Question

A buffer solution is 0.417 M in HCN and 0.326 M in NaCN . If K_a for HCN is 4.0×10^-10, what is the pH of this buffer solution?

A buffer solution is 0.418 M in H2SO3 and 0.253 M in NaHSO3. If K_a1 for H2SO3 is , what is the pH of this buffer solution?

pH =

Answer 1

Answer – a) We are given, [HCN] = 0.417 M and [NaCN] = [CN^-] = 0.326 M

K_a(HCN) = 4.0 x 10^-10

We are given acid and its conjugate base , so we can use Henderson Hasselbalch equation-

pH = pKa + log [Conjugate base] / [acid]

pH = pKa + log [CN^-] / [HCN]

Now we need to calculate the pKa from the given Ka

We know formula,    

pKa = - log Ka

        = - log 4.0 x 10^-10

        = 9.39

Now we need to plug the values in the above formula

pH = 9.39 + log 0.326 / 0.417

     = 9.29

so, the pH of this buffer solution is 9.29.

b) We are given, [H₂SO₃] = 0.418 M and [HSO₃^-] = 0.253 M

K_a(HCN) = 1.54 x 10^-2

We are given acid and its conjugate base , so we can use Henderson Hasselbalch equation-

pH = pKa + log [Conjugate base] / [acid]

pH = pKa + log [HSO₃^-] / [H₂SO₃]

Now we need to calculate the pKa from the given Ka

We know formula,    

pKa = - log Ka

        = - log 1.54 x 10^-2

        = 1.81

Now we need to plug the values in the above formula

pH = 1.81 + log 0.253/ 0.418

     = 1.59

so, the pH of this buffer solution is 1.59.