How many grams of water are made from the reaction of 4.0 grams of hydrogen gas?

N H2 = mH2 / MH2
= 4.0g / 2gmol-1
= 2 mol
therefore n H2O = 1 mol

mH2O = nH2O * MH2O
= 1mol * 18 gmol-1
=18 g

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