using the equation of formation of water:
H2+O2---->H2O
Balancing the equation,
2H2+O2---->2H2O
So now use the ratio from the balance equation, we know that 2 mol of H2 are producing 2 mol of H2O so the ratio will be 1:1.
So as 2 mol of H2O are produced by 2 mol of H2.
That implies 1 mol of H2O are produced by 1 mol of H2.
That is 0.253 mol of H2O are produced by 0.253 mol of H2.