What are possible structures for an organic molecule whose mass spectrum gives #m"/"z=128, 85, 71, 57, 43#?

What is the melting point of the hydrocarbon? What are the melting points of its derivatives? You will not identify an unknown organic compound with only a mass spectrum.

You know that the compound is a hydrocarbon.

Let's assume that it's an alkane. Then its molecular formula is #C_nH_(2n+2)#.

Its molecular mass is

#12n + 2n+2 = 128#

#14n = 126#

#n=9#

So the molecular formula is #"C"_9"H"_20#.

The base peak at #"m/z" = 43# corresponds to a 3-carbon fragment, almost certainly an isopropyl group, because that is the more stable cation (2°).

The other fragments must be less stable (1°?).

The peaks at #"m/z" = 57, 71#, and #85# correspond to radical cations with formulas #"C"_4"H"_9#, #"C"_5"H"_11#, and #"C"_6"H"_13#.

A possible structure is 2-methyloctane

If the molecule cleaves between #"C-2"# and #"C-3"#,

we get an isopropyl fragment (#43#) as the base peak and a less stable 1° 6-carbon fragment (#85#).

If the molecule cleaves between #"C-3"# and #"C-4"#,

we get a 1° isobutyl fragment (#57#) and a less stable 1° 5-carbon fragment (#71#).

The isobutyl cation should be more stable because of the inductive effect of the isopropyl group.