The ionization energy of sodium in kJ mol-1, if electromagnetic radiation of wavelength 242 nm is just sufficient to ionize sodium atom is .
Answer
494
Solution
Energy per mole of photons: E = NA hc
= 6.022 1023 6.626 10-34 2.998 108242 10-9 J mol-1
= 4.943 105 J mol-1 494 kJ mol-1
∵ Radiation "just sufficient" to ionise ⇒ photon energy = ionisation energy
Answer: 494