What mass of hydrogen is needed to react with 40g of copper oxide CuO plus H2-- Cu plus H20?

To find the mass of hydrogen needed to react with 40g of copper oxide (CuO), first calculate the molar mass of CuO. Then, use stoichiometry to determine the moles of CuO present in 40g. From the balanced chemical equation CuO + H2 -> Cu + H2O, you can determine the mole ratio between CuO and H2. Finally, use the molar mass of hydrogen to calculate the mass of hydrogen needed to react.