What do i need to convert mass of product to ma's of a reactant

Idon’t know how to solve this question

Which compound contain an iron with a 3+ charge

Which of the following natural processes is most likely to support the formation of an underwater sinkhole? a pollution buildup from deposited minerals b limestone cave collapsing due to changes in sea level c erosion of large amounts of sand moved by ocean waves d oxidation of rock formed by chemical weathering