equivalent weight

equivalent weight

equivalent weight. The equivalent weight of an element or radical is equal to its atomic weight or formula weight divided by the valence it assumes in compounds. The unit of equivalent weight is the atomic mass unit; the amount of a substance in grams numerically equal to the equivalent weight is called a gram equivalent. Hydrogen has atomic weight 1.008 and always assumes valence 1 in compounds, so its equivalent weight is 1.008. Oxygen has an atomic weight of 15.9994 and always assumes valence 2 in compounds, so its equivalent weight is 7.9997. The sulfate radical (SO4) has formula weight 96.0636 and always has valence 2 in compounds, so its equivalent weight is 48.0318. Some elements exhibit more than one valence in forming compounds and thus have more than one equivalent weight. Iron (atomic weight 55.845) has an equivalent weight of 27.9225 in ferrous compounds (valence 2) and 18.615 in ferric compounds (valence 3). The weight proportion in which elements or radicals combine to form compounds can be determined from their equivalent weights. For example, hydrogen can combine with oxygen to form water; the weight proportion of oxygen to hydrogen in water is the same as the proportion of their equivalent weights, 7.9997 to 1.008 or 7.946 to 1; there is 1 weight of hydrogen for every 7.946 weights of oxygen, or water is about 11.2% hydrogen (by weight). Iron forms two oxides: ferrous oxide (FeO), in which there are 27.9225 weights of iron for each 7.9997 weights of oxygen, and ferric oxide (Fe2O3), in which there are 18.615 weights of iron for every 7.9997 weights of oxygen.

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.

When choosing primary standards in analytical chemistry, compounds with higher equivalent weights are generally more desirable because weighing errors are reduced or minimized.

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

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