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Topic: Chemistry Equilibrium Question Help  (Read 5664 times)

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Offline kraver0

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Chemistry Equilibrium Question Help
« on: November 29, 2012, 10:16:02 PM »
A solution that contains 1.48 M hydrofluoric acid, HF (Ka = 7.2E-4), and 3.00 M hydrocyanic acid, HCN (Ka = 6.2E-10).

What is the pH of this mixture of weak acids?

a) 1.49
b) 2.97
c) 4.52
d) 9.04
e)12.51

The correct answer is a. But I don't know how to get this answer. Can someone help me? Thanks!

Offline Hunter2

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Re: Chemistry Equilibrium Question Help
« Reply #1 on: November 30, 2012, 03:17:44 AM »
Check this:

its in german but the formulas you can read

http://www.chemieunterricht.de/dc2/mwg/ph-saeuremischung.htm

Offline AWK

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Re: Chemistry Equilibrium Question Help
« Reply #2 on: November 30, 2012, 03:57:34 AM »
Could you calculate pH based on the HF present in this solution? This is quite sufficient result in this case.
AWK

Offline vin

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Re: Chemistry Equilibrium Question Help
« Reply #3 on: December 02, 2012, 11:17:15 AM »
use the formula [H+] = square root (Ka x C)
to find out the concentrations of H+ of both the acids. Total the concentrations and use the formula pH = -log[H+]
You get the answer....
step - 1
H+ ions from HCN = 4.31 x 105-
step - 2
H+ ions from HF = 3268.66 x 105-
Add the above concentrations and find negative log... you get 1.48
Hope this helped you.... good luck

Offline Borek

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Re: Chemistry Equilibrium Question Help
« Reply #4 on: December 02, 2012, 01:01:26 PM »
use the formula [H+] = square root (Ka x C)
to find out the concentrations of H+ of both the acids. Total the concentrations and use the formula pH = -log[H+]
You get the answer....
step - 1
H+ ions from HCN = 4.31 x 105-
step - 2
H+ ions from HF = 3268.66 x 105-
Add the above concentrations and find negative log... you get 1.48
Hope this helped you.... good luck

And it is completely wrong.
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Offline vin

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Re: Chemistry Equilibrium Question Help
« Reply #5 on: December 02, 2012, 01:24:41 PM »
can you tell how it is wrong?

Offline Borek

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Re: Chemistry Equilibrium Question Help
« Reply #6 on: December 02, 2012, 01:31:29 PM »
First of all - you can't simply sum concentrations, as H+ produced in one reaction shifts the equilibrium in the other reaction.

Second - you are using an approximate method not checking if it can be used.
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