Final answer:
The most likely explanation for the final pressure being 1.00 atm after adding 1.25 atm of gas is that the gas was in equilibrium, with the rate of vaporization and condensation balancing each other out.
Step-by-step explanation:
If you added 1.25 atm of gas to the flask and the final pressure was 1.00 atm, the most likely explanation is that the gas was in equilibrium. In thermodynamics, an equilibrium state means that the rate at which the gas molecules escape from the liquid to become gas is equal to the rate of gas molecules condensing back into the liquid. This dynamic process means that the vapor pressure of the gas above the liquid is constant, as the vaporization and condensation continue at the same rate. Hence, despite adding gas, the final pressure remains the same because some of the added gas has condensed, thus maintaining a state of equilibrium.