JEE Chemistry 2025 Question with Solution

Given: Reaction $A\text{(g)} \rightarrow 2B\text{(g)} + C\text{(g)}$ is first order and starts with pure $A$. The total pressure is $160 \, \text{mm Hg}$ at $t = 10 \, \text{min}$ and $240 \, \text{mm Hg}$ at $t \to \infty$.

Find: Which statement is incorrect.

At completion, $1$ mole of $A$ gives $3$ moles of gaseous products, so the final pressure is three times the initial pressure.

So statement (1) is correct.

For a first-order reaction, the reactant amount decreases exponentially and approaches zero asymptotically. Hence statement (2) is taken as correct in the given solution.

At $t = 10 \, \text{min}$, use

Substituting the values,

Therefore, the rate constant is $0.0693 \, \text{min}^{-1}$, not $1.693 \, \text{min}^{-1}$. So statement (3) is incorrect.

Also, the partial pressure of $A$ after $10 \, \text{min}$ is $40 \, \text{mm Hg}$, so statement (4) is correct.

Therefore, the correct option is C.

Given: Initial total pressure is due only to $A$. At $t \to \infty$, total pressure is $240 \, \text{mm Hg}$.

Find: Check the four statements using reactant pressure.

Since

let the pressure of $A$ remaining after $10 \, \text{min}$ be $P_A$. If $x$ pressure equivalent of $A$ decomposes, then total pressure becomes

At $t = 10 \, \text{min}$,

Hence,

Now apply the first-order law:

Thus statement (4) is correct, while statement (3) is incorrect.

Therefore, the incorrect option is C.