According to the Bronsted-Lowry theory of acids and bases, an acid molecule donates a single proton to a water molecule, creating an H3O+ ion and a negatively-charged ion known as as "conjugate base." While acids such as sulfuric (H2SO4), carbonic (H2CO3) and phosphoric (H3PO4) have multiple protons (i.e. hydrogen atoms) to donate, each proton donated counts as a separate acid-conjugate base pair. For example, phosphoric acid only has one conjugate base: dihydrogen phosphate (H2PO4-). Meanwhile, hydrogen phosphate (HPO4 2-) is the conjugate base of dihydrogen phosphate and phosphate (PO4 3-) is the conjugate base of hydrogen phosphate.
Count the total number of hydrogen atoms in the acid.
Count the acid molecule's total number of charges (the charge of an ionic molecule is expressed as an integer followed by a positive or negative sign). Therefore, a molecule of hydrogen phosphate (HPO4 2-) would have a charge of "-2" while a molecule of phosphoric acid (H3PO4) would have a charge of "0."
Subtract "1" from the total number of hydrogen atoms. For example, if sulfuric acid has two hydrogens, then its conjugate base will have only one hydrogen atom.
Add "-1" to the molecule's total charge. So, if the acid hydrogen sulfate has a charge of "-1," its conjugate base will have a charge of "-2."
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