José Rodríguez Alvira
Measures 19 and 20 are exactly the same as measures 5 - 6. They are a recapitulation before a coda to end the prelude.
In measure 21, Chopin starts an ascending progression that goes to B minor, C# minor and finally G minor. In measure 22, the V7 of G minor is used as a German augmented sixth chord of F# minor to prepare a coda in this key:
Notice the use of the Neapolitan sixth chords in measure 24:
Measures 27 and 28 are repeated in F# major before a final cadence that uses another Neapolitan sixth chord: