Given:
- Initial conditions:
- Volume of gas (V1) = ?
- Temperature (T) = 300 K
- Pressure (P1) = 20 atm
- Number of moles of gas (n) = ?
- Final conditions:
- External pressure (Pext) = 1 atm

To Find:
- Work done by the gas (W)

Formula:
The work done by a gas during an isothermal expansion can be calculated using the following formula:

W = -nRT ln(V2/V1)

where:
- W is the work done by the gas
- n is the number of moles of the gas
- R is the gas constant (8.314 J/(mol·K))
- T is the temperature in Kelvin
- V1 and V2 are the initial and final volumes of the gas, respectively.

Solution:

Step 1: Calculate the initial volume of the gas (V1)
To calculate the initial volume (V1), we can use the ideal gas law equation:

PV = nRT

Rearranging the equation, we get:

V1 = (nRT) / P1

Step 2: Calculate the final volume of the gas (V2)
Since the process is an isothermal expansion, the final temperature (T2) will also be 300 K. We can use the ideal gas law equation to calculate the final volume (V2):

V2 = (nRT) / Pext

Step 3: Calculate the work done by the gas (W)
Using the formula mentioned above:

W = -nRT ln(V2/V1)

Substituting the values:

W = -nRT ln((nRT/Pext) / (nRT/P1))

Simplifying the equation:

W = -nRT ln(P1/Pext)

Step 4: Calculate the number of moles of gas (n)
To calculate the number of moles (n), we can use the formula:

n = mass / molar mass

Given that the mass of N2 gas is 2.8 g and the molar mass of N2 is approximately 28 g/mol:

n = 2.8 g / 28 g/mol
n = 0.1 mol

Step 5: Calculate the work done by the gas (W)
Substituting the values:

W = - (0.1 mol) * (8.314 J/(mol·K)) * (300 K) * ln(20/1)
W = - 249.42 J ln(20)

Final Answer:
The work done by the gas is approximately -249.42 J ln(20).