How to find molarity of a fertilizer?
You probably mean 50kgN/ha, which sounds like a plausible number for fertilizer application.

I don't know what you are doing exactly, but you need to know how much surface area your soil sample represents. If you do planting tests, it will be simply the surface of your bed.

Then it's just mulitplying the surface area by 50kgN/ha (mind the units!). Now you know how much N you need.

If you buy fertilizer, the fertilizer value in kg/t, kg/m³ or some similar measure will be given for the important macronutrients NPK. Else you need to know how much N is in Ammonia per weight but I think that is the easy part.

Disclaimer: I've never done any soil science, but I worked with farmers for a while and was in a field where fertilizer etc. matters.