In the important industrial process for producing ammonia (the Haber Process), the overall reaction is:

N2(g) + 3H2(g) 2NH3(g) + 24,000 calories

A yield of ammonia, NH3 of approximately 98% can be obtained at 200o C and 1000 atmospheres of pressure.

How many grams of N2 must react to form 1.7 grams of ammonia?

First, you have to see that the ammonia will have a mass of 1.7g, and that represents 98% of the ammonia produced from a fixed amount of Nitrogen.

Find the number of moles of ammonia you have in 1.7g. (let that be x moles)

The molar ratio of nitrogen to ammonia is 1:2. So, you'd have 0.5*x to have the number of moles of nitrogen if the yield was 100%. Since x represents the 98% formed, find 100%. From this new value, multiply by 0.5 to have the number of moles of Nitrogen required. Find the mass of nitrogen corresponding to that number of moles.

Hope it helped! :)