I read 3.5 into 10 raise to minus 3 kg to be 3.5 x 10^-3 kg. See how easy that is?
Molar mass N2 = 28g/mol
3.5*10^-3kg = (3.5*10^-3) *1000 = 3.5g
Mol N2 in 3.5g = 3.5g / 28g/mol = 0.125 mol N2
1 mol N2 at STP has volume = 22.4L
0.125 mol N2 at STP has volume = 0.125mol x 22.4L/mol = 2.8L
Volume = 28 litres.
