Consider the following sequence of reactions to give the major...

Step 1: The first reaction is a Friedel-Crafts alkylation. Benzene reacts with $CH_3Cl$ in the presence of anhydrous $AlCl_3$ to form toluene.

Step 2: The second reaction is electrophilic aromatic substitution. Toluene reacts with $Cl_2$ in the presence of $FeCl_3$ to form a mixture of o-chlorotoluene and p-chlorotoluene. Due to steric hindrance at the ortho position, the major product is p-chlorotoluene.

Step 3: The third reaction is the oxidation of the alkyl side chain. p-chlorotoluene is oxidized by $K_2Cr_2O_7 / H_2SO_4$ to form p-chlorobenzoic acid, which is the major product $(X)$.

The molecular formula of p-chlorobenzoic acid is $C_7H_5ClO_2$.
Molar mass of $(X) = (7 \times 12) + (5 \times 1) + 35.5 + (2 \times 16) = 84 + 5 + 35.5 + 32 = 156.5 \text{ g mol}^{-1}$.

Step 4: Reaction of p-chlorobenzoic acid with $NaHCO_3$ liberates $CO_2$ gas:
$C_6H_4ClCOOH + NaHCO_3 \rightarrow C_6H_4ClCOONa + H_2O + CO_2 \uparrow$

From the stoichiometry, $1$ mole of p-chlorobenzoic acid produces $1$ mole of $CO_2$ gas.
Volume of $CO_2$ liberated at STP = $11.2 \text{ dm}^3$.
Moles of $CO_2$ liberated = $\dfrac{11.2}{22.4} = 0.5 \text{ mol}$.

Therefore, moles of p-chlorobenzoic acid reacted = $0.5 \text{ mol}$.
Mass of $(X)$ reacted, $P = 0.5 \text{ mol} \times 156.5 \text{ g mol}^{-1} = 78.25 \text{ g}$.

Answer: $78.25$