There are two ions in ferric carbonate salt: ferric ion or Fe(III) ion and carbonate ion.
Given volume of salt = 5 lt
And concentration of salt = 1.25 M
Concentration is given as moles per liter or M. In ferric carbonate, there are 2 moles for every mole of salt. That is, 1.25 M salt contains 2 x 1.25 = 2.5 M ferric ion.
Similarly, each mole of salt contains 3 moles of carbonate ion. Thus, 1.25 M salt contains 3 x 1.25 M = 3.75 M carbonate ions.
Remember that concentration is not a function of how many liters of salt there is, that is, it is independent of the quantity of salt. 1 ml of salt can have the same concentration as 10 lt of it.
We can calculate the moles of each ion using the given volume. For 5 lt of salt containing 2.5 M ferric ion, there are 5 x 2.5 moles = 12.5 moles of ferric ion. Similarly, there are 18.75 moles of carbonate ions.
Hope this helps.