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8.9: Arrhenius Acids and Bases: Writing Chemical Formulas and Names of Polyatomic Arrhenius Acids

  • Page ID
    226548
  • Learning Objectives
    • Write chemical formulas of polyatomic, "HNPoly," Arrhenius acids.
    • Name polyatomic, "HNPoly," Arrhenius acids.

    As stated previously, an Arrhenius acid must, by definition, contain hydrogen, which ionizes to form a proton, H+1and a component that ionizes to produce a stable, negatively-charged particle.  Furthermore, because all Arrhenius acids contain hydrogen, the chemical formula and name of a specific Arrhenius acid is dependent on the type of anion that it contains.  The previous section of this chapter presented and applied the patterns for determining the chemical formulas and names of the hydrohalogenated, or "HX," Arrhenius acids, which contain monatomic, or single-atom, anions.  The following paragraphs will present and apply the patterns for determining the chemical formulas and names of Arrhenius acids that contain polyatomic, or multi-atom, anions.

    Writing Chemical Formulas of Polyatomic (HNPoly) Arrhenius Acids

    As stated in Section 3.11, polyatomic, or multi-atom, ions have defined formulas, names, and charges.  Furthermore, these types of ions are inherently highly stable and, therefore, can exist independently in solution as indivisible charged entities.  However, only the polyatomic anions that are named using "-ate ion" or "-ite ion" suffixes can successfully bond with protons to form Arrhenius acids.  

    Based on the bonding combinations that were presented in Chapter 3, non-metals, such as hydrogen, and polyatomic anions are electronically-incompatible and, therefore, should not bond with one another.  However, an Arrhenius acid generates, by definition, a hydrogen ion, H+1, when dissolved in water.  Because a proton is a positively-charged cation, an Arrhenius acid that also contains a polyatomic anion is categorized as an ionic compound.  Therefore, the chemical formula of an Arrhenius acid that contains protons and a polyatomic anion can be derived by applying the Chapter 3 rules for determining ionic chemical formulas.  In order to develop a formula pattern that can be used to represent these types of acids, which are known as "polyatomic Arrhenius acids," a polyatomic anion will be generically symbolized as "Poly–N" in the following paragraphs.  In this notation, the elemental symbols and subscripts that are present in a polyatomic ion are represented as "Poly," and "N" corresponds to the numerical portion of the charge of that ion. 

    Recall that the symbol for the cationic component of an ionic compound is written first in an ionic chemical formula, and that, after removing the "+" and "–" signs from each of the constituent ion symbols, the subscripts in an ionic base formula can be derived using either the "Ratio Method" or the "Criss-Cross Method."  Because a polyatomic ion is an indivisible unit, its chemical formula must be enclosed inside of two parentheses when incorporated into an ionic chemical formula, and the subscript that specifies how many of that ion are present within the corresponding compound must be written after the closing parenthesis.  The subscripts in the base formula must be reduced to the lowest-common ratio of whole numbers, if possible, and any explicitly-written "1"s must be removed.  Finally, if no subscript is present after the closing parentheses in the resultant chemical formula, the parentheses surrounding the polyatomic ion should be removed.

    By applying these rules to the cationic, H+1, and anionic, Poly–N, components of a polyatomic Arrhenius acid, a formula pattern of "HNPoly".  Since the original cation, H+1, bears a +1 charge, the subscript that is written after the parentheses that enclose the chemical formula of the polyatomic ion has a value of "1," which should not be explicitly-written in the chemical formula that is being developed.  As a result, the parentheses surrounding the chemical formula of the polyatomic ion should also be eliminated, and the corresponding polyatomic Arrhenius acid is understood to contain one polyatomic anion, Poly–N.  However, because polyatomic anions that are named using "-ate ion" or "-ite ion" suffixes, which can successfully bond with protons to form Arrhenius acids, can bear –1, –2, or –3 charges, the corresponding acids must contain 1, 2, or 3 protons, respectively, in order to achieve charge-balance in the final compound.  Finally, to obtain the chemical formula of a particular polyatomic Arrhenius acid, the "Poly" and "N" portions of the pattern that is shown above can be replaced with the chemical formula of a specific polyatomic anion and the absolute value of the charge of that ion, respectively.

