Normality is a concentration unit that uses chemical equivalent or gram equivalent term. This concentration term is not very commonly used in today’s laboratories. However, it is used in some analysis like wastewater treatments. The titrimetric analysis also uses this term often and thus it is imperative we study it.
Normality is defined as no of gram equivalents present in one liter of solution.
What is a chemical/gram equivalent/equivalent weight?
Gram equivalent or equivalent weight is the amount of substance that reacts with or supplies 1mol of H+ ions or 1mol of e– s . Additionally, the equivalent weight of a compound can be calculated by dividing the molecular mass by the number of positive or negative electrical charges that result from the dissolution of the compound.
The formula for equivalent weight is –
Molecular mass and molecular weight are synonymous terms.They indicate the mass of one molecule of a substance. However, molar mass is a different term.It means mass of 1 mole of that substance.
What exactly does chemical equivalents mean?
The concept of chemical equivalents helps us to compare apples with bananas. To understand this concept better, let us consider an example- Suppose we have 20 rupees. We can buy 12 bananas, 1 apple , 20 chocolates or 25 pencils with this money. So, all the items mentioned above are equivalent in value to a fixed price(20 rupees).
12 bananas ≡ 1 apple ≡ 20 chocolates ≡ 25 pencils ≡ 20 rupees.
Similarly, in a chemical compound each element has a fixed weight which combines with or displaces a fixed weight of the other element. A standard value considered in this case is 8 parts by weight of oxygen or 1.008 parts by weight of hydrogen or 35.5 parts by weight of chlorine.
Thus, equivalent weight can also be defined as-
the weight of an element which can combine with or displace 8 parts 8 parts by weight of oxygen or 1.008 parts by weight of hydrogen or 35.5 parts by weight of chlorine.
e.g.- Sodium reacts with chlorine tp form NaCl. 23g (1mole) of sodium reacts with 35.5g(1 mole) of chlorine and thus the equivalent weight of sodium is 35.5.(Na + Cl → NaCl)
Before trying to understand what equivalent weight means, let us first learn about the n-factor or valency factor.
|Type of compound||n-factor|
|Acids||# of H+ ions produced by 1 mole of acid in a reaction|
|Bases||# of OH– ions produced by 1 mole of a base in a reaction|
|Ions||Charge on the ion|
i) where the oxidation number does not change after the reaction. ii) Redox reaction (where oxidation number changes)
|Modulus of total charge on either cation or anion. Change in the oxidation number per mole of the substance|
What does this mean? Let us understand this with examples.
In general, acids are compounds that yield H+ ion/s on dissociation (we shall learn in detail about acids later).
Example1 – Consider 1 mol of hydrochloric acid(HCl).This is a monoprotic/monobasic acid i.e it has one replaceable proton(H+). If this acid dissociates, it will yield 1mole of H+ ions and 1 mol of Cl– ions as follows –
According to the above table, n-factor for acid is – # of H+ ions produced by 1 mole of acid in a reaction. Thus, the n-factor for HCl is 1.
Now, let us find the molecular weight of this acid-
M.wt of HCl = Atomic weight of hydrogen + Atomic weight of chlorine = 1+35.5=36.5.
For this acid the molar mass is 36.5.
Equivalent weight = Molecular weight / nf = 36.5/1 = 36.5.
The gram equivalent will be 36.5 g/eq.
∴ the equivalent weight of HCl = Molecular weight of HCl.
Example 2 – Consider 1 mol of sulphuric acid (H2SO4).If this acid dissociates it will furnish 2 moles of H+ ions and 1 mole of SO4– ions as follows-
Basicity and n-factor for acid are NOT always equal.
Basicity is the number of hydrogen ions, which can be produced by one molecule of the acid. However, just because it CAN produce doesn’t necessarily mean it will produce all available H+ ions.
From the above equation, we can conclude that the basicity of sulphuric acid is 2. However, its n-factor can be 1 or 2 depending on the reaction.
1)H2SO4 + NaOH → NaHSO4 + H2O
2)H2SO4 + 2NaOH → Na2SO4 + 2H2O
For reaction(1) of sulphuric acid, its Eq.Wt=M.Wt.
For reaction (2), we need the amount of acid that will give us 1mol H+ ions. So, half a mole of this acid will give us 1 mole of H+ ions. Thus, we have to divide the molar mass by 2.
M.Wt of H2SO4 = 2(1) + 32 + 4(16) = 2+32+64 = 98g/mol.
Equivalent weight = M.Wt / 2 = 98 / 2 = 49g/eq.
∴ the equivalent weight of H2SO4 = M.Wt /2.
In the next post we will study some more acids and find out their equivalent weights. Till then,
Be a perpetual student of life and keep learning…