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### 4b practice midterm

Posted: Sun May 06, 2018 5:54 pm
" 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"

So sometimes to find frequency we use the equation (c(speed of light)/wavelength), which is what I when given wavelength, however I know that sometimes we also use the equation h*v(frequency)-work function=1/2*m*velocity^2. In what circumstances do you use either equation because I'll use (c(speed of light)/wavelength) and it ends up wrong to find frequency. This specifically pertains to question 4b on the practice midterm.

### Re: 4b practice midterm

Posted: Sun May 06, 2018 7:46 pm
$c = \lambda \nu$ is for electromagnetic radiation (light) only. The wavelength mentioned in this problem is that for a moving electron (something with mass), so you have to use de Broglie equation to get the desired information.

### Re: 4b practice midterm

Posted: Sun May 06, 2018 9:18 pm
I believe that you have to use the De Broglie equation for this problem, since the wavelength is given for a moving electron in this problem, in order to get the correct answer.

### Re: 4b practice midterm

Posted: Sun May 06, 2018 10:32 pm
How does work function fit into the problem? Is it only necessary in order to show us that this is a moving electron?

### Re: 4b practice midterm

Posted: Sun May 06, 2018 10:46 pm
It is a photoelectric question so you will need the photoelectric equation. You just need to use the de Broglie equation to get the velocity of the electron, hence the kinetic energy of the ejected electron in order to solve the entire problem.

### Re: 4b practice midterm

Posted: Tue May 08, 2018 9:54 pm
Do we need to take into account the fact that it says the work function is kJ*mol^-1? I know we have to convert the energy to J, but do we need to cancel out the moles?

### Re: 4b practice midterm

Posted: Tue May 08, 2018 11:55 pm
EllenRenskoff-1C wrote:Do we need to take into account the fact that it says the work function is kJ*mol^-1? I know we have to convert the energy to J, but do we need to cancel out the moles?

Yes, use Avogadro's number to do so.

### Re: 4b practice midterm

Posted: Tue May 08, 2018 11:58 pm
I am still very confused and keep getting the wrong answer for this problem. I found v from wavelength=h/mv and then did 1/2m (mass of an electron) v^2 plus the converted work function and got 3.00x10^14

### Re: 4b practice midterm

Posted: Wed May 09, 2018 12:04 am
Update: I forgot to add the kinetic energy to the work function before diving by h!

### Re: 4b practice midterm

Posted: Sun Jun 10, 2018 6:26 pm
I struggled with this one because I wasn't sure how to convert the work function. I just had to remember that it was in kJ/mol, and needed to be in J/ atom. This was confusing for me because it is normally just written as J. So I had to multiply it by 1000 and divide by 6.022*10^23

### Re: 4b practice midterm

Posted: Tue Nov 05, 2019 6:41 pm
How do we find the frequency once we've found the Energy using E = phi + 1/2mv^2 ? because the ones that have frequency we aren't supposed to use if there is mass involved i thought

### Re: 4b practice midterm

Posted: Tue Nov 05, 2019 7:06 pm
For this problem, you must use the De Broglie equation. De Broglie is typically used for an electron's wavelength with mass, while c=λν or E=hv is used for massless particles, like photons.