The decomposition of mercuric oxide (HgO) on heating, resulting in the formation of mercury and oxygen, is represented by the following chemical equation
$2 H g O + 180 k J x r i g h t a r r o w 2 H g + O_{2}$
Which one of the following represents the heat of formation of mercuric oxide?
A) $5.66 k J m o l^{- 1}$
B) $90 k J m o l^{- 1}$
C) $180 k J m o l^{- 1}$
D) $900 k J m o l^{- 1}$

Answer

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Hint: Heat of formation, also called standard heat of formation, enthalpy of formation, or standard enthalpy of formation, the amount of heat absorbed or evolved when one mole of a compound is formed from its constituent elements, each substance being in its normal physical state (gas, liquid, or solid).

Complete answer:
The Heat of Reaction (also known as Enthalpy of Reaction) is the change in the enthalpy of a chemical reaction that occurs at a constant pressure. It is a thermodynamic unit of measurement useful for calculating the amount of energy per mole either released or produced in a reaction. Since enthalpy is derived from pressure, volume, and internal energy, all of which are state functions, enthalpy is also a state function.
Mercuric oxide gives mercury and oxygen on heating.

2 H g O + 180 k J \to 2 H g + O_{2}

The heat of formation of mercuric oxide can be represented as follows :

H g + \frac{1}{2} O_{2} \to H g O + 90 k J

Hence, the heat of formation is the enthalpy change for the formation of one mole of HgO.

So the correct answer is B).

Note: While determining the heat of formation we must have the balanced chemical equation, if it is not balanced then the answer would become incorrect and it is always measured at the standard conditions of temperature and pressure.