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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?

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

tryst with freedom
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    Related: https://chemistry.stackexchange.com/questions/50684/how-can-i-predict-if-a-reaction-will-occur-between-any-two-or-more-substances – Tyberius Feb 10 '20 at 19:35

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$\ce{P4O10}$ 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

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

andselisk
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Maurice
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  • how do you know if compound has dehydrating properties or not? – tryst with freedom Feb 09 '20 at 10:01
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    The only use of $P_4O_{10}$ is as a dehydrating agent. It is probably the most efficient dehydrating agent. – Maurice Feb 09 '20 at 10:33
  • oh so it's like a general knowledge thing? – tryst with freedom Feb 09 '20 at 11:10
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    Chemistry contains a lot of empirical knowledge, that cannot be derived from basic principles. Some of them can be derived by quantum chemistry calculations, but it is not the path you want to follow. E.g. computation of correct mercury melting point has waited for 50 years until recently, when powerful enough computers were available. – Poutnik Feb 09 '20 at 12:27