Question

Give likely chemical formulas corresponding to the following names: (a) Phosphorous acid (b) Hydroselenic acid

Short answer

Phosphorous acid is H₃PO₃, and hydroselenic acid is H₂Se.

Step-by-step solution

Understanding Chemical Naming

Before we begin, it's essential to understand that chemical names follow a set of rules. Phosphorous acid and hydroselenic acid are names for specific acids. The name indicates what elements are present and their general composition.

Phosphorous Acid

Phosphorous acid is related to phosphorus and is different from phosphoric acid. The suffix "-ous" in chemistry often indicates a lower oxidation state compared to "-ic". Phosphorous acid is an oxyacid of phosphorus with the chemical formula H₃PO₃. It consists of three hydrogen atoms, one phosphorus atom, and three oxygen atoms.

Hydroselenic Acid

"Hydroselenic acid" is formed when selenium is combined with hydrogen to form an acid. The "hydro" prefix typically signifies that the acid is formed between hydrogen and a nonmetal (without oxygen present). Thus, hydroselenic acid has the chemical formula H₂Se, indicating it contains two hydrogen atoms and one selenium atom.

Key concepts

Naming Acids

Naming acids can seem complicated, but once you understand the patterns and rules, it becomes much easier. Acids can be broadly divided into two types: binary acids and oxyacids.
Binary acids consist of hydrogen and one other nonmetal element, often a halogen. They are named with the prefix "hydro-" followed by the root of the nonmetal element and the suffix "-ic." For example, when hydrogen combines with selenium, it forms hydroselenic acid (H₂Se).
Oxyacids, on the other hand, contain hydrogen, oxygen, and another element (often a nonmetal). The naming of oxyacids is based on the polyatomic ion they contain. If the ion ends in "-ate," the acid name will be the root of the ion with "-ic acid". If the ion ends in "-ite," the acid name will be the root of the ion with "-ous acid." For example, phosphorous acid, H₃PO₃, contains the "phosphite" ion.

Oxidation States

Oxidation states are a way of keeping track of electrons in chemical compounds. They help us understand the electron distribution in molecules and ions.
In simplest terms, oxidation states refer to the charge an atom would have if the compound was composed of ions. For example, in phosphorous acid (H₃PO₃), phosphorus has an oxidation state of +3. This is lower than phosphoric acid (H₃PO₄), where phosphorus is in a +5 state. The suffix "-ous" in phosphorous acid indicates this lower oxidation state compared to "-ic."
Understanding oxidation states can help in predicting the formula of the acids and in balancing chemical equations, as they give insight into how atoms gain or lose electrons in a reaction.

Chemical Nomenclature

Chemical nomenclature refers to the system of names used to distinguish different chemical compounds. This system ensures that each compound is given a unique name that describes its composition and structure.
The International Union of Pure and Applied Chemistry (IUPAC) sets the standard rules for chemical nomenclature. However, traditional naming rules are often used in addition to IUPAC names. For example, common names like "water" for H₂O or "ammonia" for NH₃ are widely recognized and used.
In naming acids, we follow specific conventions. Binary acids use "hydro" plus the element name with "-ic," such as hydroselenic acid for H₂Se. Oxyacids follow patterns based on the polyatomic ions, like phosphorous acid for H₃PO₃.

Oxyacids

Oxyacids are a type of acid that contains oxygen, hydrogen, and another element, usually a nonmetal. Their chemical properties largely depend on the structure and oxidation state of the central atom.
These acids differ from binary acids because they include oxygen in their composition. The naming of oxyacids depends on the oxidation state of the central atom and the composition of the polyatomic ion. For instance, phosphorous acid, with the formula H₃PO₃, contains the phosphite ion.
Oxyacids play a critical role in chemistry, often acting as intermediates in reactions or as compounds forming important salts. They provide insight into how nonmetals react when bonded to oxygen and hydrogen, showcasing a variety of coordination and reactivity.