Track 10 on the standard edition of Unorthodox Jukebox is "If I Knew." It closes the album's original ten-song sequence, following "Money Make Her Smile," and gives the record a softer, more retro finish after its more dance-oriented material.
The song is a Sam Cooke-inspired ballad built around soul and doo-wop influences. Its lyrics focus on regret, fitting the reflective role of a closing track. That placement is why some descriptions call it the album's final song rather than simply its tenth track.
The main source of confusion is the deluxe edition. That later edition added bonus material, including "Old & Crazy," a duet with Esperanza Spalding, so its expanded running order can make the standard album's structure less familiar. The question specifically limits the answer to the standard edition.
Other choices are songs from earlier positions: "Show Me" is track 8, "Money Make Her Smile" is track 9, and "Moonshine" appears much earlier in the sequence.