The decomposition of mercuric oxide (HgO) on heating, resulting in the formation of mercury and oxygen, is represented by the following chemical equation:

2HgO + 180 kJ → 2Hg + O₂

Which one of the following represents the heat of formation of mercuric oxide?

A) 5.66 kJmol⁻¹
B) 90 kJmol⁻¹
C) 180 kJmol⁻¹
D) 900 kJmol⁻¹

To determine the heat of formation of mercuric oxide (HgO), we need to understand the concept of heat of formation and use the information provided in the reaction.

The heat of formation (\( \Delta H_f^\circ \)) of a compound is the change in enthalpy when one mole of the compound is formed from its elements in their standard states.

The given reaction is:

\[ 2 \text{HgO} \xrightarrow{180\; \text{kJ}} 2 \text{Hg} + \text{O}_2 \]

Here’s how to solve the problem step by step:

1. **Identify the reaction and its enthalpy change:**
- The reaction shows that 2 moles of mercuric oxide (HgO) decompose to form 2 moles of mercury (Hg) and 1 mole of oxygen gas (O\(_2\)).
- The heat absorbed during this reaction is 180 kJ for 2 moles of HgO.

2. **Determine the heat of formation for HgO:**
- The reaction provided is the decomposition of HgO, which is the reverse of the formation reaction of HgO.
- The heat absorbed (or released in the reverse reaction) is \( 180 \text{ kJ} \) for 2 moles of HgO. Therefore, the heat of formation for 1 mole of HgO would be half of this value.

3. **Calculate the heat of formation:**
- For 2 moles of HgO, the heat of formation is \( \frac{180 \text{ kJ}}{2} = 90 \text{ kJ/mol} \).

Thus, the heat of formation of mercuric oxide (HgO) is:

**B) 90 kJ/mol**