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Zoeva123
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I've just started my a levels so this may seem easy but I've been struggling on this question for a while. Any help is massively appreciated x

A sample of bromine was analysed in a TOF mass spectrometer and found to contain two isotopes, bromine-79 and bromine-81.
After electron impact ionisation, all of the ions were accelerated to the same kinetic energy and then travelled through a flight tube that was 0.950m long.

The bromine-79 ions took 6.69x10^-4 to travel through the flight tube

1)Calculate the mass in kg of one ion of bromine-79
2) Calculate the time taken for the bromine-81 ions to travel through the same flight tube
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Guarddyyy
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This took me a few minutes to calculate - I don't blame you for not being able to calculate it. I'm in year 13 so after a year's worth of practising chemistry on an A-level standard, you will be able to spot key points.

1) 1 mole of bromine-79 ions weigh 79 grams. Convert the grams to kg and divide by Avogadro's constant (6.02*10^23).

2) Use the information about the same kinetic energy to solve the question. KE = 1/2 mv^2.

You have the mass of 1 bromine-79 ion, the TOF and the length of the tube. Using the TOF and the length of the tube, you can calculate speed (v=s/t) Substitute the values of v and s in to calculate its velocity, and substitute the velocity of it into KE = 1/2 mv^2, as well as the mass of 1 ion of bromine-79.

Now you use the fact that the EK's are the same, so the EK of bromine-81 is the same. You can do what you did in part 1 to calculate the mass (in kg again) of the bromine -81, and rearrange the EK to give you v. Afterwards you can plug the value of v back into the equation of (t=s/v) and you should have an answer of TOF.


If you have any questions, let me know! I'll be happy to help (or try to).
Last edited by Guarddyyy; 4 months ago
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Zoeva123
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(Original post by Guarddyyy)
This took me a few minutes to calculate - I don't blame you for not being able to calculate it. I'm in year 13 so after a year's worth of practising chemistry on an A-level standard, you will be able to spot key points.

1) 1 mole of bromine-79 ions weigh 79 grams. Convert the grams to kg and divide by Avogadro's constant (6.02*10^23).

2) Use the information about the same kinetic energy to solve the question. KE = 1/2 mv^2.

You have the mass of 1 bromine-79 ion, the TOF and the length of the tube. Using the TOF and the length of the tube, you can calculate speed (v=s/t) Substitute the values of v and s in to calculate its velocity, and substitute the velocity of it into KE = 1/2 mv^2, as well as the mass of 1 ion of bromine-79.

Now you use the fact that the EK's are the same, so the EK of bromine-81 is the same. You can do what you did in part 1 to calculate the mass (in kg again) of the bromine -81, and rearrange the EK to give you v. Afterwards you can plug the value of v back into the equation of (t=s/v) and you should have an answer of TOF.


If you have any questions, let me know! I'll be happy to help (or try to).
Thank you sooo much. I literally sat down for ages trying to work it out but my answer to question 2 was way too big. Turns out I forgot to convert the grams into kg. massively appreciate it xx
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