How many grams of hydrogen are necessary to react completely with 50.0 grams of nitrogen?

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You should have included a reaction equation or some product so we can tell the balanced coefficients of nitrogen and hydrogen.  Assuming you are producing ammonia, 3 moles of hydrogen are required for each one mole of nitrogen.  These gases are diatomic. 50g Nitrogen / 28 g Nitrogen per mole = 1.786 moles of Nitrogen, therefore, 5.358 moles of Hydrogen gas.  Thus, you need  5.358 moles times 2.0 g per mole = 10.7 g of Hydrogen