Question

What is the pH of 40.0 mL of a solution that is 0.15 M in CN� and 0.29 M in HCN? For HCN, use Ka = 4.9 * 10^-9

Answer 1

This is a buffer solution because it consists of a weak acid (HCN) and its conjugate base (CN-)
Since it's a buffer solution, the pH is equal to the pKa of the solution, you would use the henderson-hasslebach equation:
pH = pKa - log([acid]/[base])
pH = -log(4.9*10-9) - log(.29/.15)
pH = about 8