Show Unsaturation answer
Rule 2, omit O, gives C5H10
5 - 10/2 + 1 = 1 degree of unsaturation.
Look for 1 pi bond or aliphatic ring.
Show IR answer
The band at 1727 indicates a carbonyl, probably an aldehyde; an aldehyde is also suggested by the band at 2719 which is likely the C-H stretch of the H-C=O group. The bands at 3000-2850 indicate C-H alkane stretches.
Show Structure answer
This is the structure. See if you can assign the peaks on your own.
Show NMR answer