When Mercurial Chemister's second ability is copied, both the original and its copy will refer to the same discarded card (copying an ability also copies "all decisions made for" that ability, such as the choice of which card to discard [C.R. 706.10; see also C.R. 706.2]). Therefore, when each resolves, Mercurial Chemister (not the ability [C.R. 608.2g]) will deal as much damage as that card's converted mana cost.
Mercurial Chemister + Illusionist's Bracers