    For example, write the chemical formula of the Arrhenius acid that contains a sulfate ion.

    As stated above, an Arrhenius acid contains a proton, H+1, by definition, and a component that ionizes to produce a stable, negatively-charged particle.  Because the indicated anion, the sulfate ion, SO4–2, is a polyatomic anion, the corresponding electrolyte is a polyatomic Arrhenius acid, which can be generically-symbolized as "HNPoly."  In order to obtain the chemical formula of a particular polyatomic Arrhenius acid, which contains one polyatomic anion, Poly–N, and one or more protons, H+1, the "Poly" and "N" portions of the pattern that is shown above can be replaced with the chemical formula of a specific polyatomic anion and the absolute value of the charge of that ion, respectively.  Because the given polyatomic anion bears a –2 charge, the corresponding polyatomic Arrhenius acid must contain 2 protons, in order to achieve charge-balance in the final compound.  Therefore, the chemical formula of the Arrhenius acid that contains a sulfate ion, SO4–2, is H2SO4.

    Exercise \(\PageIndex{1}\)

    Write the chemical formula of the Arrhenius acid that contains

    1. an acetate ion.
    2. a carbonate ion.
    3. a nitrate ion.
    4. a nitrite ion.
    5. a sulfite ion.
    6. a phosphate ion.
    7. a phosphite ion.
    Answer a
    Because the indicated anion, the acetate ion, C2H3O2–1, is a polyatomic anion, the corresponding electrolyte is a polyatomic Arrhenius acid, which can be generically-symbolized as "HNPoly."  In order to obtain the chemical formula of a particular polyatomic Arrhenius acid, which contains one polyatomic anion, Poly–N, and one or more protons, H+1, the "Poly" and "N" portions of the pattern that is shown above can be replaced with the chemical formula of a specific polyatomic anion and the absolute value of the charge of that ion, respectively.  Because the given polyatomic anion bears a –1 charge, the corresponding polyatomic Arrhenius acid must contain 1 proton, in order to achieve charge-balance in the final compound.  However, since values of "1" are usually implicitly-understood in chemistry, a subscript that corresponds to this hydrogen count should not be written in the formula that is being developed.  Therefore, the chemical formula of the Arrhenius acid that contains an acetate ion, C2H3O2–1, is HC2H3O2.
    Answer b
    Because the indicated anion, the carbonate ion, CO3–2, is a polyatomic anion, the corresponding electrolyte is a polyatomic Arrhenius acid, which can be generically-symbolized as "HNPoly."  In order to obtain the chemical formula of a particular polyatomic Arrhenius acid, which contains one polyatomic anion, Poly–N, and one or more protons, H+1, the "Poly" and "N" portions of the pattern that is shown above can be replaced with the chemical formula of a specific polyatomic anion and the absolute value of the charge of that ion, respectively.  Because the given polyatomic anion bears a –2 charge, the corresponding polyatomic Arrhenius acid must contain 2 protons, in order to achieve charge-balance in the final compound.  Therefore, the chemical formula of the Arrhenius acid that contains a carbonate ion, CO3–2, is H2CO3.
    Answer c
    Because the indicated anion, the nitrate ion, NO3–1, is a polyatomic anion, the corresponding electrolyte is a polyatomic Arrhenius acid, which can be generically-symbolized as "HNPoly."  In order to obtain the chemical formula of a particular polyatomic Arrhenius acid, which contains one polyatomic anion, Poly–N, and one or more protons, H+1, the "Poly" and "N" portions of the pattern that is shown above can be replaced with the chemical formula of a specific polyatomic anion and the absolute value of the charge of that ion, respectively.  Because the given polyatomic anion bears a –1 charge, the corresponding polyatomic Arrhenius acid must contain 1 proton, in order to achieve charge-balance in the final compound.  However, since values of "1" are usually implicitly-understood in chemistry, a subscript that corresponds to this hydrogen count should not be written in the formula that is being developed.  Therefore, the chemical formula of the Arrhenius acid that contains a nitrate ion, NO3–1, is HNO3.
