The result of dividing molar mass by equivalent mass is the n-factor (or valence). This factor represents the number of equivalents of a substance that can participate in a given chemical reaction.
Understanding Molar Mass and Equivalent Mass
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Molar Mass: This is the mass of one mole of a substance, expressed in grams per mole (g/mol). It's essentially the sum of the atomic weights of all atoms in a molecule.
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Equivalent Mass: This is the mass of a substance that can combine with or displace one mole of hydrogen ions (H⁺) or one mole of electrons in a chemical reaction. It's calculated by dividing the molar mass by the n-factor. The n-factor varies depending on the type of reaction and the substance involved.
- For acids: The n-factor is the number of replaceable hydrogen ions (H⁺) per molecule of the acid. For example, HCl has an n-factor of 1, while H₂SO₄ has an n-factor of 2.
- For bases: The n-factor is the number of replaceable hydroxide ions (OH⁻) per molecule of the base. For example, NaOH has an n-factor of 1, while Ca(OH)₂ has an n-factor of 2.
- For redox reactions: The n-factor is the number of electrons gained or lost per molecule or ion.
Calculating the n-factor
The relationship can be expressed as:
n-factor = Molar Mass / Equivalent Mass
Therefore, if you know the molar mass and equivalent mass of a substance, you can determine its n-factor, which is crucial in various stoichiometric calculations. Conversely, if you know the molar mass and n-factor, you can calculate the equivalent mass.
Examples
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HCl: The molar mass of HCl is approximately 36.5 g/mol. Since it has one replaceable H⁺ ion, its n-factor is 1. Therefore, its equivalent mass is also 36.5 g/eq.
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H₂SO₄: The molar mass of H₂SO₄ is approximately 98 g/mol. It has two replaceable H⁺ ions, so its n-factor is 2. Its equivalent mass is 98 g/mol / 2 = 49 g/eq.
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K₂Cr₂O₇ (in acidic medium): The n-factor for K₂Cr₂O₇ in acidic medium is 6 (due to the change in oxidation state of chromium). Knowing the molar mass, one can calculate the equivalent mass.
