## How to calculate the kinetic energy, energy and frequency? [ENDORSED]

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

### How to calculate the kinetic energy, energy and frequency?

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? If you can, could you give me a step by step explanation on how to solve this?

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

### Re: How to calculate the kinetic energy, energy and frequenc  [ENDORSED]

The steps to solve the problem are:
1. Use Ek = 1/2 mv^2 2.

2. Divide the total energy (work function) by Avogadro's number.

3. Use: kinetic energy of e- = energy of incoming photon – work function ****In addition to covering this in class, the course reader has detailed examples as does the textbook.

derek1d
Posts: 19
Joined: Wed Jun 28, 2017 3:00 am

### Re: How to calculate the kinetic energy, energy and frequency?

Won't the mass of the electron need to be given in order to solve A) of this problem?

diegomartinez1F
Posts: 4
Joined: Wed Nov 18, 2015 3:00 am

### Re: How to calculate the kinetic energy, energy and frequency?

Mass of electron is given:

Me= 9.10938 x 10^-31 kg

Jacinda Wollenweber 1D
Posts: 22
Joined: Wed Nov 16, 2016 3:02 am

### Re: How to calculate the kinetic energy, energy and frequenc

Chem_Mod wrote:The steps to solve the problem are:
1. Use Ek = 1/2 mv^2 2.

2. Divide the total energy (work function) by Avogadro's number.

3. Use: kinetic energy of e- = energy of incoming photon – work function ****In addition to covering this in class, the course reader has detailed examples as does the textbook.

Why do we have to divide the work function by Avogrado's number? I thought the energy was given simply by adding the kinetic energy and threshold energy together.

Sarah_Wilen
Posts: 62
Joined: Fri Jun 23, 2017 11:39 am
Been upvoted: 4 times

### Re: How to calculate the kinetic energy, energy and frequency?

This is because the work function's energy is given in kJ/mol. You need to solve for the energy of a single unit. That was very poorly explained, sorry. Refer to this other answer:
viewtopic.php?f=14&t=21193

Kevin Morden 1E
Posts: 7
Joined: Fri Sep 29, 2017 7:04 am
Been upvoted: 1 time

### Re: How to calculate the kinetic energy, energy and frequency?

diegomartinez1F wrote:Mass of electron is given:

Me= 9.10938 x 10^-31 kg

Is the mass of an electron always going to be the same?

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

### Re: How to calculate the kinetic energy, energy and frequency?

Yes, the mass of an electron is always going to be the same.

Sohini Halder 1G
Posts: 58
Joined: Thu Jul 13, 2017 3:00 am
Been upvoted: 1 time

### Re: How to calculate the kinetic energy, energy and frequency?

As Dr. Lavelle said in lecture, it always helps to write out the equation in words first.
For example:
Energy of incoming photon = Workfunction of metal (or energy needed to remove electron) + Kinetic energy of the electron emitted
After this, you can rearrange whatever way you need to for the problem.

Kevin Hernandez 3A
Posts: 21
Joined: Fri Sep 29, 2017 7:06 am

### Re: How to calculate the kinetic energy, energy and frequenc

Chem_Mod wrote:The steps to solve the problem are:
1. Use Ek = 1/2 mv^2 2.

2. Divide the total energy (work function) by Avogadro's number.

3. Use: kinetic energy of e- = energy of incoming photon – work function ****In addition to covering this in class, the course reader has detailed examples as does the textbook.

For Part c, would you subtract the work function of the sodium metal or subtract the work function of the one sodium atom?

gabbym
Posts: 76
Joined: Wed Apr 11, 2018 3:00 am

### Re: How to calculate the kinetic energy, energy and frequency?

This was so helpful, thank you!

Ashley P 4I
Posts: 64
Joined: Wed Nov 15, 2017 3:04 am

### Re: How to calculate the kinetic energy, energy and frequency?

At last I finally understood what this problem was asking, thank you so much!

Liza Hayrapetyan-3K
Posts: 31
Joined: Fri Sep 28, 2018 12:25 am

### Re: How to calculate the kinetic energy, energy and frequency?

Thank you so much! This was extremely helpful.

Andrew Bennecke
Posts: 62
Joined: Fri Sep 28, 2018 12:15 am

### Re: How to calculate the kinetic energy, energy and frequency?

Yiting_Gong_4L
Posts: 69
Joined: Fri Sep 28, 2018 12:25 am

### Re: How to calculate the kinetic energy, energy and frequency?

Thank you for this explanation! This was very helpful.