2. Rust forming on a piece of iron is an example of which of the following: A. A physical change because the process can be reversed
B.
A chemical change because oxygen atoms from the atmosphere bonded with the iron atoms
C.
A violation of the laws of physics because new mass has been created

B.

Explanation:

Rust is a chemical change because you can not undo it. Rusting of iron is a chemical change because a new substance iron oxide is formed. The presence of oxygen and water or water vapour is essential for rusting.

1. for the first question, we must find the mass of the anhydrous salt, mgso₄. the molar mass for mgso₄·7h₂o is 246.47 g/mol, while that of mgso₄ is 120.37 g/mol.7.834 g mgso₄·7h₂o * 1 mol mgso₄·7h₂o/246.47 g * 1 mol mgso₄/1 mol mgso₄·7h₂o * 20.37 g mgso₄/mol = 0.647 g mgso₄

2. mass of water = mass of sample - mass of anhydrous salemass of water = 7.834 - 0.647 = 7.19 gpercent water in hydrate = 7.19/7.834 * 100 = 91.74%