Question and answers in Redox and electrochemistry

Find the oxidation numbers for O in H2O, K in KI, Ca in CaS, H in MgH2, F in HF, and Fe in Fe(s).


Solutions: 
Oxygen will be 2− as it is quite often.
Potassium is located in group 1 and will have an oxidation number of 1+.
Calcium is in group 2 and will have a charge of 2+.
Hydrogen is bonded with a metal in a metal hydride and will have an oxidation number of 1−.
Fluorine will have an oxidation number of 1−. Iron is a lone element and will have an oxidation state of zero.