finding pH?

Question

pH of a 0.10 M HCN solution with pH=5.2

to find [H+] i'm supposed to do 10^5.2?

how could i find [CN] and [HCN]?

Answer 1

(H+)= 10^-5.2 = 0.00000631 M

The reaction is

HCN <>H + + CN-
Initiali concentration
0.10 .... ..0... ... ..0
at equilibrium
0.10 -x.....x.... .... x

But we know x =0.00000631 M = Concentration H+ = Concentration CN-

Concentration HCN = 0.1 - 0.00000631 = 0.09999 M

Answer 2

No HCN is a weak acid incompletely dissociated so

HCN <---> H+ CN-

K= [H+][CN-] /[HCN]

lets x the fraction of HCN which is dissociated and c the concentration
You have
[H+]=[CN-] = cx
and HCN = c (1-x) as x is small so you have c(1-x) =c

K = c^2x^2/c

as cx = [H+] you have K = [H+]^2/c

and [H+]^2 = Kc or [H+] = (Kc)^0.5

1/[H+]= (1/K*1/c)^0.5

pH = log 1/[H+] pK = log 1/K


pH = (pK-logc)/2

here pH = 5.2 c=0.1 log c =-1

so 5.2=( pK +1)/2

pK = 9.4 and K =10^(-9.4)

Answer 3

pH=log[H+]
H+=10^-pH

hope u benefit:)