The internal energy of a gas is given by the equation: U = (3/2) * n * R * T, where n is the number of moles, R is the gas constant, and T is the temperature in Kelvin. For diatomic nitrogen gas (N2), the gas constant is 8.314 J/(molK). Plugging in the values: U = (3/2) * 4.00 mol * 8.314 J/(molK) * 455 K = 55957.76 J.