    Answer d
    Because the indicated anion, the nitrite ion, NO2–1, is a polyatomic anion, the corresponding electrolyte is a polyatomic Arrhenius acid, which can be generically-symbolized as "HNPoly."  In order to obtain the chemical formula of a particular polyatomic Arrhenius acid, which contains one polyatomic anion, Poly–N, and one or more protons, H+1, the "Poly" and "N" portions of the pattern that is shown above can be replaced with the chemical formula of a specific polyatomic anion and the absolute value of the charge of that ion, respectively.  Because the given polyatomic anion bears a –1 charge, the corresponding polyatomic Arrhenius acid must contain 1 proton, in order to achieve charge-balance in the final compound.  However, since values of "1" are usually implicitly-understood in chemistry, a subscript that corresponds to this hydrogen count should not be written in the formula that is being developed.  Therefore, the chemical formula of the Arrhenius acid that contains a nitrite ion, NO2–1, is HNO2.
    Answer e
    Because the indicated anion, the sulfite ion, SO3–2, is a polyatomic anion, the corresponding electrolyte is a polyatomic Arrhenius acid, which can be generically-symbolized as "HNPoly."  In order to obtain the chemical formula of a particular polyatomic Arrhenius acid, which contains one polyatomic anion, Poly–N, and one or more protons, H+1, the "Poly" and "N" portions of the pattern that is shown above can be replaced with the chemical formula of a specific polyatomic anion and the absolute value of the charge of that ion, respectively.  Because the given polyatomic anion bears a –2 charge, the corresponding polyatomic Arrhenius acid must contain 2 protons, in order to achieve charge-balance in the final compound.  Therefore, the chemical formula of the Arrhenius acid that contains a sulfite ion, SO3–2, is H2SO3.
    Answer f
    Because the indicated anion, the phosphate ion, PO4–3, is a polyatomic anion, the corresponding electrolyte is a polyatomic Arrhenius acid, which can be generically-symbolized as "HNPoly."  In order to obtain the chemical formula of a particular polyatomic Arrhenius acid, which contains one polyatomic anion, Poly–N, and one or more protons, H+1, the "Poly" and "N" portions of the pattern that is shown above can be replaced with the chemical formula of a specific polyatomic anion and the absolute value of the charge of that ion, respectively.  Because the given polyatomic anion bears a –3 charge, the corresponding polyatomic Arrhenius acid must contain 3 protons, in order to achieve charge-balance in the final compound.  Therefore, the chemical formula of the Arrhenius acid that contains a phosphate ion, PO4–3, is H3PO4.
    Answer g
    Because the indicated anion, the phosphite ion, PO3–3, is a polyatomic anion, the corresponding electrolyte is a polyatomic Arrhenius acid, which can be generically-symbolized as "HNPoly."  In order to obtain the chemical formula of a particular polyatomic Arrhenius acid, which contains one polyatomic anion, Poly–N, and one or more protons, H+1, the "Poly" and "N" portions of the pattern that is shown above can be replaced with the chemical formula of a specific polyatomic anion and the absolute value of the charge of that ion, respectively.  Because the given polyatomic anion bears a –3 charge, the corresponding polyatomic Arrhenius acid must contain 3 protons, in order to achieve charge-balance in the final compound.  Therefore, the chemical formula of the Arrhenius acid that contains a phosphite ion, PO3–3, is H3PO3.

    Naming Polyatomic (HNPoly) Arrhenius Acids

    The name of an ionic compound is based solely on the identities of the ions that it contains.  Since the subscripts in an ionic chemical formula are the result of achieving charge-balance between the compound's constituent ions, referencing subscripts in an ionic chemical name is considered redundant.  Therefore, the names of ionic compounds do not include any numerical prefixes.

