Question

In a reaction, calcium phosphate is producing calcium hydrogen phosphate. The equivalent weight of calcium phosphate, in this process, is \[\left( {Ca = 40, P = 31} \right)\]
A. \[310\]
B. \[155\]
C. \[103.33\]
D. \[51.67\]

Answer 1

Hint: The chemical formula for calcium phosphate is \[C{a_3}{\left( {P{O_4}} \right)_2}\] and for calcium hydrogen phosphate is \[CaHP{O_4}\]. \[CaHP{O_4}\] is a salt of a strong base and weak acid.

Complete step by step solution:
Equivalent weight is defined as the mass of an element/compound/ion which displaces 1 part of hydrogen or 8 parts of oxygen or 35.5 parts of chlorine by mass.
\[Equivalent{\text{ }}weight,E = \dfrac{{molecular{\text{ }}weight}}{{valence{\text{ }}factor}} = \dfrac{M}{x}\]
Hence, the equivalent weight is mainly dependent on valence factor.
Here, we are dealing with compounds.
The compounds of alkali earth metals and alkaline earth metals are ionic and they can dissociate in ionic compounds in aqueous solution. So, the valence factor is equal to the number of electric charges on either cation or anion.
\[Equivalent{\text{ }}weight,E = \dfrac{{molecular{\text{ }}weight{\text{ }}of{\text{ }}compound}}{{numbers{\text{ }}of{\text{ }}electronic{\text{ charge }}on{\text{ }}cation{\text{ }}or{\text{ }}anion}}\]
The numbers of electronic charge on cation of \[C{a_3}{\left( {P{O_4}} \right)_2}\] is \[2\]\[\left( {3C{a^{2 + }} + 2P{O_4}^{3 - }} \right)\].
Hence, equivalent weight of \[C{a_3}{\left( {P{O_4}} \right)_2}\]is \[\dfrac{{\left( {3 \times 40 + 2 \times \left( {31 + 4 \times 16} \right)} \right)}}{{3 \times 2}}\]\[ = \dfrac{{310}}{6} = 51.67\]

Hence, the correct answer is D) \[51.67\]

Additional information:
Dibasic calcium phosphate, \[CaHP{O_4}\], is also called calcium monohydrogen phosphate, dicalcium orthophosphate, or secondary calcium phosphate. It is usually found in the form of hydrate, such as \[CaHP{O_4}.2{H_2}O\]. It does not melt, instead decomposes when heated to \[{109^ \circ }C\].
Preparation: \[{H_3}P{O_4}\; + {\text{ }}Ca{\left( {OH} \right)_2}\; \to {\text{ }}CaHP{O_4}\]
Calcium hydroxide reacts with phosphoric acid to produce calcium hydrogen phosphate which is anhydrous.
\[CaC{l_2}\; + {\text{ }}{\left( {N{H_4}} \right)_2}HP{O_4}\; \to {\text{ }}CaHP{O_4} .2{H_2}O\]
Calcium chloride is treated with Ammonium phosphate to form calcium hydrogen phosphate which is dihydrate.
The overall reaction is,
\[C{a_3}{\left( {P{O_4}} \right)_2}\; + {\text{ }}Ca{\left( {{H_2}P{O_4}} \right)_2} .{H_2}O{\text{ }} + {\text{ }}7{\text{ }}{H_2}O{\text{ }} \to {\text{ }}4{\text{ }}CaHP{O_4} .2{H_2}O\]
This reaction shows the increased production of calcium monohydrogen phosphate (hydrate).

Note:
We should know the chemical formula of calcium phosphate should be known and how it is dissociated to ionic form. Calcium hydrogen phosphate dihydrate is used in the manufacture of pharmaceutical tablets and as a source of calcium and phosphorus in nutritional supplements.