Q:What is equivalent weight ?
Ans:-Weight of one equivalent of a substance is called equivalent weight.Its units are grams per equivalent.Here we will discuss only practical aspect of this concept of equivalent weight.
Q:- What are equivalents and how do we calculate their weight?
Ans:-Equivalents are nothing just a term given to the substances taking part in the chemical reactions.If a substance is acid then we consider how many replaceable hydrogen atoms it has per mole. For example in case of sulfuric acid we have two replaceable hydrogen atoms per mole.Therefore we can say that sulfuric acid has two equivalents per mole.
And if the substance under consideration is a base then we see how many replaceable hydroxy groups it has per mole. For example in case of Calcium hydroxide we have two replaceable hydroxy groups per mole.Hence we will say that Calcium hydroxide has two equivalents per mole.And if we have some salt then we consider valency of metal ion present in that salt.For example in case of Ferric chloride iron has valency 3.Therefore it is understood that ferric chloride has three equivalents per mole.
similarly if we are dealing with some sort of redox reactions then we calculate the number of equivalent per mole on the basis of electrons gained or lost during the redox reaction.
The equivalent weight of a substance is not constant and it may have different values in different chemical reactions depending on the medium of reaction that weather it is acidic ,basic or neutral.
Hence one thing we should always keep in mind that the EQUIVALENT WEIGHT is always less than or equal to the MOLECULAR WEIGHT.
EW greater than or equal to MW.
As in all the above examples the equivalent weight is less than molar mass.But if we consider examples like HCl , NaOH , NaCl etc then the value of equivalent weight will be same as that of their molar masses.
Now let us reconsider the second question again :
Q:- What are equivalents and how do we calculate their weight?
Ans:-If Weight of one equivalent of a substance is called equivalent weight then in order to calculate equivalent weight we divide the molar mass of the substance with number of equivalents per mole as follow :
Eq Wt = Molar Mass / No.of equivalents per mole
For example equivalent wt of sulfuric acid can be calculated as follow:
Eq Wt = 98 g /mol / 2 equiv / mol
Eq Wt = 49 g /equiv
Q: What if we are given some mass of the substance other than the molar mass and asked to calculate the number of equivalents in that given mass of the substance ?
Ans:-For that we use equivalent weight and calculate the number of equivalents in the given mass of the substance as follow:
We divide the given mass of the substance with equivalent weight of the substance :
Number of equivalents = Given weight/Eq wt
Q:- Calculate the number of equivalents in 35 gram of sulfuric acid ?
Ans:-
Number of equivalents = Given weight/Eq wt
Number of equivalent = 35 g /49 g / Equiv
Number of equivalent = 0.71 Equiv
Q:- Calculate equivalent weight and number of equivalents of Ca(OH)2 in 6.32 g of Ca(OH)2.
Ans :-
eq wt = 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠/𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡𝑠 𝑝𝑒𝑟 𝑚𝑜𝑙
= 74.09/2 = 37.05 gram/eq
Number of equivalents = 𝑔𝑖𝑣𝑒𝑛 𝑤𝑒𝑖𝑔ℎ𝑡 𝑖𝑛 𝑔𝑟𝑎𝑚𝑠/𝐸𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑤𝑒𝑖𝑔ℎ𝑡
= (6.32 𝑔𝑟𝑎𝑚)/(37.05 𝑔𝑟𝑎𝑚/𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡)
= 0.171 equivalents
Practice
1. Determine the equivalent weights of the folowing acids and bases:
a) Ca(OH)2
b) H2C2O4
c) KOH
d) H2CO3
2. How many equivalents is:-
15.0 g of:
a) H2SO4
b) Ca(OH)2
c) Al(OH)3?
Ans:
1. a) 37.05
b) 45.04
c) 56.11
d) 31.01:
2.
a) 0.306 eq
b) 0.405 eq
c) 0.577 eq
0 Comments