In one H₂O molecule, there are two hydrogen atoms along with one oxygen atom. So for every 1 oxygen atom (of which there are 2 of in an oxygen gas molecule) there needs to be 2 hydrogen atoms per oxygen( which in the equation of 2H2 +O2–> 2H2, that 2 H2 is broken down into 4 hydrogen atoms which means 2 hydrogen atoms have to bond to 1 oxygen atom and with 4 hydrogen atoms, there will be 2 atoms of oxygen which creates 2 molecules of water.
So to solve the question,
10.5g/2=5.25g of oxygen required to fully react with hydrogen to make water