[Borek]

Simple one, and an open one - that is, I think I know the answer, but I am ready for better solutions.

What is the lightest (lowest molar mass) chiral compound made of C and H only?

What is the lightest chiral compound (no restriction to CH)?

[cth]

What is the lightest (lowest molar mass) chiral compound made of C and H only?

I think it's C₅H₈:

H₃C          H
   \          /
    C=C=C
   /          \
  H           CH₃

What is the lightest chiral compound (no restriction to CH)?

Does the compound have to be known and stable, or something more "exotic" would do?

For example, a cyclopropane with 2 trans-substituted lithium atoms C₃H₄Li₂ (I haven't check if this particular compound exists):

     CH₂   
     /\
Li-C-C-H
  /      \
 H       Li

[fledarmus]

Personally I was thinking of the ammonia analog NHDT for the smallest chiral compound -

      N
    / | \
   H D T

Of course, at a temperature cold enough to freeze out the umbrella transition...

MW ~ 20

But the methyllithium analog Li-CHDT (MW~21) might be a better choice, being more stably chiral.

[sjb]

Personally I was thinking of the ammonia analog NHDT for the smallest chiral compound -

      N
    / | \
   H D T

Of course, at a temperature cold enough to freeze out the umbrella transition...

MW ~ 20

perhaps you can go one further and utilise the ¹²N isotopologue (or the ¹⁰N, if you want really exotic)?

But the methyllithium analog Li-CHDT (MW~21) might be a better choice, being more stably chiral.

Similarly, there are lighter isotopes of Li and C???

[AWK]

But the methyllithium analog Li-CHDT (MW~21) might be a better choice, being more stably chiral.

CH3Li is an ionic compounds, hence  chiral stability of Li-CHDT is questionable.

[fledarmus]

But the methyllithium analog Li-CHDT (MW~21) might be a better choice, being more stably chiral.

CH3Li is an ionic compounds, hence  chiral stability of Li-CHDT is questionable.

Not really ionic - carbon-lithium bonds actually have a surprising amount of covalent character. Methyllithium in particular does not tend to dissociate; in both solutions and solids it tends to form aggregate structures with the lithiums in the center and the methyl groups on the outside. This would tend to keep the methyl groups from flipping. At least, I would predict them to be more stable than ammonia...

[Ann1234]

Simple one, and an open one - that is, I think I know the answer, but I am ready for better solutions.

What is the lightest (lowest molar mass) chiral compound made of C and H only?

for this one so far I have:

                     T
                     /
                D -C*-H
                   /
                  C*H3

* Being C-8 (lowest-mass C isotope I found) both carbons in the molecule. Molar mas is around 25 g/mol.

[AWK]

But C-8 has half life of 2.0 x 10⁻²¹ s.
If we are talking about chirality we should measure it. in this case and in the case of CHDTLi the measurement is impossible because of stability.
Chirality of CHDTCOOH is known - I think this is the lowest weight molecule for which chirality is proved.
We can expect a stable chirality  for example from CHDTBH₂, CHDTCH₃ CHDTNH₂.
From theoretical point of view CHDT⁴H should be chiral - ⁴H shows half life 1.39  × 10⁻²² s - in this case  ⁸C can also be used.

[Borek]

H₃C          H
   \          /
    C=C=C
   /          \
  H           CH₃

Nice one. I was thinking about 1,3-dimethylcyclopropene, but it has one hydrogen more.

I guess to say which of the other answers is best I should define the question much more precisely first. I was thinking about adding some remarks about stability and "isolability" to the question, but they are not obvious, besides, I have a feeling as long as we don't have a sample of H,D,T substituted compound it is hard to predict if we would be able to separate chiral isomers.

[AWK]

Chiral acetate
http://pubs.acs.org/doi/abs/10.1021/ed080p333
http://www.ch.ic.ac.uk/local/organic/pericyclic/ja992498e.pdf