As there are two nitrogen atoms in N2O, so, in 44 (1 mole) of N2O there are 2 moles of Nitrogen ( = 28 g). So, according to this reference,
44 g of N2O = 1 mole
0.173 g = x
solving for x,
x= 1x0.173/44
x=0.0039 mole
as 1 mole contains 2 moles of N,
so, 0.0039 mole will contain,
=2x0.0039/1
=0.0078
So, 0.173 g will contain 0.0078 moles of N