What is the temperature of 1.2 moles of helium gas at 2.57 ATM if it occupies 15.5L of volume

We will assume helium to behave as an ideal gas and apply the ideal gas law:

PV = nRT

For pressure measured in atmospheres and volume measured in liters, the value of the molar gas constant is 0.082. Therefore:

T = PV / nR

T = (2.57 x 15.5) / (1.2 x 0.082)

T = 404.8 Kelvin