What is the actual formula for iron rust ??

1 Answer
May 28, 2017



Rust is just iron oxide, so it is made up of iron and oxygen. We can find the empirical formula (simplest ratio of a compound's components) by looking at the ions each element forms.
Oxygen forms #O^(2-)# ions and iron forms #Fe^(3+)# ions. Now iron can actually form different ions because it is a transition element, but we usually stick to it being 3+ (if it forms a different ion then it will have the charge represented by a roman numeral after iron, e.g. iron(II) forms 2+ ions).

Compounds have an overall neutral charge, so we need to find a combination of iron and oxygen which results in their charges balancing out. This basically involves finding the lowest common multiple of their two charges - so for 2 and 3 this is 6, and so we have #6/2=3# oxygen ions and #6/3=2# iron ions.
This gives us the final formula #Fe_2O_3#

Hope this helped; let me know if I can do anything else:)