1. Identify the process

The gas expands isothermally into vacuum.

Expansion into vacuum is called free expansion.

2. Work done in free expansion

For expansion against external pressure:

$w = -P_{\text{ext}}\Delta V$

Since it is expansion into vacuum,

$P_{\text{ext}} = 0$

So,

$w = 0$

3. Use the first law of thermodynamics

$\Delta U = q + w$

For an ideal gas, internal energy depends only on temperature. Since the process is isothermal,

$\Delta U = 0$

Thus,

$0 = q + 0$

$q = 0$

4. Final answer

The amount of heat absorbed is

$\boxed{0\ \text{L atm}}$

Note: The stated volumes seem inconsistent for an expansion ($4.0\,\text{L}$ to $2.0\,\text{L}$ is compression), but this does not affect the conclusion for a free process of an ideal gas under isothermal condition: $q=0$.