At very high temperature water vapour dissociates significantly :
$2H_2O(g)$ $2H_2(g)$ $+$ $O_2(g)$
If $n$ moles water are heated until $x$ moles have been dissociated, then the quantities at equilibrium are:
.......... mole(s) $H_2O$
.......... mole(s) $H_2$
.......... mole(s) $O_2$
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