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I understand that $\ce{H2SO4}$ is a very good dehydrating agent because it is a strong acid, so that it quickly donates $\ce{H+}$ to hydroxide ions in water to form $\ce{HSO4-}$ and $\ce{H3O+}$ .But we know that there are a lot of other acids stronger than $\ce{H2SO4}$ like $\ce{HClO4}$ which are not used as dehydrating agents. They can also readily give $\ce{H+}$ to hydroxide ions in water and act as dehydrating agents but we don't use them for this purpose. Why is that?

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Nitric and perchloric acids are sold at 70% aqueous, so most of their dehydrating ability has already been consumed, while sulfuric acid is sold as 98%. At 98% concentration, nitric and perchloric acids are likely to be very good dehydrating agents, but these are also very strong oxidizers, less stable, etc.

Likewise, commercial hydrochloric acid is sold as a 38% solution in water, so it already has had its dehydrating ability consumed. However, I imagine that liquid hydrogen chloride would be a very good dehydrating agent.

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    $\begingroup$ H2so4 also has strong oxidising properties. If the oxidising nature of nitric acid is the reason we do not use it as dehydrating agent then h2so4 should also not be used. $\endgroup$ – Arishta Dec 6 '16 at 17:33
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    $\begingroup$ Oxidizing properties of H2SO4 may be called strong, but they are nowhere near those of HNO3 and HClO4. $\endgroup$ – Ivan Neretin Dec 6 '16 at 17:45
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    $\begingroup$ I dried $\ce{NiCl2 . 6 H2O}$ in a dry $\ce{HCl}$ gas stream in an inorganic lab course to give anhydrous $\ce{NiCl2}$. So yes, $\ce{HCl}$ even in gaseous form is a good dehydrating agent. $\endgroup$ – Jan Dec 6 '16 at 23:40

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