Question

White phosphorus reacts with sulphuryl chloride and forms a solid substance A and a gas D. When A is heated at high temperature, it decomposes to give a colorless oily liquid 'X' and a gas C. The shape of X is:

• A. Pyramidal
• B. Trigonal bipyramidal
• C. Square planar
• D. Tetrahedral

Hint:

$PCl_3$ structure is similar to ammonia ($NH_3$) with a lone pair on the central atom.

Answer 1

The Correct Option is A

Step 1: Identify the unknowns from the clues.
White phosphorus reacts with sulphuryl chloride to give a solid A and a gas D. Heating A gives an oily liquid X and a gas C. We need the shape of X.

Step 2: Track the chemistry to find X.
The reaction route leads to phosphorus chloride chemistry. The oily, colourless liquid formed here is phosphorus trichloride, $PCl_3$. So X is $PCl_3$.

Step 3: Count the electron pairs on phosphorus.
In $PCl_3$ the central phosphorus has $5$ valence electrons. Three are used to bond three chlorine atoms, leaving one lone pair. So there are $3$ bond pairs and $1$ lone pair.

Step 4: Apply VSEPR.
Four electron domains arrange in a tetrahedral direction, but one corner is a lone pair, not an atom. So the shape of the atoms is not a full tetrahedron.

Step 5: Find the molecular shape.
With three bonded atoms and one lone pair, the molecule takes a pyramidal (trigonal pyramidal) shape, much like ammonia.

Step 6: State the answer.
The shape of X ($PCl_3$) is pyramidal.
\[ \boxed{\text{Pyramidal}} \]