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Sorry, I have to give up. I think its the quadratic thing that I don't get.

@ace , the only mistake which you made was that you used the mass of a single N atom instead of the mass of N2.
I feel a little bit sorry that you didn't win the prize because you were the first one with a nearly correct solution. At least I upvoted all your comments with 100 % as compensation. :-)

Understood. I'm off because of N2. All for the fun anyway so it's ok. That why i have the details so someone could figure out the right answer and enlighten me. So, my maths is still good . My chem still no good. :-)

Thanks. I think I took into account of those.
Will relook a bit.