Ah, I do so love Mozart. This piece, as stated, begins in A-major. The piece is in rounded binary form, although this piece is far from being a nice simple, symmetrical work. The A section in particular lacks a consistant periodic feel. Each of what I figure to be two parts are of different lengths. The first one feels to me like a contrasting period last 10 bars, and definately contrasts, with a half cadence after 4 bars, followed by a change of texture and no repetition of melody anywhere to be found. The second half of this phrase is 6 measures and ends on a nice PAC. This seems like a nice place to end our A section, but Mozart has other plans. Afetr the PAC we're greeted by what I considered to be a transitional section, as Mozart plays around for 8 bars, using them to modulate to E-Major through a series of virtuosic runs and sequences. This lacks the definition of a phrase group, since it's a cadenza-esque passage. The ending of this section does end in a different key, making the overall passage continuous. We repeat the section to convince our ears that we've modulated.
Now the B section feels relatively straightforward after our meandering A section. This B material is still mostly transitional, as we begin we quickly move to the parallel minor (e-minor) and continue to sequence through this minor key for 12 bars, ending on what seems to be a PAC in e-minor, though we quickly return to our A section, with it's initial 10 bar phrase, complete with large block-chord entrance (which really hammers home "HEY WE'RE IN THE A SECTION AGAIN"). The entire section is repeated, including our cadenza passage, this time ending in our original key, capped off by a PAC. This entire section is again repeated, adding a finality to the PAC in A-major.