Well friends i have something for you over here. You see when we write the Ksp of a compound such as PbS for example we say its Ksp is about 10^(-40), but practically speaking when i have a concentation of the order of 10^(-10) In a litre flask for the sulphide ion[ which can be easily attained] ,I need to have a concentration of 10^(-30) for the lead ion in the same litre but practically speaking the lowest concentration for a litre of any solution is when 1 atom of it is dissolved in a litre i.e 1/N where N is the avagadro number. so how do i acheve it .