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Competency #7

Answer and Explanation

The correct answer is D.

When iron rusts, it combines with the oxygen in the air to make rust. This process is described in the following chemical reaction:

4Fe (s) + 3O₂ (g) 2Fe₂O₃ (s)

How would you expect the mass of iron before a reaction to compare to the mass of the rust after this process is complete?

1. The rust would have less mass than the iron, because there is less rust than iron.
   
   
   The rust contains oxygen and iron atoms, combined. Although there are fewer rust particles than iron particles, the mass of each rust particle is much higher than the mass of each iron particle. Rust has all the iron and the oxygen combined, so the mass is much greater.
   
   
   
2. The rust and the iron would have to have the same mass according the Law of Conservation of mass because oxygen is a gas, and gases do not weigh anything.
   
   
   The statement that gases do not weigh anything is false. All matter has mass, and the mass of an empty balloon is less than the mass of balloon filled with air.
   
   
   
3. The rust would have a higher mass than the iron, this reaction is an exception to the Law of Conservation of Mass.
   
   
   There are no exceptions to the Law of Conservation of Mass for ordinary chemical reactions. While it is true that the rust has a higher mass, this is not a violation of the Law of Conservation of Mass.
   
   
   
4. The rust would have a higher mass than the iron, because it includes the mass of the iron and the mass of the oxygen.
   
   
   This is correct, the mass of the rust is the mass of the iron plus the mass of the oxygen.
   
   
   
5. It would depend on the amount of iron you started with, a small piece of iron would weigh less than the rust, but a large piece of iron would weigh more than the rust.
   
   
   If the reaction goes to completion (as stated in the problem), this cannot be true. Chemical reactions always proceed according to established ratios and the results have to be proportional based on the amount reacting. At the end of the reaction, the amount of rust produced will always have to have a greater mass that the iron consumed because the mass or rust will always be equal to the mass of the iron plus the mass of the oxygen.