P2O5 reacts with water to form phoshoric acid. Did you mean pyrophosphate (P2O74-)?

Phosphate solubility depends on the counterion and the temperature. You can purchase 1 M stock solutions of NaH2PO4, Na2HPO4, KH2PO4 and K2HPO4, so they are all soluble to at least 1 M.

Dear Dhiraj Bhalachandra Thakur thank you for your interesting technical question. In order to give you a qualified answer you should try to be a bit more specific. First of all, please note that there is no such compound as P2O5. The most common oxide of phosphorus is P4O10 (see attached schematic formula). As mentioned by Adam B Shapiro P4O10 easily reacts with water under formation of phosphoric acid, H3PO4. As for the phosphate, it would be helpful of you could specify which phosphate salts you are interested in. The solubility of phosphates in water ranges from insoluble to highly soluble.

Dear Frank T. Edelmann, Thank you for your fruitful answer. I am looking for phosphates of Ca, Zn, Mg, Fe and Al, as these are present in the significant concentration in our solution of pH 5.5. I will be please to know the solubility of these salts at pH 5.5.

Thank You Adam B Shapiro for fruitful answer

In my solution there is acidity( H2SO4) around 100 ppm at pH ~5.5. As P2O5 is soluble in H2SO4, is it possible that P2O5 will have~100 ppm solubility at pH~5.5?

If you can find a copy of the CRC Handbook of Chemistry and Physics in your library, look up the solubility products of phosphate salts. This can be used to calculate their solubilities in water. I think you will find that the salts you mentioned have very, very low aqueous solubility. The solubility product does not take pH into account. In the case of phosphate, lowering the pH results in the formation of hydrogen phosphate ions in this order as pH decreases PO43-, HPO42-, H2PO4-, H3PO4. This means that the salts may dissolve in acid, but the ion is not simply phosphate (PO43-).

As for P2O5 (more properly P4O10), if you add it to water I think you will just get H3PO4, and a lot of heat, whether or not there is any H2SO4. Depending on the final pH, you will have an equilibrium mixture of the various hydrogen phosphate ions mentioned above.

Dear Dhiraj Bhalachandra Thakur as mentioned by Adam B Shapiro the CRC Handbook of Chemistry and Physics is a good source of solubility data. For Fe and Al please also have a look at the following useful article entitled

Solubility of Iron(III) and Aluminum Phosphates in Aqueous Solutions

https://goldschmidtabstracts.info/2002/285.pdf

(see attached pdf file)