That is the case because you don't know the identity of the solute, and consequently can't use its molar mass to figure out how many moles you have in that sample.
However, the concentration of the initial solution is not important here.
The important thing to keep in mind here is that the
You then dilute the aliquot to a total volume of
This factor is called the dilution factor
#color(blue)(|bar(ul(color(white)(a/a)"D.F." = V_"diluted"/V_"concentrated" = c_"concentrated"/c_"diluted"color(white)(a/a)|)))#
In your case, the dilution factor is equal to
#"D.F." = (100 color(red)(cancel(color(black)("mL"))))/(5color(red)(cancel(color(black)("mL")))) = 20#
This means that the concentration of the resulting solution, i.e. the diluted solution, will be
#c_"diluted" = 1/20 * c_"concentrated"#