# How much oxygen gas can I make from a 9*kg mass of water?

${H}_{2} O \left(g\right) \rightarrow \frac{1}{2} {O}_{2} \left(g\right) + {H}_{2} \left(g\right)$
You have $90 \times {10}^{3} \cdot g$ of water. This represents a MOLAR quantity of (90xx10^3*g)/(18*g*mol^-1)=??mol.
The number of moles of oxygen gas, ${O}_{2}$, is half this molar quantity by the stoichiometric equation.
And of course each mole of oxygen gas has a mass of $32 \cdot g$. From where am I getting these masses? Do you have to learn them?