pre-1982 definition STP: 12.0 m^3
post-1982 definition STP: 12.2 m^3
First, determine the number of moles of nitrogen gas you have.
Atomic weight nitrogen = 14.0067
Molar mass N2 = 2 * 14.0067 = 28.0134
Moles N2 = 15000 g / 28.0134 g/mol = 535.4580308 moles
Now the answer to this question is a bit ambiguous because the definition of STP changed in 1982 and a lot of text books are still using the 35 year obsolete definition of STP. The two definitions are:
pre-1982: 273.15K @ 1 atm (101325 Pascals).
post-1982: 273.15K @ 100,000 Pascals.
Because of the differences in pressure, the volume for 1 mole of an ideal gas is either 22.414 liters/mol or 22.71098 liters/mol, for the old and new standard respectively.
To get the volume of the tank, just multiply the number of moles by the molar volume. So
pre-1982: 535.4580308 mol * 22.414 l/mol = 12001.76 liters
post-1982: 535.4580308 mol * 22.71098 l/mol = 12160.78 liters
Since 12000+ liters is a rather inconvenient size, divide by 1000 to get m^3 and round to 3 significant figures. So:
pre-1982: 12.0 m^3
post-1982: 12.2 m^3
