How to predict the products of an inorganic dehydration reaction?

For example, this reaction is a dehydration reaction


$$\ce{ HNO3 + P4O10 -> N2O5 + HPO3} $$


If I were only given the reactants side of the equation, how would I predict the product's side of it?


What I have seen already:
[How can I predict if a reaction will occur between any two (or more) substances?](https://chemistry.stackexchange.com/questions/50684/how-can-i-predict-if-a-reaction-will-occur-between-any-two-or-more-substances)


I wish to know how many $ \ce{H2O}$ molecules a dehydrating agent would remove.




$\ce{P4O10}$ is known to be a dehydrating agent. It removes two hydrogens and one oxygen from any substance it may encounter.


In order to withdraw $\ce{H2O}$ from nitric acid, it must find at least two $\ce{H}$ atoms. One molecule $\ce{HNO3}$ is not enough. So you have to use at least two molecules of $\ce{HNO3}.$ What remains from two $\ce{HNO3}$ after removing one $\ce{H2O}?$ By subtraction you find that it remains $\ce{N2O5}.$


What is more difficult to discover is the molecule made with $\ce{P4O10}$ reacts with water. It could be $\ce{HPO3}$ or $\ce{H3PO4}.$ By simply counting the atoms before and after the arrow, you may discover that $\ce{HPO3}$ is formed. So, the equation may be


$$\ce{4 HNO3 + P4O10 -> 2 N2O5 + 4 HPO3}.$$


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