Caffeine contains 28.9% by mass of nitrogen . If molecular mass of caffeine is 194, then the number of nitrogen atoms present in one molecule of caffeine is ___?

100 g of caffeine contains 28.9 gm of N

194 g of caffeine (194/100) × 28.9 = 56.06 g of N 

The number of N atoms present in one molecule of caffeine = 56.06/14 = 4.