The final volume of the gas will be 2.58 L.
We can use Charles' law to calculate the final volume of the gas. Charles' law states that the volume of a fixed mass of an ideal gas is directly proportional to its absolute temperature, when pressure is constant.
In this case, the initial volume of the gas is 3.00 L, the initial temperature is 80 C, and the final temperature is 30 C. We can convert the initial and final temperatures to Kelvin by adding 273.15 to each temperature. This gives us an initial temperature of 353.15 K and a final temperature of 303.15 K.
We can now use Charles' law to calculate the final volume of the gas:
V2 = V1 * (T2 / T1)
V2 = 3.00 L * (303.15 K / 353.15 K)
V2 = 2.58 L
Therefore, the final volume of the gas will be 2.58 L.