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

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

This was so helpful, thank you!

Ashley P 4I
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### 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
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### Re: How to calculate the kinetic energy, energy and frequency?

Thank you so much! This was extremely helpful.

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

Yiting_Gong_4L
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### Re: How to calculate the kinetic energy, energy and frequency?

Thank you for this explanation! This was very helpful.

AustinMcBrideDisc3C
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### Re: How to calculate the kinetic energy, energy and frequency?

diegomartinez1F wrote:Mass of electron is given:

Me= 9.10938 x 10^-31 kg

Do we need to convert this to grams or can we keep it as kg?

Moura Girgis 1D
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### Re: How to calculate the kinetic energy, energy and frequency?

AustinMcBrideDisc3C wrote:
diegomartinez1F wrote:Mass of electron is given:

Me= 9.10938 x 10^-31 kg

Do we need to convert this to grams or can we keep it as kg?

We need to keep it in kg, as all of the other units include kg, such as Joules (kg.m^2/s^2)

clairehathaway 3L
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### Re: How to calculate the kinetic energy, energy and frequency?

derek1d wrote:Won't the mass of the electron need to be given in order to solve A) of this problem?

This might be kinda obvious, but kinetic energy only applies to electrons right? So whenever we use the equation $KE = \frac{1}{2}mv^{2}$
the m will always represent the mass of an electron?

Chudi Onyedika 1C
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### Re: How to calculate the kinetic energy, energy and frequency?

clairehathaway 3L wrote:
derek1d wrote:Won't the mass of the electron need to be given in order to solve A) of this problem?

This might be kinda obvious, but kinetic energy only applies to electrons right? So whenever we use the equation $KE = \frac{1}{2}mv^{2}$
the m will always represent the mass of an electron?

The kinetic energy equation can be applied to any object, but in this case, the mass should represent the mass of an electron.

Giselle_zamora_3E
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### Re: How to calculate the kinetic energy, energy and frequenc

So for part A, are there any additional steps to take after doing the 1/2 mv? Because I did that and I got a fairly large number that wasn't any of the options but I don't know where I went wrong.
Kevin Hernandez 3A wrote:
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?

Giselle_zamora_3E
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### Re: How to calculate the kinetic energy, energy and frequenc

for part B, is there any particular reason why we should divide by Avogadro's number? I thought that the work function was the minimum amount needed to free the electron so that it would be a given?
Kevin Hernandez 3A wrote:
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?