chemistry pH question?

[fashion_fever]

what is the pH of a 0.140M NaOCN solution? ka HOCN=3.5x10^-4
(rule of 100 can simplify 0.140-x to 0.140)

i know that ka=HOCN*OH^-/OCN^-=x^2/0.140
but i don't know the left hand side...i don't know what to do with HOCN=3.5x10^-4..

[Borek]

OCN- is a weak, conjugate base of HOCN. Calculate Kb, calculate pOH from known Kb and concentration, convert to pH.

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