[Sophia7X]

Which of these compounds is amphoteric?
I.) Al(OH)3    II.) Ba(OH)2   III.) Zn(OH)2

The answer is supposed to be I and III.

Why is aluminum hydroxide and zinc hydroxide amphoteric, and not barium hydroxide? Is it because barium hydroxide is a strong base?



How many atoms are covalently bonded to the chromium atom in Cr(NH3)4Cl3
A.) 3    B.) 4   C.) 6   D.) 7
I chose B because I thought it was a Cr(NH3)4⁺³ ionically bonded to 3 Cl^- so 4 NH3's are bonded to the chromium. The answer is supposed to be C.

[Schrödinger]

Al₂O₃ and ZnO are amphoteric oxides. The hydroxides, hence, are also expected to be so. Barium on the other hand belongs to the alkali metal group. All hydroxides of alkali metals and alkaline earth metals are basic/alkaline.

[Borek]

First of all - what does it mean "amphoteric"?

[XGen]

Here is another thread that asked the same question:

http://www.chemicalforums.com/index.php?topic=46032.0

I assume that it would indeed be in the form [Cr(NH₃)₄Cl₂]Cl and that it's just one of those questions that you have to "know beforehand" to know.

[Sophia7X]

First of all - what does it mean "amphoteric"?

Acid and base properties

How could a metal hydroxide act like an acid? Could it react with a base to form a complex?

Thanks XGen. I guess it would make sense for Cr+3 to have 6 ligands since coordination number is usually the charge of the ion * 2.




Sorry, I've got another question. (I know, they never end.  )

Determine the [H+] needed to just prevent precipitation by H2PO4^- in a 0.25 M H3PO4 solution that has [Ca+2] = 0.15 M. The Ka₁ of H3PO4 is 7.1x10^-3. Ksp for calcium dihydrogen phosphate is 1.0x10^-3

Here's what I have.

Ksp eqn: Ca(H2PO4)2 --> Ca⁺² + 2 H2PO4^-
K = Q in order to initiate precipitation so...
1x10^-3 = 0.15 M Ca⁺² * [H2PO4^-]²
[H2PO4^-] = 0.0816 M (amount needed to initiate precipitation).

Ka₁ eqn: H3PO4 --> H⁺ + H2PO4^-
             0.25 M       0     0.0816 M
                   -x      +x             +x
             0.25-x        x    0.0816+x

x = 0.0168 M H⁺

I know 0.0168 M is the amount to start precipitation, but what does it mean if you want to prevent precipitation? Do you need more than that amount or less, and why?

[Borek]

First of all - what does it mean "amphoteric"?

Acid and base properties

How could a metal hydroxide act like an acid? Could it react with a base to form a complex?

You better read more, as you are missing a lot. That is - yes, you are right about the meaning, substance has both acid and base properties at the same time.

Think this way - you have a (R)X-O-H, where X is some element and R is some group (which may, or may not exist, I put it there just for the completness). Depending on which bond is stronger (R)X-O-H can dissociate into (R)X⁺ + OH^- or (R)XO^- + H⁺. There is a whole spectrum of possibilities, starting with strong acids (think sulfuric acid with (R)S-O-H), ending with strong bases (like K-O-H). - and everything in between.

[Sophia7X]

Alright, that makes sense. We haven't really discussed amphoterism much in my chem class, except the definition and a couple examples like water. I wasn't aware of the several possibilities.

Yeah, nothing better than curling up in bed with a general chem book for some light reading. 
I'll be sure to put that on the list of topics I need to read about.