}_n$ is $\frac{3n}{2}$.

The general formula for the number of $\pi$-bonds (double bonds) in a fullerene C${}_n$ is $\frac{n}{2}$.

For C₉₀ fullerene:

Number of C-C linkages = $\frac{3\times 90}{2}=\frac{270}{2}=135$.

Number of $\pi$-bonds = $\frac{90}{2}=45$.

The statement claims 270 C-C linkages and 60 $\pi$-bonds, which contradicts our calculations. Therefore, this statement is INCORRECT.

C. Rhombic sulfur transforms into monoclinic sulfur above 369 K which is soluble in CS₂.

Sulfur exists in several allotropic forms. Rhombic sulfur ($\alpha$-sulfur) is the most stable allotrope at room temperature. Above 369 K (96 °C), rhombic sulfur slowly transforms into monoclinic sulfur ($\beta$-sulfur). Both rhombic and monoclinic sulfur are soluble in carbon disulfide (CS₂). This statement is CORRECT.

D. SO₃ has three allotropic forms $\alpha$-SO₃, $\beta$-SO₃ and $\gamma$-SO₃.

Sulfur trioxide (SO₃) exists in three main allotropic forms:

$\alpha$-SO₃: An asbestos-like polymer, stable below 288 K.

$\beta$-SO₃: A polymeric form, consisting of chains of SO₄ tetrahedra.

$\gamma$-SO₃: A trimeric form, (SO₃)₃, also known as ice-like SO₃, which is the most volatile form.

This statement is CORRECT.

Since statement B is incorrect, it is the answer to the question.

The final answer is $\boxed{B}$.