What mass of water is formed by the interaction of 1.00 g of hydrogen and 4.00 g of oxygen?

Let’s write the reaction equation: 2H2 + O2 = 2H2O
Let’s find the amount of substances:
(O2): 4/32 = 0.125
(H2): 1/2 = 0.5
According to the reaction equation, we see that 0.125 mol of oxygen will require 0.25 mol of hydrogen. From here we see that there is an excess of hydrogen, the calculation is based on the lack.
Weight of water: 0.25 * 18 = 4.5 (g)