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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: kraver0 on November 29, 2012, 10:16:02 PM

Title: Chemistry Equilibrium Question Help
Post by: kraver0 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!
Title: Re: Chemistry Equilibrium Question Help
Post by: Hunter2 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
Title: Re: Chemistry Equilibrium Question Help
Post by: AWK 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.
Title: Re: Chemistry Equilibrium Question Help
Post by: vin 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
Title: Re: Chemistry Equilibrium Question Help
Post by: Borek on December 02, 2012, 01:01:26 PM
Quote from: vin 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

And it is completely wrong.
Title: Re: Chemistry Equilibrium Question Help
Post by: vin on December 02, 2012, 01:24:41 PM
can you tell how it is wrong?
Title: Re: Chemistry Equilibrium Question Help
Post by: Borek 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.