Question

Sodium pyrophosphate is represented by which of the following formulas.
(a) $N{a}_2{P}_2{O}_4$
(b) $N{a}_4{P}_2{O}_5$
(c) $N{a}_4{P}_2{O}_7$
(d) $N{a}_4{P}_2{O}_7$

Answer 1

Hint: Pyrophosphate is the phosphorus oxyanions with a P-O-P linkage. It has two phosphorus atom presents in it. If we know the chemical formula of pyrophosphate, we can find the formula of sodium pyrophosphate.

Complete step by step solution: If we know the formula of pyrophosphate, we can find the solution. The pyrophosphate contains two phosphorus atoms and seven oxygen atoms.
The chemical formula of pyrophosphate is $P_2{O_7}^{4-}$
Now, let us check the structure of pyrophosphate.

We can see in the structure of pyrophosphate, with P-O-P linkage. The valency of phosphorus is five, hence one double-bonded oxygen atom is attached and one P-O-P linkage is available. The remaining two are bonded with oxygen ions, which are ready to make a bond. In pyrophosphate we have two phosphorus hence a total valency of pyrophosphate is 4. From this, we can understand that it can form an ionic bond between them. Hence four sodium atoms are used to make the compound sodium pyrophosphate. It can also be called tetrasodium pyrophosphate.

Thus, the formula for tetrasodium pyrophosphate is $N{a}_4{P}_2{O}_7$.
i.e., option(c) is correct.

Additional Information: Sodium pyrophosphate as salt, it is a white water-soluble solid. It has uses in buffering agents, an emulsifier, a dispersing agent and importantly used as a food additive. It is sometimes used in detergents to prevent similar deposition on clothing, but since it has phosphate content it causes eutrophication of water, promoting algae growth.

Note: Pyrophosphate can also form as disodium pyrophosphate, where two sodium atoms and two hydrogen atoms are attached to respective oxygen atoms. It has the formula of $N{a}_2{H}_2{P}_2{O}_7$ .It also has similar uses as the sodium pyrophosphate.