No. of moles of NH3 = 1 / 17
No. of molecules of NH3 = (1/17) x NA
No. of N atoms in 1 molecule of NH3 = 1
No. of N atoms in (1/17) x NA molecules of NH3 = (1/17) x NA x 1
No. of moles of N2 = 1 / 28
No. of molecules of N2 = (1/28) x NA
No. of N atoms in 1 molecule of N2 = 2
No. of N atoms in (1/28) x NA molecules of NH3 = (1/28) x NA x 2
∴ Desired ratio = 14 : 17