## Chemistry: The Central Science (13th Edition)

(NH$_{4}$)$_{2}$HPO$_{4}$ has 9 hydrogen atoms because NH$_{4}$ has 4 hydrogen molecules and there are 2 NH$_{4}$ molecules in this compound, so 2$\times$4=8. There is also 1 hydrogen atom in HPO$_{4}$, so 8+1=9.