How many of the following are heterocyclic aromatic compounds ?
benzene, cyclopentadiene, oxirane, tetrahydrofuran, naphthalene, cyclopentane, tropolone, azulene, diphenyl.

Explanation

To determine how many of the given compounds are heterocyclic aromatic compounds, we need to analyze each compound based on the definitions of heterocyclic and aromatic compounds.

### Step-by-Step Solution:

1. **Benzene**:
- Benzene is a cyclic compound consisting only of carbon atoms.
- It is aromatic due to its planar structure and the delocalization of 6 π electrons (follows Huckel's rule).
- **Conclusion**: Not heterocyclic (homocyclic).

2. **Cyclopentadiene**:
- Cyclopentadiene is a five-membered ring with two double bonds.
- It is not planar due to sp³ hybridization of one of the carbons, and it does not have delocalized π electrons.
- **Conclusion**: Not aromatic and not heterocyclic.

3. **Oxirane**:
- Oxirane (ethylene oxide) is a three-membered ring containing one oxygen atom.
- It is not aromatic because it does not have delocalized π electrons.
- **Conclusion**: Heterocyclic but not aromatic.

4. **Tetrahydrofuran**:
- Tetrahydrofuran is a five-membered ring with one oxygen atom.
- It is not aromatic because it does not have delocalized π electrons.
- **Conclusion**: Heterocyclic but not aromatic.

5. **Naphthalene**:
- Naphthalene consists of two fused benzene rings.
- It is aromatic due to delocalized π electrons (10 π electrons).
- **Conclusion**: Not heterocyclic (homocyclic).

6. **Cyclopentane**:
- Cyclopentane is a five-membered ring consisting only of carbon atoms.
- It is not aromatic because it is not planar and does not have delocalized π electrons.
- **Conclusion**: Not heterocyclic (homocyclic).

7. **Tropolone**:
- Tropolone has a seven-membered ring with one oxygen atom and three double bonds.
- It is aromatic due to the delocalization of π electrons (6 π electrons).
- **Conclusion**: Not heterocyclic (homocyclic).

8. **Azulene**:
- Azulene consists of two fused rings (5-membered and 7-membered) and has a total of 10 π electrons.
- It is aromatic due to delocalization.
- **Conclusion**: Not heterocyclic (homocyclic).

9. **Diphenyl**:
- Diphenyl consists of two benzene rings.
- It is aromatic due to delocalized π electrons (12 π electrons).
- **Conclusion**: Not heterocyclic (homocyclic).

### Summary of Findings:
- Heterocyclic aromatic compounds found: **0**
- Compounds that are heterocyclic but not aromatic: **Oxirane, Tetrahydrofuran** (2 total)
- Compounds that are neither heterocyclic nor aromatic: **Benzene, Cyclopentadiene, Naphthalene, Cyclopentane, Tropolone, Azulene, Diphenyl** (7 total)

### Final Answer:
**Total number of heterocyclic aromatic compounds: 0**