Standard pressure and temperature referred to for gases are 1 atmosphere and 273 K or 101.325 Pa and 273K .
18 kg N2 IS : 18 kg / 28 kgmol ( molecular weight of N2 ) = 0.642857142 moles of N2.
At STP, N2 gas of 1 mole would occupy 22.4 liters volume = 22.4 / 1000 liters / m3 = 0.0224 m3 , so 0.6428571moles of N2 would occupy : 0.0224 m3 / 1 mole x 0.0642657 moles =1.43955168 x 10^-3 m3.
With the addition of 10 kg addition into a fixed volume of a storage tank(at the same temperature of 273 k , the number of moles of N2 is 28 kg N2 / 28 kg N2/mole = 1 mole of N2.
Thus number of moles of N2 increase would change the final pressure of the storage tank which can be found from ideal gas law at two different conditions ; p1v1/ T1 = p2v2 / T2 , where p1 = 101.325 Pa , T1= T2 = 273K , v1 = v2 = 1.43955168x 10^-3 m3 , n1 = 0.642857142 and n2 = 1 , so P2 = 1O1.325 Pa x 1/ 0.644442857142 = 157.2288355 Pa