hydrogen gas reacts with oxygen gas to produce water if a student fills a 250-mL flask with hydrogen at STP, what volume of oxygen is necessary for the reaction?

Answer 1

You need to find the caps key on your computer and use it. While you are at it, find the period also.
2H2 + O2 ==> 2H2O

When working with gases one can use volume interchangeably as moles. Therefore, If you have 250 mL H2 you will need 250 x 1 mol (O2/2 mols H2) = 125 mL O2 at the same T and P.