Equivalent weight is the amount of an element that reacts, or is involved in reaction with, 1 mole of electrons. It is 'defined' by many texts as the weight of the element combining with 1 g hydrogen, 8 g oxygen or 35.5 g chlorine, each of which would either provide or accept one mole of electrons in a reaction. This concept is very useful in gravimetric and volumetric analysis.
For example, hydrogen, with atomic weight 1.008 and valence 1, has an equivalent weight of 1.008. Oxygen assumes a valence of 2 and has an atomic weight of 15.9994, so it has an equivalent weight of 7.9997.
Elements may assume different equivalent weights in different compounds. For example, iron (atomic weight 55.845) assumes equivalent weight 27.9225 if it is valence 2 in the compound (ferrous); or 18.615 if it is valence 3 (ferric).
The equivalent weight can also be computed for other than pure elements. For example, the carbonate radical (CO32-) has a formula (atomic) weight of 60.0092 and assumes valence 2 in compounds, so its equivalent weight is 30.0046.
The equivalent weight of a substance may be defined as that weight of it which will react with or produce one mole of hydrogen. (Or 1g of H since 1mol H=1g H)
Equivalent weight in Polymer Chemistry signifies the amount of reactive functional groups on the polymer chain . Its value denotes gram solid resin that includes one mole of functional reactive group attached to the polymer backbone.
It is widely used to indicate the reactivity of polyol, isocyanate, or epoxy thermoset resins which would undergo crosslinking reactions through those functional groups. The formula to calculate the equivalent weight of particular functional reactive groups is as follows;
% Functional reactive group (FRG) = (gr of FRG)/(100gr resin)*100
Equivalent Weight = (MW of FRG)/(% FRG) * 100
ex: For an acrylic polyol resin, MW of -OH=17;
% -OH = (gr of -OH)/(100gr acrylic resin)*100
Equivalent weight of -OH = 17/(%-OH)*100