We can use the ideal gas law to determine the volume occupied by 2.50 mole of hydrogen at -20°C and 1.5 atm. The ideal gas law is given by:

PV = nRT

Where:

P is the pressure in atmospheres (atm)

V is the volume in liters (L)

n is the number of moles of gas (mol)

R is the universal gas constant (0.08206 L atm / mol K)

T is the temperature in Kelvin (K)

First, we need to convert the temperature from °C to K. We can do this by adding 273.15 to the °C temperature:

T = -20°C + 273.15 = 253.15 K

Now we can plug in the given values into the ideal gas law equation and solve for V:

V = nRT / P = (2.50 mol)(0.08206 L atm / mol K)(253.15 K) / (1.5 atm) = 42.6 L

Therefore, 2.50 mole of hydrogen will occupy a volume of 42.6 liters at -20°C and 1.5 atm.