The following are bars 34 and 35 of Chopin's Waltz in A minor (op. posth.)
If I understand the notation correctly, the mordent in bar 34 should be played C#, D, C#. Why is it denoted differently than the appoggiatura in bar 35? Initially I thought that it's about the F# which is out of the key and there would be no way to denote it. But there's also bar 4 of the same piece:
In this case both notes are in the key so why is there an appoggiatura and not a mordent? Are both of them to be played in the same manner?