Can we write "1-Chloro-ethane" when a single chloro group is present in the ethane? I think only one position is possible for the chloro group in ethane.
In general nomenclature, the practice of omitting locants when there is no ambiguity is widespread. For preferred IUPAC names, however, the current recommendations given in Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) are prescriptive about when locants are omitted. In particular:
P-18.104.22.168 The locant ‘1’ is omitted:
(b) in monosubstituted homogeneous chains consisting of only two identical atoms;
Therefore, the preferred IUPAC name is indeed chloroethane and not the (correct, but not preferred) complete systematic name ‘1-chloroethane’.
(Nevertheless, note the correct capitalization and hyphenation of the names.)
The position can be omitted because there indeed is only one possible position for the chloro group to be. Therefore the name Chloroethane is unambiguous.
It is however not wrong to specify the position because the chloro group is in fact in position 1. Therefore it's redundant but correct.