Question

The ka for hcn is 4.9 ⋅ 10-10. What is the value of kb for cn-?

Answer 1

Answer:

kb = 2,0x10⁻⁵

Explanation:

The ka for HCN is:

HCN ⇄ H⁺ + CN⁻; ka = 4,9x10⁻¹⁰ (1)

The inverse reaction has an equilibrium constant of:

H⁺ + CN⁻ ⇄ HCN k = 1/4,9x10⁻¹⁰ = 2,0x10⁹ (2)

As the equilibrium of the water is:

H₂O ⇄ H⁺ + OH⁻; kw = 1x10⁻¹⁴ (3)

The sum of (2) + (3) gives:

H₂O + CN⁻ ⇄ HCN + OH⁻; kb = kw×k = 1x10⁻¹⁴×2,0x10⁹ =

2,0x10⁻⁶; kb = 2,0x10⁻⁵

-In fact, the general formula to convert from ka to kb is:

kb = kw / ka-

I hope it helps!