Question

What is the correct way to write the product of H+ and (OH)2 in a molecular equation? Is it H3O2 or H(OH)2?

Answer 1

\[{\rm H}_3{\rm O}_{2}^{-}\]
That will do fine. You're not generally expected to attempt to imply the structure in a small molecule, since it can be worked out by anyone with a basic understanding of bonding and structure.