Piano Concerto no. 2 in c minor, op. 18: II. Adagio sostenuto
by Сергей Васильевич Рахманинов
is most likely in the Key of D#/Eb Major
Notes in D#/Eb Major:
A#/Bb, C, D, D#/Eb, F, G, G#/Ab
Chords in D#/Eb Major:
Eb, Fm, Gm, Ab, Bb, Cm, Ddim
This key can also be known as C minor.
When it's used as a minor key, the minor key note is stressed more than the major key note. The same notes and same chords still apply.