1. Condition for zero dipole moment

A molecule has zero dipole moment when the vector sum of all bond dipoles is zero. This usually happens when:

• the molecule is symmetrical, and
• all surrounding atoms are identical so dipoles cancel.

2. Check each compound

We examine the geometry of each molecule:

(i) $\mathrm{HF}$

• Diatomic molecule with different atoms.
• Bond is polar, so dipole moment is non-zero.

(ii) $\mathrm{H_2}$

• Diatomic molecule with same atoms.
• No bond polarity.
• Dipole moment = 0.

(iii) $\mathrm{H_2S}$

• Bent shape like water.
• Bond dipoles do not cancel.
• Dipole moment is non-zero.

(iv) $\mathrm{CO_2}$

• Linear molecule: $\mathrm{O=C=O}$
• Two equal and opposite bond dipoles cancel.
• Dipole moment = 0.

(v) $\mathrm{NH_3}$

• Trigonal pyramidal.
• Dipoles do not cancel.
• Dipole moment is non-zero.

(vi) $\mathrm{BF_3}$

• Trigonal planar and symmetrical.
• Three $\mathrm{B-F}$ dipoles cancel.
• Dipole moment = 0.

(vii) $\mathrm{CH_4}$

• Tetrahedral and symmetrical.
• Four identical $\mathrm{C-H}$ bond dipoles cancel.
• Dipole moment = 0.

(viii) $\mathrm{CHCl_3}$

• Tetrahedral, but substituents are not identical ($3\,\mathrm{Cl}$ and $1\,\mathrm{H}$).
• Dipoles do not cancel.
• Dipole moment is non-zero.

(ix) $\mathrm{SiF_4}$

• Tetrahedral and symmetrical.
• Four identical $\mathrm{Si-F}$ dipoles cancel.
• Dipole moment = 0.

(x) $\mathrm{H_2O}$

• Bent molecule.
• Dipoles do not cancel.
• Dipole moment is non-zero.

(xi) $\mathrm{BeF_2}$

• Linear molecule.
• Two equal and opposite $\mathrm{Be-F}$ dipoles cancel.
• Dipole moment = 0.

3. List of molecules with zero dipole moment

These are:

• $\mathrm{H_2}$
• $\mathrm{CO_2}$
• $\mathrm{BF_3}$
• $\mathrm{CH_4}$
• $\mathrm{SiF_4}$
• $\mathrm{BeF_2}$

So total number is

$6$

4. Comparison with stored answer

Stored correct answer = $6$.

Our derived answer also = $6$. So they agree.