Determine the pressure of the gas, which, expanding isothermally by 1.5 liters, performed work of 4.5 MJ.

We translate the values from given to the SI system:
∆V = 1.5 l = 0.0015 m³.
A = 4.5 MJ = 4.5 * 10 ^ 6 J.
The first law of thermodynamics:
The amount of heat transferred to the gas is equal to the sum of the change in the internal energy of the gas and the work done by the gas:
Q = ∆U + A
In an isothermal process ∆U = 0, Q = A
The work done by the gas during expansion is equal to:
A = p * ∆V
Let us express the pressure from this expression:
p = A / ∆V
Substitute the numerical values and determine the gas pressure:
p = A / ∆V = 4.5 * 10 ^ 6 / 0.0015 = 3 * 10 ^ 9 Pa.
Answer: the gas pressure was 3 * 10 ^ 9 Pa.