According to the Russian Wikipedia, the reaction goes as follows:

$$\ce{Fe3O4 + 10HNO3 -> 3Fe(NO3)3 + NO2 + 5H2O}$$

Since $\ce{Fe3O4}$ is often written out as $\ce{Fe2O3*FeO}$, why don't we have instead

$$\ce{Fe3O4 + 8HNO3 -> Fe(NO3)2 + 2Fe(NO3)3 + 4H2O}$$

Why does the reaction evolve gaseous nitrogen dioxide?

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    $\begingroup$ Because nitric acid is a powerful oxidizer, and when it sees something to oxidize, it does not think twice. $\endgroup$ – Ivan Neretin Jun 7 '16 at 7:47
  • $\begingroup$ @IvanNeretin - thank you! Can one write an answer based on this section of the electrode potentials table, I wonder. NO3(-) more actively wants to become NO2 gas than Fe(3+) wants to become Fe(2+). $\endgroup$ – CowperKettle Jun 7 '16 at 7:55
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    $\begingroup$ You don't memorize reactions. Also, I'm not sure whether you should memorize the electrode potentials table (I, for one, do not). I just have a vague feeling about some oxidants being "strong" and the others "weak", and that's enough for most situations. $\endgroup$ – Ivan Neretin Jun 7 '16 at 8:10
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    $\begingroup$ Sorry, Mg3N2 + HCl. I looked it up, and it produces HN4Cl - I would never have guessed (or reasoned) that out. (0: $\endgroup$ – CowperKettle Jun 7 '16 at 9:57
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    $\begingroup$ What is NH3? What do you know about its properties? Will it react with HCl? Well, of course, if we don't have enough of the latter, there will be all sorts of "underreacted" ("недореагировавших") products. $\endgroup$ – Ivan Neretin Jun 7 '16 at 10:31

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