1. We need to count how many of the given molecules contain exactly two lone pairs of electrons in the whole molecule.

   Given species: $\mathrm{H_2O},\ \mathrm{N_2},\ \mathrm{CO},\ \mathrm{XeF_4},\ \mathrm{NH_3},\ \mathrm{NO},\ \mathrm{CO_2},\ \mathrm{F_2}$

2. Now check each molecule one by one.

(i) $\mathrm{H_2O}$

Oxygen has 6 valence electrons. In water, O forms two bonds with H and keeps: $2\text{ lone pairs}$ Total lone pairs in the molecule = 2.

So, $\mathrm{H_2O}$ qualifies.

(ii) $\mathrm{N_2}$

Structure: $\mathrm{:N\equiv N:}$ Each nitrogen has one lone pair. So total lone pairs in the molecule: $1+1=2$

So, $\mathrm{N_2}$ qualifies.

(iii) $\mathrm{CO}$

Structure is similar to: $\mathrm{:C\equiv O:}$ Carbon has one lone pair and oxygen has one lone pair. So total lone pairs: $1+1=2$

So, $\mathrm{CO}$ qualifies.

(iv) $\mathrm{XeF_4}$

Xenon has 8 valence electrons. In $\mathrm{XeF_4}$, Xe forms 4 bonds and retains 2 lone pairs. But each fluorine also has 3 lone pairs. So total lone pairs in the whole molecule are much more than 2: $2 + 4\times 3 = 14$

So, $\mathrm{XeF_4}$ does not qualify.

(v) $\mathrm{NH_3}$

Nitrogen forms 3 bonds and has: $1\text{ lone pair}$ Total lone pairs = 1.

So, $\mathrm{NH_3}$ does not qualify.

(vi) $\mathrm{NO}$

Total valence electrons: $5+6=11$ This is an odd-electron molecule. Its usual Lewis description has one unpaired electron and one lone pair on N, two lone pairs on O: Total lone pairs = 3.

So, $\mathrm{NO}$ does not qualify.

(vii) $\mathrm{CO_2}$

Structure: $\mathrm{O=C=O}$ Each oxygen has 2 lone pairs. So total lone pairs: $2+2=4$

So, $\mathrm{CO_2}$ does not qualify.

(viii) $\mathrm{F_2}$

Structure: $\mathrm{F-F}$ Each fluorine has 3 lone pairs. So total lone pairs: $3+3=6$

So, $\mathrm{F_2}$ does not qualify.

3. Therefore, the molecules having exactly two lone pairs in the entire molecule are: $\mathrm{H_2O},\ \mathrm{N_2},\ \mathrm{CO}$

So the number of such molecules is: $\boxed{3}$

4. Comparison with stored correct answer:

• Derived answer = 3
• Stored correct answer = 3
• They match.