We have to assume the gas behaves similar to an ideal gas and we can use the ideal gas law equation
PV = nRT
Where
P - pressure
V - volume - 7.5 L
n - number of moles - 1.0 mol
R - universal gas constant - 0.0821 L.atm/mol.K
T - temperature in kelvin
20 degrees celcius + 273 = 293 K
Substituting the values in the equation
P x 7.5 L = 1.0 mol x 0.0821 L.atm/mol.K x 293 K
P = 3.2 atm
The pressure exerted is 3.2 atm