HCN dissociates as:
HCN -----> H+ + CN-
5.5*10^-2 0 0
5.5*10^-2-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)*5.5*10^-2) = 5.191*10^-6
since c is much greater than x, our assumption is correct
so, x = 5.191*10^-6 M
So, [H+] = x = 5.191*10^-6 M
use:
pH = -log [H+]
= -log (5.191*10^-6)
= 5.2847
Answer: 5.28
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