C6H5NO2+? = C6H5N2Cl

I doubt this conversion can be done in a single step.

Here is a three-step procedure which might work. If one of the other tutors has a better idea, I am sure it would be welcome:
Reduce the nitrobenzene to aniline with Fe/HCl, then treat the aniline with H2SO4/HNO2 to get the phenyl diazonium cation. Finally, add HCl to get the phenyl diazonium chloride salt.

Solution give me

To balance the chemical equation C6H5NO2 + ? = C6H5N2Cl, we need to determine the missing reactant.

First, let's identify the elements and their respective atoms in each molecule:

C6H5NO2: 6 carbon (C) atoms, 6 hydrogen (H) atoms, 1 nitrogen (N) atom, 2 oxygen (O) atoms.
C6H5N2Cl: 6 carbon (C) atoms, 5 hydrogen (H) atoms, 2 nitrogen (N) atoms, 1 chlorine (Cl) atom.

Comparing the number of atoms for each element on both sides of the equation, we can see that the number of carbon (C) and hydrogen (H) atoms are already balanced. However, the number of nitrogen (N) atoms is 1 on the left side and 2 on the right side.

To balance the nitrogen atoms, we need to add a molecule or compound that contains nitrogen atoms. One possible compound is HNO2, which has 1 nitrogen atom.

Therefore, the balanced chemical equation would be:

C6H5NO2 + HNO2 = C6H5N2Cl

Please note that there may be other compounds that can also balance the equation. The choice of the missing reactant may depend on the specific reaction being considered or the conditions in which the reaction occurs.