Page 1 of 1

Midterm Dino Nuggets worksheet 8b

Posted: Tue Nov 05, 2019 5:24 pm
by Mansi_1D
A newly designed laser pointer with a certain frequency is pointed at a sodium metal surface. An electron is ejected from the metal surface with wavelength 1.10 nm. What is the frequency of the light from the laser pointer? The work function of sodium is 150.6 kJ∙mol-1.



Can someone walk me through the steps to solve this problem please? The correct answer is 6.78x10^14 HZ and I tried different ways but I'm not getting to the answer. Please help!

Re: Midterm Dino Nuggets worksheet 8b

Posted: Tue Nov 05, 2019 11:35 pm
by nicolely2F
In broad terms, you need to figure out the total energy of one photon from the incident light to then discover the light's frequency. In more detail:
1. You're given the ejected electron's wavelength, and this tells you that there is excess energy (i.e. kinetic energy). Use this wavelength to calculate the electron's velocity using the De Broglie's equation. You'll get V(ejected electron) = 6.68e5 m/s
2. Use this velocity V to figure out the electron's kinetic energy. You'll get E(kinetic of ejected electron) = 2e-19J
3. Next, you should plug that into the general equation E(photon) = E(threshold) + E(kinetic). However, the E(threshold) you're given is for each mole of sodium, not each atom. Convert 150.6 kJ/mol to J/atom by using Avogadro's number. You'll get E(threshold of electron) = 2.5e-19J.
4. Now plug the numbers from #2 and #3 into the equation E(photon) = E(threshold) + E(kinetic). E(photon) is also equal to h.v, so h.v = = E(threshold) + E(kinetic). Solve that and you'll get 6.78e14 Hz.

Re: Midterm Dino Nuggets worksheet 8b

Posted: Tue Nov 05, 2019 11:40 pm
by Micah3J
nicolely3B wrote:In broad terms, you need to figure out the total energy of one photon from the incident light to then discover the light's frequency. In more detail:
1. You're given the ejected electron's wavelength, and this tells you that there is excess energy (i.e. kinetic energy). Use this wavelength to calculate the electron's velocity using the De Broglie's equation. You'll get V(ejected electron) = 6.68e5 m/s
2. Use this velocity V to figure out the electron's kinetic energy. You'll get E(kinetic of ejected electron) = 2e-19J
3. Next, you should plug that into the general equation E(photon) = E(threshold) + E(kinetic). However, the E(threshold) you're given is for each mole of sodium, not each atom. Convert 150.6 kJ/mol to J/atom by using Avogadro's number. You'll get E(threshold of electron) = 2.5e-19J.
4. Now plug the numbers from #2 and #3 into the equation E(photon) = E(threshold) + E(kinetic). E(photon) is also equal to h.v, so h.v = = E(threshold) + E(kinetic). Solve that and you'll get 6.78e14 Hz.


Thanks, this helped a lot.