Question

How to calculate the n factor of HCL, HNO3 and H3PO4?

Answer 1

Acids
Acids are the species which furnish H+ ions when dissolved in a solvent.
For acids, n-factor is defined as the number of H+ ions replaced by 1 mole of acid in a reaction. Note that the n-factor for acid is not equal to its basicity; i.e. the number of moles of replaceable H+ atoms present in one mole of acid.
For example, n-factor of HCI = 1,
n-factor of HNO3 = 1,
n-factor of H2SO4 = 1 or 2, depending upon extent of reaction it undergoes.
H2SO4 + NaOH → NaHSO4 + H2O
Although one mole of H2SO4 ahs 2 replaceable H atoms but in this reaction H2SO4 has given only one H+ ion, so its n-factor would be 1.
H2SO4 + 2NaOH → Na2SO4 + 2H2O
The n-factor of H2SO4 in this reaction would be 2.
Similarly,
n-factor of H2SO3 = 1 or 2
n-factor of H2CO3 = 1 or 2
n-factor of H2PO4 = 1 or 2 or 3
n-factor of H3PO3 = 1 or 2 because one of the H is not replaceable in H3PO3. This can be seen using its structure .
The H atoms which are linked to oxygen are replaceable while the H atom linked directly to central atom (P) is nonreplaceable.
n-factor of H3BO3 = 1
In H3BO3, although all three H are linked to oxygen, yet all 3 H are not replaceable. Here, boron atom is electron deficient, so it acts as a Lewis acid. When H3BO3 is added to water, then oxygen atom of H2O through its lone pair attack the boron atom, as follows

The net reaction is H3BO3 + 2H2O → [B(OH)4]– + H3O+.
Thus, one mole of H3BO3 in solution gives only one mole of H+, so its n-factor is 1.
Bases
Bases are the species, which furnish OH– ions when dissolved in a solvent. For bases, n-factor is defined as the number of OH– ions replaced by 1 mole of base in a reaction. Note that n-factor is not equal to its acidity i.e. the number of moles of replaceable OH– ions present in 1 mole of base.
For example,
n-factor of NaOH = 1
n-factor of Zn(OH)2 = 1 or 2
n factor of Ca(OH)2 = 1 or 2
n factor of AI(OH)3 = 1 or 2 or 3
n factor of NH4(OH) = 1