Q.Define equivalent mass.
Concept understanding — Gram Equivalent Mass
The gram equivalent concept runs parallel to the mole concept, but is built on equivalent mass rather than molecular mass. The gram equivalent mass of an element, compound or ion is defined as the mass of it that combines with, or displaces, 1.008 g of hydrogen, 8 g of oxygen, or 35.5 g of chlorine.
Since referencing every reaction back to H/O/Cl directly is impractical, the general working formula is:
Gram equivalent mass = (Molar mass, g mol⁻¹) ÷ (Equivalence factor n, eq mol⁻¹)
The equivalence factor n takes a different specific meaning for each type of chemical entity:
- Acid: n = basicity = number of moles of ionisable H⁺ per mole of acid (H₂SO₄: n = 2, gram equivalent mass = 98/2 = 49 g eq⁻¹).
- Base: n = acidity = number of moles of ionisable OH⁻ per mole of base (KOH: n = 1, gram equivalent mass = 56/1 = 56 g eq⁻¹).
- Oxidising/reducing agent: n = number of moles of electrons gained/lost per mole of reagent in the redox half-reaction (KMnO₄ in acid medium, MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O: n = 5, gram equivalent mass = 158/5 = 31.6 g eq⁻¹).
Equivalent mass has no unit; gram equivalent mass has the unit g eq⁻¹.
Why use this alongside the mole concept? The mole concept needs a fully balanced equation before you can relate reactant amounts. The gram equivalent concept does not -- by definition, equivalent amounts of any two reacting substances always react in a 1:1 ratio, whatever the underlying reaction. This makes it especially convenient for redox reactions (where balancing the full equation is laborious), while the mole concept remains the natural choice for non-redox reactions.
Gram equivalent mass = molar mass ÷ equivalence factor (basicity for acids, acidity for bases, electrons transferred for redox agents); equivalents always react 1:1, without needing a balanced equation.
Equivalent mass is defined relative to a fixed combining/displacing capacity against hydrogen, oxygen or chlorine.
Equivalent mass is the mass of an element, compound or ion that combines with or displaces 1.008 g of hydrogen, 8 g of oxygen, or 35.5 g of chlorine.
Step 1. Recall the gram equivalent concept parallels the mole concept but is built on equivalent mass instead of molecular mass.
Step 2. By definition, the (gram) equivalent mass of a substance is the mass of it that combines with or displaces a fixed reference amount of hydrogen, oxygen or chlorine.
Step 3. State the three reference amounts precisely: 1.008 g hydrogen, 8 g oxygen, or 35.5 g chlorine.
Equivalent mass = the mass of a substance that combines with or displaces 1.008 g hydrogen, 8 g oxygen, or 35.5 g chlorine.
- Confusing equivalent mass (unitless) with GRAM equivalent mass (unit g eq⁻¹) -- both are valid to mention, but they are not the same thing
- Forgetting the three alternative reference substances (giving only one of H/O/Cl)
- CBSE 2024Set ANNUAL2 marksQ.Define equivalent mass.
›Reveal solutionSolution
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.
Unlike molar mass, equivalent mass can vary for the same substance depending on the specific reaction it undergoes (since the n-factor can differ by context), which is why it is always defined with respect to a particular reaction when precision matters.
✓Final answerThe equivalent mass (equivalent weight) of a substance is the mass of it that combines with, displaces, or is chemically equivalent to 1.008 g of hydrogen, 8 g of oxygen, or 35.5 g of chlorine; it equals molar mass divided by the n-factor (valency/number of replaceable H, charge, or electrons transferred).
- CBSE 2022Set ANNUAL2 marksQ.Define Gram equivalent mass.
›Reveal solutionSolution
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.
When this equivalent mass is expressed in grams, it is called the gram equivalent mass. Operationally, one gram equivalent mass of a substance is the mass that reacts exactly with, or displaces, one gram equivalent (1.008 g) of hydrogen — this is why equivalent masses of different substances always react with each other in a 1:1 equivalent ratio, which is the basis of volumetric (titration) calculations.
✓Final answerGram equivalent mass is the equivalent mass (molar mass ÷ valency, or ÷ number of H+/electrons involved) of a substance expressed in grams — the mass that reacts with or is equivalent to one gram-equivalent of hydrogen.
- CBSE 2020Set ANNUAL2 marksQ.Define basicity. Find the basicity of ortho-phosphoric acid.
›Reveal solutionSolution
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.
Ortho-phosphoric acid has the structure (HO)3P=O — all three hydrogen atoms are present as -OH groups bonded to the central phosphorus through oxygen, and all three can be successively replaced/ionised:
H3PO4 -> H+ + H2PO4^-
H2PO4^- -> H+ + HPO4^2-
HPO4^2- -> H+ + PO4^3-
Since it can donate 3 protons, ortho-phosphoric acid is tribasic, with a basicity of 3.
✓Final answerBasicity of an acid = number of ionisable H+ ions per molecule; the basicity of ortho-phosphoric acid (H3PO4) is 3.
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