In the equation:2H2 + O2 → 2H2O100 molecules of hydrogen react with ........... molecules of oxygen.

Answer:
[tex]50\text{ molecules}[/tex]Explanation:
Here, we want to know the number of molecules of oxygen that was reacted with
From the question, we can see that the mole ratio is 2 to 1
Thus, the number of molecules of oxygen reacted will be half that of hydrogen
Hence, the number of molecules of oxygen will be:
[tex]\rightarrow\frac{100}{2}\text{ = 50 molecules}[/tex]