The number of moles of hydrogen that is needed to produce 0.253 mol of water is ...

The chemical formula of water is written as H2O we can just use the ratio of the atoms since we are not given the reaction. Hydrogen is a diatomic substance so that it exists as H2. We calculate as follows:

0.253 mol H2O (1 mol H2 / 1 mol H2O) = 0.253 H2 needed

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