What mass of HBr (in g) would you need to dissolve a 3.7 −g pure iron bar on a padlock?

Answer 1

dissolve? You are kidding.
The reaction would be 2 HBr + Fe>>Fe(Br)2 + H2
so you need twice the moles of HBr as is moles Fe.
Moles Fe= 3.7/55.8
Moles HBr= twice that.
Mass HBr= molesHBr* (1+80)