Answer:
The standard enthalpy of formation of HgO is -90.7 kJ/mol.
Explanation:
The reaction between Hg and oxygen is as follows.

From the given,
Molar mass of HgO = 216.59 g/mol
Mass of HgO decomposed = 18.5 g
Amount of heat absorbed = 7.75 kJ
From the reaction,
The standard enthalpy of formation = 
During the decomposition of 1 mol of HgO , 90.7 kJ of energy absorbed.
For the formation of 1 mol of HgO , 90.7 kJ of energy is release
Therefore, the enthalpy of formation of mercury(II)Oxide is -90.7 kJ/mol
The reaction CO₂(g) → C(s) +O₂(g) is a decomposition reaction is true
Explanation
A Decomposition reaction is a reaction in which involve breaking down a compound into two or more simpler substances.
The reaction above is a decomposition reaction since CO₂ molecule is broken down into C (an element) and O₂ (a molecule).
Electrons are both gained and lost.