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I want to know what is an equivalent and how is it different from equivalent mass. How to calculate and why is it used in chemistry for knowing concentration?

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    $\begingroup$ In a simple way, you can consider the literal meaning of equivalent. A molecules of $\ce{H2SO4}$ is equivalent to two molecules of $\ce{HCl}$, in the sense of how much hydrogen ions released by them. Not much to get confused about, it's rather easy to understand. $\endgroup$ – Pritt Balagopal Jul 11 '17 at 16:14
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In reaction stoichiometry, it is the amount of one substance that reacts with one mole of another substance.

Equivalent mass is molar-mass divided by the n factor which isn't always dependent on the other reactant. As a rule ot the thumb "One equivalence of one reactant reacts completely with one equivalence of another." These concept of equivalence is extremely important in acid base titration to determine the neutralisation point.

$xM_1V_1 = yM_2V_2$

here $M_i$ is molarity.

For a relation in terms of equivalence use normality.

NOTE: Normality is a measure of concentration that is equal to the gram equivalent weight per liter of solution. Gram equivalent weight is a measure of the reactive capacity of a molecule.

As an example a solution containing $\pu{1M}$ $\ce{H2SO4}$ is actually $\pu{2N}$ as each molecule dissociates to give 2 $\ce{H+}$ ions.

I hope that clears it.

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