How many grams of O2 are produced when 0.065 mol of water decompose ?

The answer is 0.52 grams.

2H2O->2H2+O2

So, we can say that,

2 moles of water= 16 gm of O2

Then

0.065 moles of water=16/2*0.065 gm

                                   =0.52 gm

Hope you understood:)