Question
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ASSERTION: The heat of neutralisation of perchloric acid,$HCl{{O}_{4}}$ with NaOH is the same as that of HCl with NaOH.
REASON: Both HCl and $HCl{{O}_{4}}$ are strong acids.
A. Both assertion and reason are correct and reason is the correct explanation of assertion.
B. Both assertion and reason are correct but reason is not the correct explanation of assertion.
C. Assertion is correct but reason is incorrect.
D. Both assertion and reason are incorrect.

Answer
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Hint: When an acid and a base react, a neutralisation reaction occurs. If in both the reactions the reactant and product side is same, then the reactions are similar, and thus will have the same heat of neutralisation.

Complete answer:
Energy changes always accompany chemical reactions. If energy, within the sort of heat, is liberated the reaction is exothermic and if energy is absorbed the reaction is endothermic. Thermochemistry is anxious with the measurement of the quantity of warmth evolved or absorbed. The warmth (or enthalpy) of neutralization (∆H) is that the heat evolved when an acid and a base react to make a salt plus water. Heat measurements are performed by ending the reaction in a very special container called a calorimeter. The warmth (Q) given off by the neutralisation is absorbed by the reaction solution and therefore the calorimeter. Both the answer and calorimeter increase in temperature because of the absorbed heat and this increase may be measured with a thermometer. ∆H is negative if heat is evolved and vice versa if heat is absorbed.
In our question , we have been given two acids, HCl and $HCl{{O}_{4}}$. Both are strong acids, and as well as NaOH is a strong base. So, they will undergo complete dissociation. As strong bases and strong acids react to form neutral salt and water, the reactions in both the cases are same i.e,
                    ${{H}^{+}}+O{{H}^{-}}->{{H}_{2}}O+salt$
As the constituent elements are same, hence the heat of neutralisation will be same for both the reactions, and it was possible because both of them are strong acids. Hence, both the assertion and reason are correct and reason is the correct explanation of assertion, which gives A as the correct option.

Note:
In case there was a strong acid and a weaker acid, then the ${{H}^{+}}$ ion would only have been given by the stronger acid. Same is the case for $O{{H}^{-}}$ions for a stronger/weaker base. In that case, we would have obtained different values of heat of neutralisation.