A lovely piece. The original key is Eb major. Originally written for voice and piano. Franz Schubert (1797 - 1828) was an Austrian composer. He wrote some 600 Lieder, nine symphonies, liturgical music, operas, and a large body of chamber and solo piano music. He is particularly noted for his original melodic and harmonic writing. Watch a performance of this arrangement on YouTube by the duo TenStrings.
Tender treatment of Schubert's Litanei by TenStrings.