How many grams of water do you need to weigh if a reaction requires 5 moles of h2o

One mole of water weighs 18 grams. H₂O is composed of 2H= 2 and 1 0=16 adding gives you 18. number of moles= mass/ Relative Molecular Mass

Therefore, mass= Relative Molecular Mass×number of moles

                           = 18×5 moles

                           = 90 grams

if you mean hydrogen and oxygen then water is the product