Rust forms when 2 atoms of fe reacts with oxygen to form iron oxide as follows.
4Fe2 + 302 ==[]::::::::::::::::> 2Fe2O3.
So basically 4 atoms of oxygen reacts with 3 atoms of oxygen to produce the product. We need to find the number of moles of Fe that reacts with Oxygen. Number of moles is the concentration of iron multiplied by the molarity. So we have 55 * 0.035 = 1.925 moles.
Hence 1.925*4 atoms of iron which gives 7.7 moles will actually react with 3 *1.925 = 5.775 of Oxygen
Since we know the number of moles of oxygen we can solve for its mass. No of moles = mass/molar mass. Mass = no of moles * molar mass = 5.775 * 32 = 184.5
