Q.Define basicity. Find the basicity of ortho-phosphoric acid.

Equivalent mass is the mass of a substance that reacts with or is equivalent to 1.008 g hydrogen, 8 g oxygen, or 35.5 g chlorine, calculated as molar mass divided by the n-factor.

The equivalent mass (or equivalent weight) of a substance is defined as the mass of that substance which combines with, or displaces, or is otherwise chemically equivalent to, one gram-equivalent of a reference element — conventionally taken as 1.008 g of hydrogen, 8 g of oxygen, or 35.5 g of chlorine.

It is calculated as:

Equivalent mass = Molar mass (M) / n-factor

where the n-factor depends on the type of substance/reaction:

• For an acid: n-factor = basicity = number of replaceable H+ ions (e.g. for H2SO4, n = 2).
• For a base: n-factor = acidity = number of replaceable OH- ions (e.g. for NaOH, n = 1).
• For a salt: n-factor = total positive (or negative) charge that gets exchanged in the reaction.
• For a redox reactant: n-factor = number of electrons gained or lost per formula unit. …

Gram equivalent mass is the equivalent mass of a substance expressed in grams; it is the quantity of the substance that combines with, displaces, or is chemically equivalent to one gram-atom (1.008 g) of hydrogen.

Equivalent mass (equivalent weight) of a substance is defined as its molar mass divided by its valency (or the number of electrons/protons it gains, loses, or shares per formula unit in a given reaction):

Equivalent mass = Molar mass / n

where n is the valency for an element, the number of replaceable H+/OH- ions for an acid/base, or the number of electrons transferred for a redox reaction.

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Basicity is the number of replaceable (ionisable) hydrogen atoms per molecule of an acid; ortho-phosphoric acid, H3PO4, has a basicity of 3.

The basicity of an acid is defined as the number of hydrogen ions (H+) that can be furnished (i.e., replaced by a base or a metal) by one molecule of the acid on complete ionisation. Only hydrogen atoms directly bonded to a highly electronegative atom like oxygen (i.e., -OH hydrogens) are ionisable; hydrogens bonded directly to carbon or phosphorus are usually not.

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