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Topic: AP Chem: Calculating pH  (Read 6198 times)

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kissoftalons

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AP Chem: Calculating pH
« on: August 27, 2005, 06:11:37 PM »
Hi. I'm doing a package for AP chem and I'm having some problems with calculating pH. There are 3 questions. Step by step answer would be GREATLY appreciated!!

1. Calculate the pH of 0.025 mol/L of HCL (aq)
2. Give the Hydronium ion concentration in a solution with a pH of 3.
3. What is the concentration of sulfate ions in a 2 mol/L solution of Aluminum Sulfate.

If anyone has any good sites to help I'm kind of lost with this section. Anything to clearify would be great!

Thanks!
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Offline Borek

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Re:AP Chem: Calculating pH
« Reply #1 on: August 27, 2005, 06:40:17 PM »
In general:

http://www.chembuddy.com/?left=pH-calculation&right=toc

For questions 1 and 2 check equation 7.6:
http://www.chembuddy.com/?left=pH-calculation&right=pH-strong-acid-base#7.6
and example question:
http://www.chembuddy.com/?left=pH-calculation-questions&right=pH-strong-acid-q1

3rd question has nothing to do with pH calculation (well, to be precise - it can have something to do, but I doubt as the first two questions are pretty easy, while the last is a real hard one).
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ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Blueshawk

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Re:AP Chem: Calculating pH
« Reply #2 on: August 29, 2005, 10:14:52 AM »
Quote from: kissoftalons on August 27, 2005, 06:11:37 PM

3. What is the concentration of sulfate ions in a 2 mol/L solution of Aluminum Sulfate.


assuming that (Al)2(SO4)3 completely dissociates.

(Al)2(SO4)3 -> 2Al+3 + 3(SO4-2)

so assume you have 1 liter solution, so you have 2 mols of aluminum sulfate

So by the reaction equation..for every 2 mols of starting material, you get 2 mols of Al ion and 3 mols of sulfate ion.  

From there you can find what you are looking for.

What this has to do with pH i dont know...but maybe something...my guess is the solution is acidic.
« Last Edit: August 29, 2005, 10:16:54 AM by Blueshawk »
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kissoftalons

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Re:AP Chem: Calculating pH
« Reply #3 on: August 29, 2005, 10:28:23 AM »
Quote from: Blueshawk on August 29, 2005, 10:14:52 AM

so assume you have 1 liter solution, so you have 2 mols of aluminum sulfate

So by the reaction equation..for every 2 mols of starting material, you get 2 mols of Al ion and 3 mols of sulfate ion.  


Do you mean for every ONE mol of starting material you get 2 mols of Al and 3 mols of sulfate....and since you have 2 mols of aluminum sulfate..you'd get 4 mols of Al and 6 mols of sulfate?

And since concentration is mols/L ...wouldn't the concentration of sulfate ions be 6 mols/L..? I don't think this seems right.
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Blueshawk

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Re:AP Chem: Calculating pH
« Reply #4 on: August 29, 2005, 02:20:03 PM »
Quote from: kissoftalons on August 29, 2005, 10:28:23 AM
Do you mean for every ONE mol of starting material you get 2 mols of Al and 3 mols of sulfate....and since you have 2 mols of aluminum sulfate..you'd get 4 mols of Al and 6 mols of sulfate?

And since concentration is mols/L ...wouldn't the concentration of sulfate ions be 6 mols/L..? I don't think this seems right.

I am very sorry for a typo.  for every one mol of starting material you get 2 mols Al and 3 mols of sulfate.  Again I am sorry if I confused you.

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Blueshawk

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Re:AP Chem: Calculating pH
« Reply #5 on: August 29, 2005, 02:27:44 PM »
When I am calculating the [] of sulfate, I am assuming that aluminum sulfate completely dissociates.

If it does.....by the reaction equation, one mol Al sulfate = 2 mol Al and 3 mol sulfate.

So for 2 mols Al sulfate, yes 6 mols of sulfate ion is produced.

If it does not completely dissociate, then less then 6 mols of sulfate is produced.
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kissoftalons

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Re:AP Chem: Calculating pH
« Reply #6 on: August 29, 2005, 03:11:56 PM »
So it is possible that 6 mols of Sulfate ions are present in a 2 mol/L aluminum sulfate compound? I'm just not quite sure that it means by "concentration of sulfate ions" so there are 6 mols of sulfate ions for every L...but the original compound (aluminum sulfate)'s concentration is only 2 mol/L.
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Blueshawk

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Re:AP Chem: Calculating pH
« Reply #7 on: August 29, 2005, 03:45:13 PM »
There is nothing wrong with getting 6 mols from 2 mols.

look at these examples.

(1)CaCl2 -> (1)Ca+2 + (2)Cl-    from one mol of calcium chloride, you get one mol calcium II ion and 2 mols of chloride ion.

2(H2O) + NRG  ->  2H2 + O2   2 mols water after electrolysis gives 2 mols hydrogen gas and 1 mol oxygen gas.





 
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