Which of the following is not one of the main ionic compounds to know?

Ethanol (C2H5OH) is not considered one of the main ionic compounds because it is a molecular compound rather than an ionic one. Ionic compounds are typically formed from the transfer of electrons between a metal and a non-metal, resulting in the formation of positively charged cations and negatively charged anions. Ethanol, on the other hand, is an organic compound consisting of carbon, hydrogen, and oxygen, and it is held together by covalent bonds rather than ionic bonds. As such, it does not fit the category of common ionic species that are typically emphasized in these studies.

In contrast, the other choices represent ions commonly encountered in various chemical contexts. Sulfate (SO4²⁻) is a polyatomic ion widely found in ionic compounds like salts and acids. Ammonium (NH4⁺) is another prevalent cation, often present in fertilizers and ammonium salts. Hydroxide (OH⁻) is frequently associated with bases and plays a critical role in acid-base chemistry. Understanding these ions is essential for studying chemical reactions and properties of substances in the context of ionic compounds.