Find the ph of 0.135m nacn solution. for hcn, ka=4.9⋅10−10,

NaCN is considered as salt of weak acid (HCN) and strong base (NaOH):
we have salt concentration Cs = 0.135 M NaCN and Ka = 4.9 x 10⁻¹⁰ 
use this formula to calculate pH:
pH = 1/2 pKw + 1/2 pKa - 1/2 pCs
pKw = - log Kw,    pKa = - log Ka        and pCs = - log Cs
pKw = - log (1 x 10⁻¹⁴) = 14
pKa = - log (4.9 x 10⁻¹⁰) = 9.31
pCs = - log (0.135) = 0.87
pH = (1/2 x 14) + (1/2 x 9.31) - (1/2 x 0.87) = 11.22 
Note that it is basic solution (pH more than 7) because it consists of strong base and weak acid