Given the balanced equation representing a reaction: 2h2 + o2 ==> 2h2o
what is the mass of h2o produced when 10.0 grams of h2 react completely with 80.0 grams of o2?
(1) 70.0 g (2) 90.0 g (3) 180. g (4) 800. g

The answer is (2) 90.0 g. The reaction equation represents the ratio of mole number of every compounds. So we need to transfer the mass to mol first. There is 5 mol H2 and 5 mol O2. According to equation, we can know production is 5 mol, too. So the mass is 90 g.

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