B. What mass of water can be made if 6.00 g of hydrogen reacts with 40.00 g of oxygen?

Question

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Answer 1

2 H2 + O2 ==> 2 H2O

40g of oxygen is 40/32 = 1.25 moles of oxygen gas

6g of oxygen is 6/2 = 3 moles of hydrogen gas

Thus oxygen is your limiting reagent.  The maximum amount of water you could form, then, would be 2 * 1.25 moles = 2.5 moles of water.

Molecular weight of water ≈ 18 g/mole, thus you could make

2.5 moles * 18 g/mole = 45 grams of water

Answer 2

22.525g of water.  you have excess hydrogen and 2.5 mols of oxygen so divide mols of o2 by the stoichometric coefficent and multiply by the molecular weight of water, 18.02. and it gives the you the mass of water yeilded by this reaction.