Awesome! $$9 \, \mathrm{gm}$$ of Water splits into $$1 \, \mathrm{gm}$$ of Hydrogen. $$18 \, \mathrm{gm}$$ of Water is double $$9 \, \mathrm{gm}$$ of Water. So, we should get double the amount of Hydrogen, that is $$2 \, \mathrm{gm}$$. Similarly, we should get double the $$8 \, \mathrm{gm}$$ of Oxygen, that is, $$16 \, \mathrm{gm}$$.
In the previous Chapter, we discussed mixtures. We said that a cup of tea is a mixture of milk, tea leaves and sugar. Your father and your mother make one cup of tea each. You take $$10 \, \mathrm{gm}$$ of tea from each cup and split it into tea leaves, milk and sugar.