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N2 + 3H2 --> 2NH3

how many grams of hydrogen are needed to react with 50 grams of nitrogen?

Three moles of H2 per one mole of N2. How many moles of N2 is 50 grams?

1.79 moles is 50 grams.

You do these problems in steps.
Step 1. Write a balanced chemical equation. You have that.

N2 + 3H2==> 2NH3

Step 2. Convert grams of what you have to mols remembering that mols = g/molar mass. mols N2 = 50/28 = 1.79.

Step 3. Convert mols of what you have (in this case N2) to mols of what you want (in this case H2) using the balanced equation from step 1. The coefficients do the trick if you make the units cancel.
mols H2 = 1.79 mols N2 x [3 mols H2/1 mol N2] = ?? mols H2.
Note that mols N2 cancel leaving you with mols H2 (and mols H2 is what you want)
Step 4. Now convert mols of what you want (from step 3) into grams using the formula from step 2 but in reverse; i.e., mols = g/molar mass and solving for g = mols x molar mass. You have mols and you use the molar mass of H2 which is 2.
g H2 = mols H2 x molar mass H2 = xx g H2 or, to watch the units;
g H2 = mols H2 x [2g H2/1 mol H2] = xx g H2.

Check my work. Check the numbers. I rounded to use whole numbers for molar masses. I hope this helps.

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