Which of the following molecules have zero dipole moment ?
cis-1,2-dichloroethene, trans-1,2-dichloroethene, 1,1-dichloroethene, trans-2-pentene, cis-2-pentene, 1-butyne, 2-butyne , trans-2-butene, cis-2-butene.

Explanation

To determine which of the given molecules have a zero dipole moment, we need to analyze the molecular geometry and the arrangement of the substituents. A molecule has a zero dipole moment if the dipole moments of the individual bonds cancel each other out due to symmetry.

### Step-by-Step Solution:

1. **Cis-1,2-dichloroethene**:
- Structure: Cl on the same side of the double bond.
- Analysis: The dipole moments from the two Cl atoms do not cancel out because they are on the same side.
- Conclusion: **Non-zero dipole moment**.

2. **Trans-1,2-dichloroethene**:
- Structure: Cl on opposite sides of the double bond.
- Analysis: The dipole moments from the two Cl atoms cancel each other out due to symmetry.
- Conclusion: **Zero dipole moment**.

3. **1,1-dichloroethene**:
- Structure: Both Cl atoms on the same carbon.
- Analysis: The dipole moment from the Cl atom does not have a counterpart to cancel it out.
- Conclusion: **Non-zero dipole moment**.

4. **Trans-2-pentene**:
- Structure: CH3 and CH2 groups on opposite sides of the double bond.
- Analysis: The dipole moments from the two different groups cancel each other out due to symmetry.
- Conclusion: **Zero dipole moment**.

5. **Cis-2-pentene**:
- Structure: CH3 and CH2 groups on the same side of the double bond.
- Analysis: The dipole moments do not cancel out as they are on the same side.
- Conclusion: **Non-zero dipole moment**.

6. **1-butyne**:
- Structure: A terminal alkyne.
- Analysis: The molecule is not symmetrical and has a dipole moment due to the terminal hydrogen and the carbon chain.
- Conclusion: **Non-zero dipole moment**.

7. **2-butyne**:
- Structure: A symmetrical alkyne with CH3 groups on both sides.
- Analysis: The dipole moments cancel each other out due to symmetry.
- Conclusion: **Zero dipole moment**.

8. **Trans-2-butene**:
- Structure: CH3 groups on opposite sides of the double bond.
- Analysis: The dipole moments from the two CH3 groups cancel each other out due to symmetry.
- Conclusion: **Zero dipole moment**.

9. **Cis-2-butene**:
- Structure: CH3 groups on the same side of the double bond.
- Analysis: The dipole moments do not cancel out as they are on the same side.
- Conclusion: **Non-zero dipole moment**.

### Summary of Molecules with Zero Dipole Moment:
- **Trans-1,2-dichloroethene**
- **Trans-2-pentene**
- **2-butyne**
- **Trans-2-butene**

### Final Answer:
The molecules that have a zero dipole moment are:
- Trans-1,2-dichloroethene
- Trans-2-pentene
- 2-butyne
- Trans-2-butene