What this excerpt lacks in length, it certainly makes up for in rapid shifts between chords within the key and modal mixture chords. In the beginning of measure 25, Mozart establishes a strong tonic feeling with an arpeggiated I chord functioning as an anacrusis to measure 26. In measure 26, Mozart begins immediately with a diminished I chord with a D# root. The harmonic rhythm occurs with each eighth note, and therefore the second chord is a ii chord, the root being an E. One of the most surprising mixture chords is apparent in the third chord of measure 26--the second beat of the measure. In this I chord is a flat 7, creating an apparent "minor" sound within the major key. The subsequent chords of measure 26 are as follows:
4th chord: ii6
5th chord: vii0
6th chord: I
Mozart re-establishes the tonic at the end of the measure because in the beginning of the next measure, another mixture chord presents itself. This chord contains both a flat 6 as well as a sharp 7, creating an interesting mix for the iv chord. This chord, because of the mixture of the sharps and flats, seems even more modal and out of the ordinary than the previous modal mixture chord. This chord is followed in the rest of the measure by:
2nd, 3rd, 4th chords: I (re-establishment of tonic)
5th and 6th chords: I 65
The next measure brings with it the last modal mixture chord, placed at the beginning of the measure. This chord is a iv chord with a flat 6, and this chord continues into the next eighth with an addition of an A in the soprano sixteenth notes.
Mozart sets up a strong tonal expectation for the listener through the use of the tonic establishments in both measures 25 and the second half of measure 26. The strong use of the I chord makes the mixture chords quite surprising to the listener. It is interesting that Mozart placed the mixture chord on the weak beat of measure 26 and one of the strong beats of measure 27. Perhaps this is used to create a sense of rhythmic unsettledness--naturally, the listener would normally expect to hear the strong beats emphasized and would therefore be "startled" by this use of mixture chords.