Chapter 2 - Atoms, Molecules, and Ions: Matter Starts Here - Problems - Page 71: 78

(c)

Option (a) is incorrect. Aluminum nitride's formula is written correctly. Aluminum has an oxidation number of +3 whereas nitrogen has an oxidation number of -3. Therefore, for a neutral compound to exist, the positive and negative charges must equal one another. In this compound, there should be one aluminum atom for every nitrogen atom. Option (b) is incorrect. Aluminum sulfate dodecahydrate's formula is written correctly. Aluminum has an oxidation number of +3 whereas the sulfate ion has an oxidation number of -2. The dodecahydrate portion just means that there are 12 molecules of water in the compound, so this portion does not have anything to do with oxidation states or with the chemical bonding between aluminum and the sulfate ion. Therefore, for a neutral compound to exist, the positive and negative charges must equal one another. In this compound, there should be two aluminum atoms for every three sulfate ions. Option (d) is incorrect. Cesium sulfate's formula is written correctly. Cesium has an oxidation number of +1 whereas the sulfate ion has an oxidation number of -2. Therefore, for a neutral compound to exist, the positive and negative charges must equal one another. In this compound, there should be two cesium atoms for every sulfate ion. Option (c) is correct. Potassium chloride's formula is written incorrectly. Potassium has an oxidation number of +1 whereas chlorine has an oxidation number of -1. Therefore, for a neutral compound to exist, the positive and negative charges must equal one another. In this compound, there should be one potassium atom for every chlorine atom.