Question

Calculate the equivalent weight of the following substance as acid or base.\[PO{\left( {OH} \right)_3}\]
If M is the molecular weight and E the equivalent weight, then:
A) \[E = \dfrac{M}{3}\]
B) \[E = \dfrac{M}{4}\]
C) \[E = \dfrac{M}{5}\]
D) None of these

Answer 1

Hint: The equivalent weight of an acid is the mass of acid required to generate one mole of hydrogen (hydronium) ion in aqueous solution. For an acid like HCl, which has only one ionizable hydrogen, the equivalent weight is the same as the molar mass.

Complete step by step answer:
We have given the compound \[PO{\left( {OH} \right)_3}\], if we rewrite it then it can be written as \[{H_3}P{O_4}\]. This compound is known as Phosphoric acid.
It is also given that M is the molecular mass of the compound and E is equivalent weight. If we take 1 mole of \[PO{\left( {OH} \right)_3}\] it has 3 moles of replaceable hydrogen ions, which indicates that its equivalent weight will equal to one third of its molecular weight.

\[Equivalent{\text{ }}Weight\;of\;an\;acid\;[E] = \dfrac{{Molar\;mass\;of\;the\;acid[M]}}{{Number\;of\;replaceable\;H\;atoms}}\] ​
i.e. $E = \dfrac{M}{3}$

Hence the correct option is A.

Additional Information: In this question it is asked to find out the equivalent weight of an acid, so to find out if we have divided molecular weight of the compound by replaceable hydrogen atoms in this compound, which is known as basicity for an acid. In case if a given compound does not contain replaceable \[{H^ + }\] then the equivalent weight of that element or radical is equal to its atomic weight or formula weight divided by the valency it assumes in compounds. The unit of equivalent weight is the atomic mass unit; the amount of a substance in grams numerically equal to the equivalent weight is called a gram equivalent.

Note: Students may confuse between acidity and basicity. The number of ionizable hydroxide \[\left( {O{H^ - }} \right)\]ions present in one molecule of base is called the acidity of bases while for acids the term basicity is used for replaceable \[{H^ + }\]ions