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Glossary:Equivalent weight

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  1. The atomic or formula weight of a material.
  2. The weight in grams of an ion or compound that combines with or replaces 1 gram of hydrogen. The atomic weight or formula weight divided by the va lence of the cations or anions which would form upon dissolution. This measure indicates how many grams of an ion or compound will supply one mole of positive or negative charges when dissolved. The mass of the dissolved cations and anions is of no consequence to the actual electrical interactions, but it is important for calculating the mass of materials participating in these reactions. The equivalent weight of a substance is the mass of the substance which will supply a standard number of positive or negative electrical charges when dissolved. For example the formula weight of anhydrous calcium chloride (CaCI2) is about 111 grams. Since calcium is divalent and there are two monovalent chloride ions, there would be two moles of positive and two moles of negative charges supplied by 111 grams of calcium chloride. Therefore 55 grams of calcium chloride is the equivalent weigh because 55 grams of calcium chloride would supply one mole of positive or negative charges when dissolved in water. The amount of chemical amendment required by a soil is based on the number of electrical charges of sodium which must be displaced and an equal number of electrical charges from the chemical amendment used to displace the sodium.