[Rutherford]

The figure represents the content of different species (expressed as molar share) in regards to pH of a boric acid solution. Total borate concentration in the system is 0.4 M. B₄O₅(OH)₄^2- is an example of one polyborate species. Determine 1, 2, 3 and 4.

How to do this?

[Big-Daddy]

Where's the full question, can you link or give the reference?

I can barely tell lines 1 and 2 apart with my eyes... and which line is B(OH)₃ and which line is B(OH)₄^-?

[Rutherford]

Sure, if you can translate . It was from the 26th Chinese Chemistry Olympiad: http://www.ccho.org.cn/en/?c=index&p=3&n=6 (task 7-2).

[Big-Daddy]

I can't translate of course ... I wonder if anyone here can. What do you think the question means? AFAIK molar fraction would be fixed at a given pH (and is not even a function of initial concentration) so lines have to refer to species, but the diagram seems to say that regions refer to species.

[Rutherford]

Then I got it wrong. Never mind then.
If someone knows Chinese, I would be thankful if he translates this .

[SinkingTako]

Lol. Let me try to translate. Not word for word, but should be understandable. (Sorry typing on a phone now so can't do the superscript and subscript)

7-2 the diagram on the right shoes a boric acid -  boric salt system, when the boron concentration is 0.4M. Mole fraction of the polyborate ions are plotted to pH. 1, 2, 3, 4 shows the 4 types of polyborate ions and the pH where they exist.

Deduce the formula of polyborate ions corresponding to 1,2,3,4.

Note: one of the ions is [B4O5(OH)4]2-, write out the formula of the other 3 ions and assign the curves. The formation rate of the ions is the same, arrangement is not affected by rate of reaction. The oxidation state of the boron does not change.

And I think you also need to understand the first part and preamble:

In water there is not only B4 ions of  [B4O5(OH)4]2-, but also B5 ion of charge -1, and B3 ions of charge -1 and -2. They are all derived from B(OH)4- and B(OH)3. The borons are connected by B-O-B bonds.

7-1: In the B5 compound, the chemical environment of all B3+ boron are the same. Draw the structure of the B5 ion.

Haha, and the y-axis is mol fraction of boron.

[Big-Daddy]

Haha, and the y-axis is mol fraction of boron.

Thanks for translating, but how can this possibly be? The graph clearly shows at some points that the curves labelled 1 and 2 are both over 0.5. Two species cannot simultaneously have more than a 50% share ... (and that's not even mentioning the grand line starting from molar share 1 and going down to 0 at around 12 - hell, pH=8.5 gives a "total molar share" of about 2.6!)

[SinkingTako]

Oh, get it. I think they are referring to the various areas (1,2,3,4). But the y axis is definitely mole fraction.

[Big-Daddy]

I think they are referring to the various areas (1,2,3,4). But the y axis is definitely mole fraction.

What do you mean?

[kriggy]

I think its that you dont add the y-axis value together like you did. Maybe example will be better:
if we look at pH 9 there is species 3 with ~25% molar share, then species 4 with ~5% molar share, then species 2 with ~30%, B(OH)₃ with 20% and 20% [B(OH)₄]^-

[Big-Daddy]

I did a bit of the problem today - I can say the formula for the 4 ions, but how do you assign them to the regions? They each have different number of O and one has -(OH)₃ rather than -(OH)₄, but how do these relate to the mole fractions?

[kriggy]

Thats good question. What speies did you identify?