Question

Write a chemical formula for each compound or ion, and indicate the oxidation state of the group 5 A element in each formula: (a) phosphate ion, (b) arsenous acid, (c) antimony(III) sulfide, (d) calcium dihydrogen phosphate, (e) potassium phosphide.

Short answer

(a) The phosphate ion has the chemical formula \( PO_4^{3-} \), with an oxidation state of +5 for phosphorous. (b) Arsenous acid has the chemical formula H₃AsO₃, with an oxidation state of +3 for arsenic. (c) Antimony(III) sulfide has the chemical formula Sb₂S₃, with an oxidation state of +3 for antimony. (d) Calcium dihydrogen phosphate has the chemical formula Ca(H₂PO₄)₂, with an oxidation state of +5 for phosphorous. (e) Potassium phosphide has the chemical formula K₃P, with an oxidation state of -3 for phosphorous.

Step-by-step solution

(a) Phosphate ion chemical formula and group 5A element oxidation state

The phosphate ion is derived from phosphoric acid, H₃PO₄. To write the formula for its ion, we can remove the 3 hydrogen ions, resulting in \( PO_4^{3-} \), leaving the phosphate ion charge as -3. In this ion, the group 5A element is phosphorous (P). Calculate the oxidation state of phosphorous (x) using the formula for the sum of the oxidation states of the elements in a compound: \( x + 4(-2) = -3 \). Solving for x, we get: \( x = +5 \), So, the oxidation state of phosphorous in phosphate ion is +5.

(b) Arsenous acid chemical formula and group 5A element oxidation state

Arsenous acid is also called arsenic(III) acid and has the chemical formula, H₃AsO₃. Here, the group 5A element is arsenic (As). To determine the oxidation state of arsenic (x), we can use the sum of the oxidation states of the elements in a compound: \( x + 3(+1) + 3(-2) = 0 \). Solving for x, we get: \( x = +3 \), So, the oxidation state of arsenic in arsenous acid is +3.

(c) Antimony(III) sulfide chemical formula and group 5A element oxidation state

Antimony(III) sulfide has a chemical formula of Sb₂S₃. Antimony(III) indicates that the oxidation state of antimony is +3. In this compound, the group 5A element is antimony (Sb). The given oxidation state is +3, so there is no need to calculate it further.

(d) Calcium dihydrogen phosphate chemical formula and group 5A element oxidation state

Calcium dihydrogen phosphate has a chemical formula of Ca(H₂PO₄)₂. In this compound, the group 5A element is phosphorous (P). Calculate the oxidation state of phosphorous (x) using the sum of the oxidation states of the elements in a compound: \( +2 + 2[2(+1) + x + 4(-2)] = 0 \). Solving for x, we get: \( x = +5 \), So, the oxidation state of phosphorous in calcium dihydrogen phosphate is +5.

(e) Potassium phosphide chemical formula and group 5A element oxidation state

Potassium phosphide has a chemical formula of K₃P. In this compound, the group 5A element is phosphorous (P). Calculate the oxidation state of phosphorous (x) using the sum of the oxidation states of the elements in a compound: \( 3(+1) + x = 0 \). Solving for x, we get: \( x = -3 \), So, the oxidation state of phosphorous in potassium phosphide is -3.

Key concepts

Understanding Chemical Formulas

Chemical formulas represent the types and numbers of atoms present in a molecule or ionic compound. They serve as a shorthand to convey detailed information about the composition of a substance. For instance, the chemical formula for water, H₂O, tells us that each molecule contains two hydrogen (H) atoms and one oxygen (O) atom. In chemical formulas for compounds, the cation (positive ion) is typically written first followed by the anion (negative ion). Additionally, subscripts are used to denote the number of atoms of each element in the compound, while parentheses are used to group atoms together, especially when there are multiple polyatomic ions. For example, in the formula Ca(H₂PO₄)₂ for calcium dihydrogen phosphate, the (H₂PO₄) group is a polyatomic ion and appears twice in the compound.

Understanding how to interpret and write chemical formulas is vital for studying the composition and reactions of substances, and by extension, is crucial in the realm of chemistry. Grasping this concept also lays the groundwork for determining the oxidation states of different elements within compounds, a key step in understanding chemical redox reactions.

Group 5A Elements and Their Properties

Group 5A elements, also known as the pnictogens, include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). These elements have five electrons in their outermost shell, which impacts their chemical behavior and bonding patterns. Nitrogen and phosphorus are nonmetals, while arsenic and antimony are metalloids, and bismuth is a metal. In compounds, these elements can exhibit various oxidation states, which is a measure of the degree of oxidation of an atom within a substance.

For instance, phosphorus can be in the +5 oxidation state as seen in the phosphate ion (PO₄³⁻), or in the -3 oxidation state as evidenced by potassium phosphide (K₃P). Similarly, arsenic can be found in the +3 oxidation state in arsenous acid (H₃AsO₃). Understanding the possible oxidation states of these elements is essential for predicting the types of chemical reactions they can undergo and the kinds of compounds they can form.

Calculating Oxidation States

Calculating the oxidation state, also known as the oxidation number, of an element within a compound involves applying certain rules. Oxidation state is a theoretical construct that helps in the understanding of the electron distribution in compounds for redox reactions. The oxidation state of an element is determined based on the assumption that the compound is composed of ions, even if it doesn't actually exist in ionic form. It is assigned a positive, negative, or zero value depending on the electron control relative to a neutral atom.

To compute the oxidation state, we start by assigning known oxidation states such as +1 to alkali metals, +2 to alkaline earth metals, and -2 to oxygen (with exceptions). For neutral molecules, the sum of oxidation states of all the atoms must equal zero; for ions, the sum must equal the charge of the ion. For example, in calcium dihydrogen phosphate (Ca(H₂PO₄)₂), calcium has an oxidation state of +2, hydrogen is +1, and oxygen is -2. Using the known oxidation states, we can solve for the oxidation state of phosphorus, which is the group 5A element in question.

If students have difficulty with this exercise, it is advisable to review the rules for assigning oxidation states and to practice with a variety of compounds to build confidence in determining the correct values through calculation or educated guesswork.