Balance: Fe + H2O -> Fe3O4 + H2. Coefficient of H2O is

1. The product Fe3O4 needs 3 Fe atoms on the left. 2. Its 4 oxygens come from 4 H2O molecules. 3. Those 4 H2O give 8 H, which become 4 H2 on the right. 4. So coefficient of H2O is 4. _Source: Selina Concise Chemistry Class 9, Ch 4 The Language of Chemistry (Mistral OCR'd PDF), section on balancing equations_