[AWK]

It doesn't matter if it's a solution (as Massimo wrote in the first post) or a solid (as he wrote in the second post, though, he didn't write that this is a correction). In both cases, the initial stage of calculations is identical. Massimo calculated the task as You and MNIO suggested and came to the conclusion that it gives the wrong result. The resulting atomic mass does not reasonably correspond to any of the known mono-, bi- or trivalent metals. So the only possibility is a salt hydrate (or salt solution) - compound crystallized from water, which means the question is:
Hydrate (solution) of which metal chloride may contain 38.7% chlorine. Such hydrate exists and contains exactly 38.74% Cl, which gives less than 0.1% error.

[Borek]

The resulting atomic mass does not reasonably correspond to any of the known mono-, bi- or trivalent metals.

Sure it does.

[AWK]

A potential compound possibly matching the problem also forms hydrates, and Massimo never wrote that the sample was dried to constant weight.
The problem results from the imprecision of the question or the ambiguity of the translation.

[DrCMS]

@AWK You are trying to apply actual real world knowledge and experience to a question that as actually fully given is solvable without it.  I agree that it would not work like the question suggests in reality but so many homework questions fail that test.  If Massimo had not asked an incorrect and incomplete question to begin with and then rounded up badly later it would have been much easier to guide them to the "correct" answer needed.