[biomed77]

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??

[Albert]

I'm sorry: it's incorrect.

4.9*10^-11 / 2

Why?!

[biomed77]

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?

[Albert]

Ka = [H+] * [CN-]
           [HCN]


I think this should lead you to the answer.

[Borek]

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.

[biomed77]

i apologise...

[Albert]

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

[biomed77]

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  

[Borek]

That's not a concentration question - but pH calculation. Check out pH cheat sheet.

[biomed77]

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