According to JMG, who got it from Billboard, Adele's tear-jerker Someone Like You is the first purely vocal and piano single to reach #1 in the 53 year history of Billboard's pop singles chart. Many similarly sparse ballads also reached #1, but according to Billboard, all had other minor accompaniment.
Well of course it's go Number 1.
It's freakin' Adele.
PS Forgive the commercial.