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