How many alkanes having a molecular weight of hundred, are chiral?
You are essentially correct.  Of the nine isomers of heptane, only two are chiral.  However, if you count each enantiomer as a separate molecule, then there are "four" chiral isomers:


(R)-3-methylhexane
(S)-3-methylhexane
(R)-2,3-dimethylpentane
(S)-2,3-dimethylpentane