# General Chem—Telluric Acid

1. Sep 4, 2014

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According to Wikipedia, the proper chemical formula for Telluric Acid is $Te(OH)_6$.

From what I've learned in chemistry thus far, acids made from polyatomic ions within the same group follow a similar pattern. For example, Selenic Acid $H_2SeO_4$ is analogous to Sulphuric acid $H_2SO_4$. If this is the case, why is it that Telluric Acid differs from this pattern? I would expect its chemical formula to be $H_2TeO_4$, but apparently this represents Metatelluric acid which is the analogue of sulfuric acid and "is unknown." (Wikipedia)

Why is this the case?

Thank you!

2. Sep 4, 2014

### Staff: Mentor

from http://tellurium.atomistry.com/telluric_acids.html
H2TeO4.2H2O can be written as Te(OH)6 making the water of hydration look like hydroxyl groups.

3. Sep 5, 2014

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Thanks, Jim.

So would $H_2TeO_4\cdot2H_2O$ then be considered a hydrate due to the presence of water molecules within it?

In addition, how are we supposed to know that telluric acid is a hydrate? (i.e., that it has a $nH_2O$)

Last edited: Sep 5, 2014
4. Sep 5, 2014

### Staff: Mentor

I feel like you are trying to describe reality with simplified formulas, which won't work.

Hydrates often have well defined water molecules in the crystal lattice (and I believe that's the way you are trying to picture the telluric acid). But that's not the case here. We call observed substances "hydrates" because they differ by a water molecule, but they are actually different (although related) compounds. This is quite common - similar thing happens for example with sodium bisulfite and sodium metabisulfite.

5. Sep 7, 2014

### snorkack

In Group IV, carbonic acid is $H_2CO_3$, but silicic acid is $H_4SiO_4$ - metasilicic acid $H_2SiO_3$ is polymeric.
In Group V, nitric acid is $HNO_3$, but phosphoric acid is $H_3PO_4$ - metaphosphoric acid $HPO_3$ is polymeric.
The reason both cases being that the bigger central atom prefers bonding to more oxygens.

Same reasoning applies to tellurium and iodine - $Te(OH)_6$ has Te bonded to 6 equal oxygens, whereas metatelluric acids are polymeric structures where some oxygens are bonded to two tellurium atoms.