    As stated above, because all Arrhenius acids contain hydrogen, the name of a specific Arrhenius acid is derived from the name of its constituent anion.  Therefore, because a polyatomic, or "HNPoly," Arrhenius acid contains a polyatomic anionPoly–N, the root term that indicates the identity of the polyatomic ion must be incorporated into the name of the corresponding Arrhenius acid.  Additionally, because the names of the Arrhenius acids that contain sulfur- and phosphorus-based polyatomic anions do not have fluid pronunciations, an additional "ur" or "or" syllable, respectively, must be incorporated into the root terms in the corresponding acid names.  Furthermore, because the substance that is being named contains one or more protons, H+1, in addition to a polyatomic ionPoly–N, the suffiof the polyatomic ion must be replaced with an acisuffix.  As stated above, polyatomic anions that are named using "-ate ion" and "-ite ion" suffixes can successfully bond with protons to form Arrhenius acids.  As has been repeatedly emphasized in several sections of this text, no two chemical formulas should share a common chemical name.  Therefore, the acisuffixes that correspond to the "-ate ion" and "-ite ion" polyatomic anions must be unique.  If a polyatomic anion that is named using an "-ate ion" suffix is present in an Arrhenius acid, that acid should be named using an "-ic acid" suffix, and an "-ite ion" polyatomic suffix should be replaced an "-ous acidsuffix when naming a polyatomic Arrhenius acid.  Finally, both the "-ate ionpolyatomic Arrhenius acids and the hydrohalogenated Arrhenius acids are named using "-ic acidsuffixes.  Therefore, in order to distinguish the names of these acids, which have distinctive chemical compositions, and consequently, should have unique chemical names, the prefix "hydro-" is not incorporated into the names of the "-ate ion" or the "-ite ion" polyatomic Arrhenius acids.

    For example, write the name of the Arrhenius acid that is symbolized as H2SO4.

    Because the given chemical formula contains two protons, H+1, and a polyatomic ionSO4–2, the corresponding molecule can be classified as a polyatomic, or "HNPoly," Arrhenius acid.  In order to name this type of acid, the suffix that is written after the root term that indicates the identity of the polyatomic ion must be replaced with an acisuffix.  Finally, because the prefix "hydro-" is only incorporated into the names of the hydrohalogenated Arrhenius acids, this word should not be written in the name of a polyatomic Arrhenius acid.  Therefore, because the molecule that is symbolized above contains a sulfate ionSO4–2, the root word "sulf" is incorporated into the acid name that is being developed.  Additionally, because the name of an Arrhenius acid that contains a sulfur-based polyatomic anion does not have a fluid pronunciation, an additional "ur" syllable is incorporated into the "sulf" root term.  The full name of the given molecule, sulfuric acid, is completed by replacing the "-ate ion" polyatomic ion suffix with "-ic acid."

    Exercise \(\PageIndex{2}\)

