Equivalent mass is the mass of an acid which donates one 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 titrations. 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. For example: Hydrochlori 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⁺) = 36.45 g/mol H⁺

For Sulfuric acid, H₂SO₄: EM = (98.07 g/mol)/(2 molH+) = 49.04 g/mol H⁺

 

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.

 

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