If $H_2$ and $Cl_2$ enclosed separately in the same vessel exert pressures of $100$ and $200$ mm respectively, their mixture in the same vessel at the same temperature will exerts a pressure of $300$ mm.

The assertion is incorrect because hydrogen and chlorine react completely to form hydrogen chloride gas, meaning the final pressure will not be the simple sum of 300 mm. The reason is correct, as Dalton's law of partial pressures applies only to non-reacting gases. Therefore, the assertion is incorrect but the reason is correct.

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