Concentration is usually expressed in four different ways.
Here the concentration is expressed as mass of the substance dissolved divided by the volume of the solution.
The mass concentration here is 1 g/l or 1 kg/m^3.
This is the number of moles of the solute in 1 liter of the solution.
The molar mass of iron is approximately 56. 1 g = 1/56 = 0.0178 mole
The molar concentration is 0.0178 mole/liter
This is determined from the number of particles of iron in 1 g of iron divided by the volume that is 1/1000 m^3.
0.0178 mole has 0.0178* 6.0221415* 10^23 = 0.1072*10^23 particles. The number concentration is 0.1072*10^26 /m^3.
This would not apply here as iron is a solid.