The density of hexane at room temp. is 0.7 g dm-3. how you produce a pure sample?

Weigh the liquid that you have. If it does not stack up with the expected weight for the volume, it is not a pure sample. Examine the liquid to see if it is a heterogeneous or homogeneous mixture. If it is heterogeneous, then mix the solution with water and perform an extraction. If it is homogeneous mixture then perform a distillation (carefully) at different temperatures before the boiling point of hexane to evaporate off impurities. Then boil (evaporate) hexane through the distillation process to leave behind heavier impurities (this is to separate hexane from other non-polar liquids). Make sure the temperatures are exact.

That should do it.