Density = mass/volume = 2000/4000 = 0.5 grams/cm3. Hope this hopes!
<u>Answer:</u> The temperature of the system will be 1622 K
<u>Explanation:</u>
The equation relating the pre-exponential factor and activation energy follows:

where,
D = diffusion coefficient = 
= pre-exponential constant = 
= activation energy of iron in cobalt = 273,300 J/mol
R = Gas constant = 8.314 J/mol.K
T = temperature = ?
Putting values in above equation, we get:

Hence, the temperature of the system will be 1622 K