Ka of HCN = 4.9*10^-10
HCN dissociates as:
HCN -----> H+ + CN-
0.25 0 0
0.25-x x x
Ka = [H+][CN-]/[HCN]
Ka = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes
Ka = x*x/(c)
so, x = sqrt (Ka*c)
x = sqrt ((4.9*10^-10)*0.25) = 1.107*10^-5
since c is much greater than x, our assumption is correct
so, x = 1.107*10^-5 M
So, [H+] = x = 1.107*10^-5 M
use:
pH = -log [H+]
= -log (1.107*10^-5)
= 4.9559
Answer: 4.96
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