The energy required to ionize a potassium ion is $419$ $\mathrm{kJ}\cdot {\mathrm{mol}}^{-1}$. What is the longest wavelength of light that can cause this ionization?

Here, we have,

$419\times {10}^3=\frac{\mathrm{hc}}{\mathrm{\lambda }}\times {\mathrm{N}}_{\mathrm{A}}$

After putting the value of planks constant and speed of light, the value energy required is equal to 285 nm.