## Q31 from online assessment

Rita Dang 3D
Posts: 31
Joined: Fri Sep 29, 2017 7:07 am

### Q31 from online assessment

Light hits a sodium metal surface and the velocity of the ejected electron is 6.61 x 105 m.s-1. The work function for sodium is 150.6 kJ.mol-1.

A. What is the kinetic energy of the ejected electron?
B. How much energy is required to remove an electron from one sodium atom?
C. What is the frequency of the incident light on the sodium metal surface?

I got the correct answer for both A and B. And it shows my answer for part C ( which is 2.27x10^38Hz) was incorrect. But I could not figure out what I did wrong. Can anyone tell me the right answer? Thanks!

Chem_Mod
Posts: 18400
Joined: Thu Aug 04, 2011 1:53 pm
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### Re: Q31 from online assessment

Hi Rita,

Can you explain/show how you attempted to solve for part C?

Rita Dang 3D
Posts: 31
Joined: Fri Sep 29, 2017 7:07 am

### Re: Q31 from online assessment

These are my steps: hv-150600J.mol^-1=1.99x10^-19J
hv=150600J
v=2.27x10^38HZ
So I basically plugged numbers into the formula E(photon)-E(energy remove e-)=E_k(e-)

An Dang 3F
Posts: 20
Joined: Fri Sep 29, 2017 7:07 am

### Re: Q31 from online assessment

First you want to identify what the problem is giving you. It says that the velocity of the electron is 6.61 x 105 m.s-1 and the work function is 150.6 kJ.mol-1. You need to convert 150.6 kJ.mol-1. to electrons by dividing 150.6 kJ.mol-1. by 6.022*10^23 convert the value into joules, so it would be 2.50*10^-19 J. Then you use the formula for kinetic energy which is E = hv - (work function). Then you can just solve for the frequency by plugging in the numbers that you solved for A and B.

Chem_Mod
Posts: 18400
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 435 times

### Re: Q31 from online assessment

Hey, Rita its Ajith,
I see what you did wrong in part C.
So first you need to convert the work function into J/atom. Do this by dividing by Avogadro's number. This is because in part B they specified the question for 1 sodium atom.
Hope this helps!