Question

when a piece of iron rusts, would you expect its mass to increase or decrease?

Answer 1

decrease.. I think...

Answer 2

Mass increases as when iron rusts, it forms 'rust' which is iron oxide.
$Fe_{2}O_{3}. H_{2}O$
As a new element is added (oxygen), its mass increases as it rusts. This is if the rust remains on the original piece of iron. If it is removed, of course the mass will decrease.
Hope this helps!