A) 10.0g of Iron is

of Iron.
One mole of dihydrogen is required to form one mole of iron, hence you'll need

of dihydrogen.
B) Likewise : 2.50g of Iron is

of Iron, hence we'll need

of Fe2O3.
Answer: The coefficient for the diatomic oxygen (O2) is 3.
Explanation:
To know the coefficient for the diatomic Oxygen, we need to balance the equation.
Fe + O2 -------> Fe2O3
LHS of the equation; Fe = 1 , O2 = 1
RHS of the equation; Fe = 2 , O = 3
∴ Multiply 'Fe' on the LHS of the equation by 4 and O2 by 3
Doing that will give the balance equation which is;
4 Fe + 3 O2 --------> 2 Fe2O3
The coefficient for the diatomic oxygen (O2) as seen from the equation is 3.