[enantiomorph]

Hi, there I'm working on some problems involving net ionic equations.  I'm not sure if they are correct so I was wondering if anyone here could please check them over.  Thanks!

Question:
a) Co(s) dissolves in dilute H2SO4(aq) to give H2(g) as a product
Co(s) + 2H+(aq) -> Co2+(aq) + H2(g)

b) Cr(s) dissolves in dilute HCl(aq) to give H2(g) as a product
2Cr(s) + 6H+(aq) -> 2Cr3+(aq) + 3H2(g)

c) Fe(s) dissolves in dilute HNO3(aq) to give H2(g) as a product
2Fe(s) + 6H+(aq) -> 2Fe3+(aq) + 3H2(g)

My answers for a) and b) are correct according to the back of my chemistry book however it says my answer for c) is wrong.  Can anyone please explain this too me?
In my equation above, I chose to use iron's common charge of 3+.  However, this led me to the wrong answer but when I use iron's 2+ charge, I got the same answer as the back of my book which is: Fe(s) + 2H+(aq) -> Fe2+(aq) + H2(g)
So, in general when I confront questions like this what charge should I use, what is the reasons behind it.  Again I would appreciate the help.  Thanks.

[Borek]

H⁺ is too weak oxidizer to oxidize Fe to Fe²⁺. However, Fe²⁺ is easily oxidized by air oxygen to Fe³⁺, that's why - in general - Fe²⁺ solutions are unstable (unless kept in airtight bottles).

[AWK]

H⁺ is too weak oxidizer to oxidize Fe to Fe²⁺. However, Fe²⁺ is easily oxidized by air oxygen to Fe³⁺, that's why - in general - Fe²⁺ solutions are unstable (unless kept in airtight bottles).

Fe reacts with HCl without oxygen
Fe + 2H+ = Fe2+ + H2

[Borek]

Sorry, a typo. Should be H+ is too weak oxidizer to oxidize Fe to Fe³⁺.