## Chemistry and Chemical Reactivity (9th Edition)

The question is asking for the incorrect chemical formula of the compounds given. All answer choices contain the Cl$^{-}$ ion which means that we only have to look at the metal ions. Choice (b) gives the correct formula because a Ca$^{2+}$ ion with a Cl$^{-}$ ion yields CaCl$_{2}$ Choice (c) gives the correct formula because a Al$^{3+}$ ion with a Cl$^{-}$ ion yields AlCl$_{3}$ Choice (d) gives the correct formula because a Rb$^{+}$ ion with a Cl$^{-}$ ion yields RbCl Therefore Choice (a) is incorrect because a Mg$^{2+}$ ion with a Cl$^{-}$ ion yields MgCl$_{2}$ not MgCl