Equivalent mass
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2008-03-28T19:36:52Z
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{{Orphan|date=August 2006}}
{{wikify|date=March 2008}}
'''Equivalent mass''' is the mass of an acid which donates one [[mole (unit)|mole]] of [[hydrogen]] [[ions]] ([[protons]] in other words) to an acceptor. Alternatively, it can be described as the mass of the acid per mole of ionizable hydrogen. Equivalent mass can be used to determine the [[volume]] or [[molarity]] of unknown substances, especially in [[titration]]s.
An acid may contain one or more hydrogen atoms in the molecule. The equivalent mass of an acid is the mass that provides one mole of hydrogen ions. It can be calculated from the molecular mass divided by the number of ionizable hydrogen atoms in a molecule, but ONLY in the case of a "strong acid," or one that exhibits complete solubility.
For example: [[Hydrochloric acid]], HCl, contains one ionizable hydrogen atom.The molecular mass is 36.45g/mol.
Equivalent Mass = (Molecular mass of acid)/(number of ionizable hydrogens)
For HCl,
EM = (36.45 g/mol)/(1 molH<sup>+</sup>) = 36.45 g/mol H<sup>+</sup>
For [[Sulfuric acid]], H<sub>2</sub>SO<sub>4</sub>:
EM = (98.07 g/mol)/(2 molH<small>+</small>) = 49.04 g/mol H<sup>+</sup>
The equivalent mass of a particular acid may be determined by titrating an acid with a standard [[solution]] of a [[strong base]], such as NaOH. This process must be undergone in the case of "weak acids" (in other words, most acids).
[[Category:Hydrogen physics]]
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