Post-assessment problem

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Post-assessment problem

Postby manasa933 » Sun Oct 08, 2017 4:31 pm

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 KE?
b. How much is the energy required to remove an electron from one sodium atom?
c. What is the frequency of the incident light?

Don't we need mass of the metal to find out KE?
The answer for 'b' isn't the work function given, but isn't the work function the energy required to remove an electron?

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Re: Post-assessment problem

Postby Chem_Mod » Sun Oct 08, 2017 7:36 pm

For part a, it is asking for the kinetic energy of the ELECTRON, so it is 1/2mv^2 of the ELECTRON. No mass for the metal is needed.
For part b, please take a careful look of the work function given, it is in kJ/mol, and the question is asking for the energy required to remove ONE SINGLE electron, and in Joules. You will need to do some conversions for this.

Ammar Amjad 1L
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Re: Post-assessment problem

Postby Ammar Amjad 1L » Fri Oct 13, 2017 7:22 pm

For part a to calculate the KE you would use the formula KE = 1/2mv^2. You need the mass of the electron which is 9.109 x 10^-31 kg. From this you can just plug in the velocity, and solve to find the answer. Hope this helps.

Cassandra Mullen 1E
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Re: Post-assessment problem

Postby Cassandra Mullen 1E » Sun Oct 15, 2017 1:40 pm

For Part C, you need to combine several equations to find the frequency. By combining KE= E-threshold, E=hv, and KE=1/2mv^2, you get

frequency= (1/2mv^2 + threshold)/ h (Planck's constant)

Remember to use the mass of an electron: 9.11 x 10^-31 kg for the mass and to square velocity when plugging into the calculator.

You should get 6.78 x 10^14 Hz.

Hope this helps!

PS work out the "combined equation" since this it might sound confusing

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