Calculate the pH of a buffer solution prepared by dissolving

Calculate the pH of a buffer solution prepared by dissolving 0.20 mole of sodium cyanate (NaCNO) and 1.0 mole of cyanic acid (HCNO) in enough water to make 1.0 liter of solution. [Ka (HCNO) = 2.0 times 10-4]

Solution

according to henderson hasselbalch equation

pH = pKa + log(Salt / Acid)

pKa = - log Ka = 3.7

Salt = 0.2 M

Acid = 1 M

pH = 3.7 + log (0.2 / 1)

=> pH = 3