Specifically how dangerous is a 10g chemical sample of pure arsenic and how do you properly dispose of it?
Treat this with very great care; don't let it come into contact with you or anyone else as it is readily absorbed through the skin, lungs and gastrointestinal tract. 

Arsenic is the classic poison known also to the ancients. Ten grammes is a huge quantity as it is deadly in quantities of approximately  0.1 g in the blood stream. The oxide and compounds such as the  arsenate, are far more poisonous than the metal as they replace phosphate in the body (which is harder for the pure metal to do) so someone can be poisoned directly by taking a large dose or by gradual exposure over a period of time. 

For a fascinating read on this and other poisons look up 'A is for Arsenic: The Poisons of Agatha Christie' by K. Harkup, publ. Bloomsbury. '