Question

How many grams of water must decompose to produce 2 grams of hydrogen and 16 grams of oxygen

Answer 1

18gms of water will decompose into 2 gms of hydrogen and 16gms of oxygen according to the law of conservation of mass which states that matter can neither be created nor be destroyed.

Answer 2

It is a stoichiometry problem.

2H2O ------> 2H2 + O2

we have been given with grams of H2 and O2.

we can start the calculations either by H2 or O2. Let's start with H2..

2 g H2 x 1 mole H2/2 g H2 x 2 mole H2O/ 2mole H2 x 18g H2O/ 1 mole H2O

on doing the calculations we get 18 g H2O.

If you want to solve it in more simpler way then you need to remember the law of mass conservation.

mass of reactant = mass of products

mass of reactant = 2 g + 16 g = 18 g