Hydrogen Gas Reacts With Copper(II) Oxide To Form Copper Metal And Water Vapor According To The Reaction: CuP + H2 ---- Cu + H2O. What Mass Of Copper Is Produced If 255 L Of Hydrogen At STP Are Used Up In This Reaction?

according to your question

hyrogen=255litres

we know 1 mole= 22.4L 

1 mole Cu=53.5 grams

CuO    +     H2------Cu +      H20

1mole      1 mole           1 mole   1 mole

here

1 mole H2 reacts to produce 1 mole Cu

i.e 22.4 L of H2 reacts to produce 53.5 grams of Cu

then 

1L of H2 reacts to produce 53.5/22.4 grams of Cu

=>255L of H2 reacts to produce (53.5/22.4)*255 grams of Cu

                                              =609 grams of Cu

hope you got how to solve it