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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: biomed77 on March 14, 2006, 02:47:52 PM

Title: concentration question
Post by: biomed77 on March 14, 2006, 02:47:52 PM

HCN <------> H+  +  CN-
If the concentration of HCN is 0.1mol dm-3 what is the concentration of cyanide ions??
Kc= 4.9 * 10^-10mol dm-3

(4.9*10^-10)*(0.1)=4.9*10^-11
4.9*10^-11 / 2
2.45*10^-11

is it right? can someone tell me??

Title: Re:concentration question
Post by: Albert on March 14, 2006, 02:51:47 PM

I'm sorry: it's incorrect.

Quote

4.9*10^-11 / 2

Why?!

Title: Re:concentration question
Post by: biomed77 on March 14, 2006, 03:03:26 PM

I DONT KNOW REALLY I SUPPOSE BECAUSE THEY HAVE THE SAME NUMBERS OF MOLES AND BECAUSE I DIDNT REALLY KNOW WHAT TO DO NEXT?? CAN YOU HELP?

Title: Re:concentration question
Post by: Albert on March 14, 2006, 03:10:15 PM

Ka = [H+] * [CN-]
           [HCN]


I think this should lead you to the answer.

Title: Re:concentration question
Post by: Borek on March 14, 2006, 03:26:28 PM

Quote from: biomed77 on March 14, 2006, 03:03:26 PM

I DONT KNOW REALLY I SUPPOSE BECAUSE THEY HAVE THE SAME NUMBERS OF MOLES AND BECAUSE I DIDNT REALLY KNOW WHAT TO DO NEXT?? CAN YOU HELP?

Please read forum rules. You violated two of them already.

Title: Re:concentration question
Post by: biomed77 on March 14, 2006, 03:51:58 PM

i apologise... :'(

Title: Re:concentration question
Post by: Albert on March 14, 2006, 03:56:32 PM

Ok, let's go back to the problem: did you solve it?

Title: Re:concentration question
Post by: biomed77 on March 14, 2006, 04:30:06 PM

yes i did....

i did the ice table
i said 4.9*10^-10=(x*x)/(0.1-x)
4.9*10^-10=x^2/0.1
4.9*10^-11=x^2
sqare root 4.9*10^-11= 2.2*10^-5 = x

thanks a lot....you were a great help  :)

Title: Re:concentration question
Post by: Borek on March 14, 2006, 04:36:01 PM

That's not a concentration question - but pH calculation. Check out pH cheat sheet (http://www.chembuddy.com/?left=BATE&right=pH-cheat-sheet).

Title: Re:concentration question
Post by: biomed77 on March 14, 2006, 04:41:32 PM

unfortunatelly it is in my equilibrium assignment as a concentration problem...its not my fault but my tutors. thanks for the help