The number of molecules having zero dipole moment is trans-but-2-ene, cis-pent-2-ene, cis-but-2-ene, propene, trans-1,2-dichloroethene, but-2-yne, cis-1,2-dichloroethene

Explanation

To determine the number of molecules having a zero dipole moment among the given compounds, we need to analyze the molecular structures and their symmetry. Here’s a step-by-step solution:

### Step 1: Identify the Structures
1. **Trans-but-2-ene**:
- Structure: CH3-CH=CH-CH3 (with CH3 groups on opposite sides of the double bond).
- Symmetry: The dipoles from the CH3 groups cancel each other out.

2. **Cis-pent-2-ene**:
- Structure: CH3-CH=CH-CH2-CH3 (with CH3 groups on the same side of the double bond).
- Symmetry: The dipoles do not cancel out, resulting in a net dipole moment.

3. **Cis-but-2-ene**:
- Structure: CH3-CH=CH-CH3 (with both CH3 groups on the same side).
- Symmetry: The dipoles do not cancel out, resulting in a net dipole moment.

4. **Propene**:
- Structure: CH2=CH-CH3.
- Symmetry: The molecule has a net dipole moment due to the presence of the CH2 and CH3 groups.

5. **Trans-1,2-dichloroethene**:
- Structure: Cl-CH=CH-Cl (with Cl atoms on opposite sides).
- Symmetry: The dipoles from the Cl atoms cancel each other out, resulting in a zero dipole moment.

6. **But-2-yne**:
- Structure: CH3-C≡C-CH3.
- Symmetry: The molecule is symmetrical, and the dipoles cancel out, resulting in a zero dipole moment.

7. **Cis-1,2-dichloroethene**:
- Structure: Cl-CH=CH-H (with Cl atoms on the same side).
- Symmetry: The dipoles do not cancel out, resulting in a net dipole moment.

### Step 2: Count the Molecules with Zero Dipole Moment
From the analysis above, the molecules with zero dipole moments are:
1. Trans-but-2-ene
2. Trans-1,2-dichloroethene
3. But-2-yne

Thus, the total number of molecules with zero dipole moment is **3**.

### Final Answer:
The number of molecules having zero dipole moment is **3**.

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