One Equivalent is defined as one mole of acidic hydrogens or one mole of basic hydroxides, in acid-base theory.

 Equivalent weight is the number of grams of a substance that will contain one equivalent of, either, acidic hydrogen or basic hydroxide.

 Consider the acid, HCl:

• The mass of one mole of HCl is 36.46 grams.
• One mole of HCl contains one mole of acidic hydrogen.
• Therefore, 36.46 grams of HCl contains one mole of acidic hydrogen.
• The equivalent weight of HCl is 36.46 grams.
• This is usually expressed as 36.46 grams/equivalent.

 Consider the acid, H3PO4:

• The mass of one mole of H3PO4 is 98.00 grams.
• One mole of H3PO4 contains three moles of acidic hydrogen.
• Therefore, 98.00 grams of H3PO4 contains three moles of acidic hydrogen.
• The equivalent weight of H3PO4 is 98.00/3, or 32.66.
• This is usually expressed as 32.66 grams/equivalent.

 Consider the base, NaOH:

• The mass of one mole of NaOH is 40.00 grams.
• One mole of NaOH contains one mole of basic hydroxide.
• Therefore, 40.00 grams of NaOH contains one mole of basic hydroxide.
• The equivalent weight of NaOH is 40.00 grams.
• This is usually expressed as 40.00 grams/equivalent.

 Consider the base, Ca(OH)2:

• The mass of one mole of Ca(OH)2 is 74.10 grams.
• One mole of Ca(OH)2 contains two moles of basic hydroxide.
• Therefore, 74.10 grams of Ca(OH)2 contains two moles of basic hydroxide.
• The equivalent weight of Ca(OH)2 is 74.10/2, or 37.05 grams.
• This is usually expressed as 37.05 grams/equivalent.