(1) ammonium nitrate contains nitrogen and belongs to nitrogen fertilizer;

Each bag of fertilizer contains at least 50kg× 34.3% =17.15kg.

So the answer is: nitrogen; 17. 15; ?

(2) The relative molecular weight of ammonium nitrate is14× 2+14+16× 3 = 80;

The mass fraction of nitrogen in NH 4 NO 3 is 28.80× 65.438+0.00% = 35% > 34.3%; Therefore, the advertisement is a real advertisement.

So the answer is: 80; That's right.