The molecular formula of water is

1. Two hydrogen atoms combine with one oxygen atom to form water. 2. Writing element symbols with their counts gives H2O. 3. HO2 and OH are not the right formula. 4. H2O2 is hydrogen peroxide, a different compound. _Source: Selina Concise Chemistry Class 9, Ch 4 The Language of Chemistry (Mistral OCR'd PDF), section on writing formulae_