In analytical chemistry and titrations, you rarely use the raw 'Molecular Weight' of a compound. Because different chemicals react with different strengths, chemists use a much more accurate concept called Equivalent Weight.
The universal master formula for calculating Equivalent Weight is: Equivalent Weight = Molecular Weight / n-factor
Master Formula: Eq. Wt = Molecular Wt / n-factor.
n-factor for Acid: Basicity (Number of replaceable H⁺ ions).
n-factor for Base: Acidity (Number of replaceable OH⁻ ions).
n-factor for Salt: Total positive charge on the cation.
Usage: It is absolutely essential for calculating the 'Normality' of a chemical solution in lab titrations.
The only difficult part of calculating the equivalent weight is finding the 'n-factor' (the denominator). The rule for finding the n-factor completely changes depending on whether the chemical is an acid, a base, or a salt.
For acids, the n-factor is its Basicity (the number of H⁺ ions the acid can release in water).
For bases, the n-factor is its Acidity (the number of OH⁻ hydroxide ions the base can release).
For ionic salts, the n-factor is the total positive charge on the metal cation.
The formula is the Molecular Weight of the compound divided by its n-factor (valency factor).
H2SO4 has a molecular weight of 98. Because it has two replaceable hydrogen ions, its n-factor is 2. Dividing 98 by 2 gives an equivalent weight of 49.
For a pure element, the equivalent weight is simply its Atomic Mass divided by its Valency. For example, Oxygen has a mass of 16 and a valency of 2, so its equivalent weight is 8.
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