    Write the name of the Arrhenius acid that is symbolized as

    1. HC2H3O2.
    2. H2CO3.
    3. HNO3.
    4. HNO2.
    5. H2SO3.
    6. H3PO4.
    7. H3PO3.
    Answer a
    Because the given chemical formula contains one proton, H+1, and a polyatomic ionC2H3O2–1, the corresponding molecule can be classified as a polyatomic, or "HNPoly," Arrhenius acid.  In order to name this type of acid, the suffix that is written after the root term that indicates the identity of the polyatomic ion must be replaced with an acisuffix.  Finally, because the prefix "hydro-" is only incorporated into the names of the hydrohalogenated Arrhenius acids, this word should not be written in the name of a polyatomic Arrhenius acid.  Therefore, because the molecule that is symbolized above contains an acetate ionC2H3O2–1, the root word "acet" is incorporated into the acid name that is being developed.  The full name of the given molecule, acetic acid, is completed by replacing the "-ate ion" polyatomic ion suffix with "-ic acid."
    Answer b
    Because the given chemical formula contains two protons, H+1, and a polyatomic ionCO3–2, the corresponding molecule can be classified as a polyatomic, or "HNPoly," Arrhenius acid.  In order to name this type of acid, the suffix that is written after the root term that indicates the identity of the polyatomic ion must be replaced with an acisuffix.  Finally, because the prefix "hydro-" is only incorporated into the names of the hydrohalogenated Arrhenius acids, this word should not be written in the name of a polyatomic Arrhenius acid.  Therefore, because the molecule that is symbolized above contains a carbonate ionCO3–2, the root word "carbon" is incorporated into the acid name that is being developed.  The full name of the given molecule, carbonic acid, is completed by replacing the "-ate ion" polyatomic ion suffix with "-ic acid."
    Answer c
    Because the given chemical formula contains one proton, H+1, and a polyatomic ionNO3–1, the corresponding molecule can be classified as a polyatomic, or "HNPoly," Arrhenius acid.  In order to name this type of acid, the suffix that is written after the root term that indicates the identity of the polyatomic ion must be replaced with an acisuffix.  Finally, because the prefix "hydro-" is only incorporated into the names of the hydrohalogenated Arrhenius acids, this word should not be written in the name of a polyatomic Arrhenius acid.  Therefore, because the molecule that is symbolized above contains a nitrate ionNO3–1, the root word "nitr" is incorporated into the acid name that is being developed.  The full name of the given molecule, nitric acid, is completed by replacing the "-ate ion" polyatomic ion suffix with "-ic acid."
    Answer d
    Because the given chemical formula contains one proton, H+1, and a polyatomic ionNO2–1, the corresponding molecule can be classified as a polyatomic, or "HNPoly," Arrhenius acid.  In order to name this type of acid, the suffix that is written after the root term that indicates the identity of the polyatomic ion must be replaced with an acisuffix.  Finally, because the prefix "hydro-" is only incorporated into the names of the hydrohalogenated Arrhenius acids, this word should not be written in the name of a polyatomic Arrhenius acid.  Therefore, because the molecule that is symbolized above contains a nitrite ionNO2–1, the root word "nitr" is incorporated into the acid name that is being developed.  The full name of the given molecule, nitrous acid, is completed by replacing the "-ite ion" polyatomic ion suffix with "-ous acid."
    Answer e
    Because the given chemical formula contains two protons, H+1, and a polyatomic ionSO3–2, the corresponding molecule can be classified as a polyatomic, or "HNPoly," Arrhenius acid.  In order to name this type of acid, the suffix that is written after the root term that indicates the identity of the polyatomic ion must be replaced with an acisuffix.  Finally, because the prefix "hydro-" is only incorporated into the names of the hydrohalogenated Arrhenius acids, this word should not be written in the name of a polyatomic Arrhenius acid.  Therefore, because the molecule that is symbolized above contains a sulfite ionSO3–2, the root word "sulf" is incorporated into the acid name that is being developed.  Additionally, because the name of an Arrhenius acid that contains a sulfur-based polyatomic anion does not have a fluid pronunciation, an additional "ur" syllable is incorporated into the "sulf" root term.  The full name of the given molecule, sulfurous acid, is completed by replacing the "-ite ion" polyatomic ion suffix with "-ous acid."
    Answer f
    Because the given chemical formula contains three protons, H+1, and a polyatomic ionPO4–3, the corresponding molecule can be classified as a polyatomic, or "HNPoly," Arrhenius acid.  In order to name this type of acid, the suffix that is written after the root term that indicates the identity of the polyatomic ion must be replaced with an acisuffix.  Finally, because the prefix "hydro-" is only incorporated into the names of the hydrohalogenated Arrhenius acids, this word should not be written in the name of a polyatomic Arrhenius acid.  Therefore, because the molecule that is symbolized above contains a phosphate ionPO4–3, the root word "phosph" is incorporated into the acid name that is being developed.  Additionally, because the name of an Arrhenius acid that contains a phosphorus-based polyatomic anion does not have a fluid pronunciation, an additional "or" syllable is incorporated into the "phosph" root term.  The full name of the given molecule, phosphoric acid, is completed by replacing the "-ate ion" polyatomic ion suffix with "-ic acid."
    Answer g
    Because the given chemical formula contains three protons, H+1, and a polyatomic ionPO3–3, the corresponding molecule can be classified as a polyatomic, or "HNPoly," Arrhenius acid.  In order to name this type of acid, the suffix that is written after the root term that indicates the identity of the polyatomic ion must be replaced with an acisuffix.  Finally, because the prefix "hydro-" is only incorporated into the names of the hydrohalogenated Arrhenius acids, this word should not be written in the name of a polyatomic Arrhenius acid.  Therefore, because the molecule that is symbolized above contains a phosphite ionPO3–3, the root word "phosph" is incorporated into the acid name that is being developed.  Additionally, because the name of an Arrhenius acid that contains a phosphorus-based polyatomic anion does not have a fluid pronunciation, an additional "or" syllable is incorporated into the "phosph" root term.  The full name of the given molecule, phosphorous acid, is completed by replacing the "-ite ion" polyatomic ion suffix with "-ous acid."