Simple concentration problem but something's wrong

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1. Oct 14, 2014

delsaber8

Ok so I think I have a suitable answer to my question but because of certain circumstances I have reason to believe my answer is wrong.

The question is: calculate the amount of phosphoric acid in mgH3PO4/L in a solution of concentration 2.92X10^-5 mol/L.

seems pretty straight forward but perhaps I've overlooked something (in which case I will be quite embarrassed)

Apologies if this is in the wrong part of the forum

2. Oct 14, 2014

sjb-2812

Well, what answer do you have? How do you know it's wrong?

3. Oct 14, 2014

delsaber8

I have an answer of 2.8616 mg/L, the reason I fear my answer is wrong is because my prof gave an answer which was several orders of magnitude different.

4. Oct 14, 2014

Staff: Mentor

What units did he want the answer in? Maybe he wanted micrograms per L.

5. Oct 14, 2014

delsaber8

This is exactly as typed in the question:
Show the calculations for determining the phosphate ion concentration in the
original Coke and converting the moles of phosphate / litre of Coke into grams of
H3PO4/355 mL can and mg of H3PO4/1 L. Now you may notice there are more questions than those that I have asked but if I figured out the problem for the mg H3PO4/L I could figure out the rest. the answers he gave ranged between 500-1000 mgH3PO4/L

I am very confused as to why this isn't working any help would be appreciated as the deadline for this approaches and I'm a little frazzled.

6. Oct 14, 2014

Staff: Mentor

Doesn't phosphoric acid dissociate in water into several different anions, only one of which is phosphate?

Chet

7. Oct 14, 2014

delsaber8

yes I believe it become 3H++PO42- -> H3PO4 but since the ratio is the same between the phosphate and the phosphoric acid they should have the same concentration. PLEASE do correct me if I am mistaken,
Thanks!

8. Oct 15, 2014

Staff: Mentor

9. Oct 15, 2014

Staff: Mentor

2.86 ppm is a correct answer (assuming we ignore the dissociation).

Doesn't make sense, that's a 0.01 M solution, not 10-5 M. Something is off, and I don't think it is related to equilibrium. Have you learned equilibrium calculations?

If you were to correct your answer for dissociation, the final result would be much lower (something like 10-16 M), so even more distant from the 500-1000 mg/L.

10. Oct 15, 2014

Staff: Mentor

Hi Borek. I'm guessing that the concentration he gave in his original post was the concentration of phosphate ions, not phosphoric acid. I think from this, he is supposed to determine the concentration of phosphoric acid. Just a guess.

Chet

11. Oct 15, 2014

Staff: Mentor

Interesting idea! But impossible. I just did some quick calculations and in phosphoric acid solutions concentration of PO43- never gets higher than about 6.5×10-14 M - for concentrated solutions pH is low, and the PO43- fraction is very low, for more diluted solutions fraction goes up, but as the whole concentration goes down concentration of PO43- goes down as well. Please remember pH of acid solution never gets higher than 7, so even in highly diluted solutions weak acids are never fully dissociated. In phosphoric acid at pH 7 PO43- fraction is 1.7×10-6.

12. Oct 15, 2014

Staff: Mentor

Thanks for setting that straight. You are soooo knowledgeable. Very impressive.

Chet

13. Oct 15, 2014

delsaber8

ok I feel really dumb about this, I realized that the concentration I was using was of coke that was heavily diluted, after going through some ratios I got a much more reasonable answer of 671mg/L

14. Oct 16, 2014

Staff: Mentor

Happens. Think about skills that you got while analyzing the problem. You definitely know more now.

I have learned so much from my mistakes I am thinking about making some new ones.