9H-xanthine

9H-xanthine is a neutral tautomer of xanthine in which the indicated hydrogen is attached to nitrogen9 of the purine framework. The oxo groups remain at positions2 and6. ChEBI identifies this particular form as CHEBI:17712, distinct from the broader tautomer-unspecified xanthine entry.^[ref]

Its molecular formula is ${\mathrm{C}}_5{\mathrm{H}}_4{\mathrm{N}}_4{\mathrm{O}}_2$. The prefix 9H is an atom-location convention, not a count of nine hydrogens: the entire neutral molecule contains four hydrogens. As a simple inventory check, it contains $5+4+4+2=15$ atoms in all.

7H-xanthine has the same elemental formula and net charge but a different indicated hydrogen position. A mass measurement consistent with that formula therefore does not alone assign the 9H form. The distinction concerns tautomerism, rather than the gain of an extra proton or a change in carbon/oxygen composition.^[ref]

Naming 9H-xanthine specifies a structural form; it does not by itself establish its fraction in a solution. That requires evidence or a model for the relevant environment and equilibrium.