The equilibrium $2 NO X_{2} (g) N X_{2} O X_{4} (g)$ is exothermic in the forward direction. A rigid sealed flask containing the equilibrium mixture is heated from $300 K$ to $350 K$ . Which statement is correct?

AThe colour deepens, the number of gas moles increases and the average molar mass falls
BThe colour deepens, and $K_{c}$ for $2 NO X_{2} N X_{2} O X_{4}$ increases
CThe colour fades and the number of gas moles decreases
DThe colour deepens and the pressure rises exactly in proportion to the absolute temperature

Answer: A

Heating shifts an exothermic forward reaction backward:

N X_{2} O X_{4}

dissociates to brown

NO X_{2}

, raising the moles of gas and lowering the average molar mass (A).

K_{c}

for the forward reaction falls (B false). Because the mole count also rises,

P

increases by more than the ratio

350/300

(D false).