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. $\endgroup$ Commented Jul 11, 2017 at 16:14

2 Answers 2


Equivalent concept is an archaic unit of measurement that was used in chemistry and the biological sciences in the era before researchers knew how to determine the chemical formula for a compound.

An equivalent (symbol: officially equiv; unofficially but often Eq) is the amount of a substance that reacts with (or is equivalent to) an arbitrary amount of another substance in a given chemical reaction. The mass of an equivalent is called its equivalent weight.

In a more formal definition, the equivalent is the amount of a substance needed to do one of the following:

  1. react with or supply one mole of hydrogen ions ($\ce{H+}$) in an acid-base reaction.
  2. react with or supply one mole of electrons in a redox reaction.

I know that it is a bit difficult to understand by the means of only definitions, so let me give you some intuition about it. The basic idea of equivalent concept is that,

$1$ eqiv of a substance reacts with exactly $1$ equiv of another substances to produce $1$ equiv of each product.

Let us start with the example of Acid-Base reaction, so for acids we define one equivalent as the amount of the acid (in moles) that supplies one mole of $\ce{H+}$ ions and for base we define it as the amount of base (in moles) that supplies 1 mole of $\ce{OH-}$.

For Example:-

  • $1$ mole $\ce{HCl}$ supplies $1$ mole $\ce{H+}$ ions, so $1$ equiv of $\ce{HCl}$ is equal to $1$ mole $\ce{HCl}$.
  • Similarly, $1$ equiv of $\ce{H2SO4}$ = $1/2$ mole $\ce{H2SO4}$, $1$ Eq $\ce{NaOH= 1}$ mole $\ce{NaOH}$, $1$ equiv $\ce{Al(OH)3}$ = $1/3$ mole $\ce{Al(OH)3}$.

Now if you look at the above closely, you will notice that since 1 equiv of acid supplies 1 mole $\ce{H+}$ ions and 1 equiv of base supplies 1 mole $\ce{OH-}$ ions so 1 equiv of an acid should completely react with 1 equiv of base to produce 1 equiv of each product ( eg. 1 mole $\ce{H2O}$, since its a neutralization reaction).

In a similar manner we can define equivalent for all other types of reactions, such that the main idea i.e. "1 eqiv of a substance reacts with exactly 1 equiv of another substances to produce 1 equiv of each product", remains unaltered.

Now lets define equivalent for other things,

  1. Element

    The equivalent of an element can be defined as the amount of the element that reacts with or supplies 1 mole electron in a reaction, so if you try a bit you can conclude that 1 equiv of an element is equal to 1/n mole of element where n is the valency of the element. (NOTE:- the equivalent of a substance can be different for different reactions, eg. 1 equiv of $\ce{Fe=}$ 1/2 mole $\ce{Fe}$ if it under goes to an oxidation state $\ce{Fe^2+}$ and it will be 1/3 mole if it undergoes to an oxidation state $\ce{Fe^3+}$).

  2. Ions

    Similar to element equivalent of ions can be defined as 1/n mole of ion where n is the charge on ion (try to understand it yourself using concept of electrons used earlier).

  3. Salt

    A salt have an equal number of cations and anions, so we need to define an equivalent in such a way that, 1 equiv of salt supplies 1 mole of cations (with +1 charge or 1/n moles of cation with +n charge) and anions (with -1 charge or 1/n moles of anion with -n charge) each so that these cations and anions react completely with another one mole of anions and cations (having -1 and +1 charge) that may have been supplied 1 equiv of another salt/ substance.

    Now if you think a bit harder you can get to the point that one equiv of a salt is equal to 1/n mole of salt where n is the total number of charge either on cationic part or on anionic part (eg. in NaCl, total cationic/anionic charge = +1 so, 1 eq of NaCl= 1 mole NaCl, now if you pay your attention, you can notice that 1 mole NaCl will give 1 mole Na+ and 1 mole Cl- which is in accordance to what we said earlier.)

    Some more examples for your help: 1 equiv of $\ce{AgCl2= 1/2 AgCl2}$ as it will supply 1/2 mole of $\ce{Ag^2+}$A and 1/2 mole of (Cl-)2, in simpler terms it will give one mole of positive charge and one mole of negative charge.

    1 equiv of AlCl3= 1/3 moles of AlCl3.

  4. Oxidizing agent/ Reducing agent

    Now if you have come this far without dozing off, so you should be able to define that 1 equiv of an Oxidizing/ Reducing agent is equal to 1/n mole of that agent where n is the number of electrons gained or lost.

Now lets get to the question " Why is equivalent concept is used in chemistry for knowing concentration? ", well you might yourself have got the answer that it is because it is much simpler to apply as you do not need the Stoichiometry of the reaction going on, equivalent concept works fine without it.

And also to know the equivalent mass you simply need to find the mass of one equivalent of a substance using mole concept.

So that was all, I knew. Lets end this with a quote " Science is nothing but simple logic which is often not so simple."

  • $\begingroup$ @Tusher. $\ce{AgCl2}$ does not exist. You should have used $\ce{CaCl2}$ instead $\endgroup$
    – Maurice
    Commented May 21, 2021 at 20:38

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 of thumb "One equivalence of one reactant reacts completely with one equivalence of another." These concepts of equivalence are 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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