The Gram equivalent (or equivalent weight) can be defined as the mass of an element that combines with or displaces 1.008 grams of hydrogen or 8.0 grams of oxygen or 35.5 grams of chlorine.
Gram equivalent is a more relevant concept when working with acid and bases.
Gram equivalents of acid:
Since acids are proton (H+) donors, the Gram equivalent of an acid can be defined as the amount of acid that supplies (or contains) 1 moles of H+ ion.
Gram equivalents of monobasic (monoprotic) acids:
For example, HCl molar mass is 36.5 g/mol and it contains 1 hydrogen atom (or can generate one H+ in solution). To calculate the HCl gram equivalent, divide 36.5 by 1 which is equal to 36.5 g. This means that 1 gram equivalent of HCl is equal to 36.5 g of HCl or 1 mole of HCl. This means that HCl equivalent weight is equal to mole of HCl.
Gram equivalents of polybasic (polyprotic) acids:
For example, H2SO4 (Molar mass: 98 g/mol) contains two hydrogen atoms (or can generate two H+ in solution), therefore the gram equivalent is equal to 98/2 = 49. In terms of mole, the gram equivalent of H2SO4 is equal to 0.5 mole.
Gram equivalents of bases:
Since bases are proton (H+) acceptors, the Gram equivalent of a base can be defined as the amount of base that combines with 1 mole of H+ ion (or contains OH—).
Gram equivalents of monoacidic bases:
For example, NaOH molar mass is 40 g/mol and it contains 1 hydroxide atom (or can generate one OH— in solution). To calculate the NaOH gram equivalent, divide 40 by 1 which is equal to 40 g. This means that 1 gram equivalent of HCl is equal to 40 g of NaOH or 1 mole of NaOH. This means that NaOH equivalent weight is equal to mole of NaOH.
Gram equivalents of polyacidic bases:
For example, Magnesium hydroxide {Mg(OH)2} (Molar mass: 58.32 g/mol) contains two hydroxide atoms (or can generate two OH— in solution), therefore the gram equivalent is equal to 58.32/2 = 29.16. In terms of mole, the gram equivalent of Mg(OH)2 is equal to 0.5 mole.
