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The energy required to ionize sodium is 496 kJ /mol. What minimum frequency of light is required to ionize sodium?
##1.24 * 10^(15)"Hz"##
Your strategy here will be to

use Avogadro's number to find the energy needed to ionize one atom of sodium

use the Planck  Einstein equation to find the frequency of light that corresponds to that specific energy
So, you know that the energy needed to ionize sodium is equal to ##"496 kJ/mol"##. As you know, one mole of any element contains exactly ##6.022 * 10^(23)## atoms of that element  this is known as Avogadro's number.
In your case, the energy needed to ionize one atom of sodium will be equal to
##496 color(red)(cancel(color(black)("kJ")))/color(red)(cancel(color(black)("mol"))) * (10^3 "J")/(1 color(red)(cancel(color(black)("kJ")))) * (1color(red)(cancel(color(black)("mol"))))/(6.022 * 10^(23)"atoms") = 8.236 * 10^(19)"J/atom"##
The relationship that exists between energy and frequency is described by the Planck  Einstein equation
##color(blue)(E = h * nu)" "##, where
##E##  the energy of the wave ##h##  , equal to ##6.626 * 10^(34)"J s"## ##nu##  the frequency of the wave
Plug in your values and solve for ##nu##, the frequency of light needed to ionize a sodium atom
##E = h * nu implies nu = E/h##
##nu = (8.236 * 10^(19) color(red)(cancel(color(black)("J"))))/(6.626 * 10^(34)color(red)(cancel(color(black)("J"))) "s") = 1.243 * 10^15"s"^(1)##
Since you have
##"1 Hz" = "1 s"^(1)##
you can say that the answer will be
##nu = color(green)(1.24 * 10^(15)"Hz") >## rounded to three