Question

Name each compound and calculate its formula mass. (a) \(\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}\) (b) \(\mathrm{P}_{2} \mathrm{O}_{3}\) (c) \(\mathrm{HClO}(a q)\) (d) \(\mathrm{Cr}\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\right)_{3}\)

Short answer

The compounds are (a) Aluminum sulfate with formula mass 342.15 u, (b) Diphosphorus trioxide with formula mass 109.94 u, (c) Hypochlorous acid with formula mass 52.46 u, and (d) Chromium(III) acetate with formula mass 229.13 u.

Step-by-step solution

Naming Compound (a)

Identify the elements and their valences. Aluminum sulfate is a compound that contains aluminum (Al) ions with a +3 charge and sulfate (SO4) ions with a -2 charge. The compound is made neutral by combining 2 aluminum ions (each with +3 charge) with 3 sulfate ions (each with -2 charge).

Calculating Formula Mass for (a)

Calculate the mass by adding the atomic masses of all atoms in the compound. Aluminum's atomic mass is approximately 26.98, and sulfate (SO4) is the sum of sulfur's mass (approximately 32.06) and four oxygen atoms (each approximately 16). The formula mass of Al2(SO4)3 = 2*(26.98) + 3*(32.06 + 4*16).

Naming Compound (b)

Identify the elements and their common compound name. Diphosphorus trioxide is a compound with two phosphorus (P) atoms and three oxygen (O) atoms.

Calculating Formula Mass for (b)

Calculate the mass by adding the atomic masses of all atoms in the compound. Phosphorus's atomic mass is approximately 30.97, and oxygen's is 16. The formula mass of P2O3 = 2*(30.97) + 3*(16).

Naming Compound (c)

Identify the elements, their valences, and the fact that the compound is an aqueous solution. Hypochlorous acid is a solution of hydrogen (H), chlorine (Cl), and oxygen (O) in water, and it exists in equilibrium with its dissolved ion, hypochlorite (ClO-).

Calculating Formula Mass for (c)

Calculate the mass by adding the atomic masses of all atoms in the compound. Hydrogen's atomic mass is approximately 1.01, chlorine's is 35.45, and oxygen's is 16. The formula mass of HClO = 1.01 + 35.45 + 16.

Naming Compound (d)

Identify the elements and their common compound name. Chromium(III) acetate is a compound containing chromium (Cr) ions with a +3 charge and acetate (C2H3O2) ions with a -1 charge. Three acetate ions are required to balance one chromium ion to make the compound neutral.

Calculating Formula Mass for (d)

Calculate the mass by adding the atomic masses of all atoms in the compound. Chromium's atomic mass is approximately 51.996, and acetate is the sum of 2 carbon atoms (each approximately 12.01), 3 hydrogen atoms (each approximately 1.01), and 2 oxygen atoms (each approximately 16). The formula mass of Cr(C2H3O2)3 = 51.996 + 3*(2*12.01 + 3*1.01 + 2*16).

Key concepts

Chemical Nomenclature

Understanding the names of chemical compounds is crucial for communication in the field of chemistry. Chemical nomenclature is the systematic method of naming chemical substances. Each part of the name gives you information about the compound's composition. For instance, the name aluminum sulfate refers to a compound composed of aluminum ions and sulfate ions. Aluminum has a valency of +3 and sulfate has a valency of -2. When naming ionic compounds, the cation (positively charged ion) is named first followed by the anion (negatively charged ion), with the anion's name often ending in '-ide' or '-ate.' With polyatomic ions like sulfate, the '-ate' suffix indicates the presence of oxygen.
Covalent compounds, on the other hand, use prefixes like 'di-' for two or 'tri-' for three to indicate the number of atoms, as with diphosphorus trioxide, which tells us that two phosphorus atoms are bonded to three oxygen atoms. These naming conventions are essential as they not only reveal the composition of the compounds but also facilitate accurate and efficient communication among scientists and students alike.

Molecular Mass Calculation

The molecular mass (or formula mass for ionic compounds) of a substance is the sum of the atomic masses of all atoms in a molecule of the substance. It is measured in atomic mass units (u). To calculate it, you simply multiply the atomic mass of each element by the number of atoms of that element in the molecule and then sum all the values. For example, the molecular mass of aluminum sulfate, \( \text{Al}_2(\text{SO}_4)_3 \), is computed by adding twice the atomic mass of aluminum to three times the sum of the atomic masses of one sulfur atom and four oxygen atoms. Knowing the molecular mass is fundamental for various applications in chemistry including stoichiometry, since it provides a bridge between the atomic and macroscopic levels of substances.

Chemical Formulas

The chemical formula of a compound is a symbolic representation of its composition. It tells you which elements are present in the compound and in what quantity. Chemical formulas like \( \text{P}_2\text{O}_3 \), for instance, indicate that a molecule consists of two phosphorus atoms and three oxygen atoms. Chemical formulas are essential for understanding the makeup of a compound and for communicating unambiguously about chemical substances. When it comes to ionic compounds, the formula is derived from the charges of the ions to ensure the overall charge is neutral, reflecting the ratio in which ions combine. For example, chromium(III) acetate is written as \( \text{Cr}(\text{C}_2\text{H}_3\text{O}_2)_3 \), implying one chromium ion combines with three acetate ions to form the neutral compound.

Stoichiometry

Stoichiometry is the field of chemistry that involves the quantitative relationships between reactants and products in a chemical reaction. It is based on the conservation of mass and the concept of moles. Using stoichiometry, chemists can predict how much product will form from a certain amount of reactants and vice versa. It requires a balanced chemical equation and knowledge of molecular masses to convert between mass and moles. For instance, if we know the formula masses of the reactants and products, stoichiometry allows us to calculate not only the amount needed to react completely but also the amount of each product formed. In educational settings, it's critical for students to grasp stoichiometry as it underpins many practical applications, such as determining the yield of a reaction or the proportion of components in a mixture.