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1. Hi im doing a chemistry worksheet and some hard question came up that i have no idea how to solve. Ive been looking at it for at least 20 minutes now and nothing

The Question

Calculate the approximate time (in seconds) an 18 O 4 molecule would take to travel the 10cm path inside a TOF mass spectrometer if initially accelerated to 4.00keV.
*18 O 4 = (superscript) 18 O (oxygen) 4 (subscript)
Useful equations:
1keV= 1.602 x 10^-19 J
Avogadros constant = 6.022 x 10^23
KE = 1/2mv^2
v=d/t
2. How much velocity will 4keV give the molecule?
3. (Original post by alow)
How much velocity will 4keV give the molecule?
I dont get what you mean by that?...
4. (Original post by Teukjae707)
I dont get what you mean by that?...
There's an equation that links energy and velocity together.

He meant how much velocity would the atom get from 4 keV energy

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5. (Original post by Teukjae707)
Hi im doing a chemistry worksheet and some hard question came up that i have no idea how to solve. Ive been looking at it for at least 20 minutes now and nothing

The Question

Calculate the approximate time (in seconds) an 18 O 4 molecule would take to travel the 10cm path inside a TOF mass spectrometer if initially accelerated to 4.00keV.
*18 O 4 = (superscript) 18 O (oxygen) 4 (subscript)
Useful equations:
1keV= 1.602 x 10^-19 J
Avogadros constant = 6.022 x 10^23
KE = 1/2mv^2
v=d/t
I think you first have to use the formula relating to Avogadros constant to find the mass, which you could use in the kinetic energy equation.
6. What exam board are you with? This is filed under AS level, and I know my AS was nowhere near as difficult as this 😂
7. well i got 3.86x10-6 seconds. The only thing is, i think your data is wrong, 1kev does not equal1.602x10^-19J. That is 1 ev, not 1 kev.

what did everyone else get?
8. (Original post by EvilScientist)
well i got 3.86x10-6 seconds. The only thing is, i think your data is wrong, 1kev does not equal1.602x10^-19J. That is 1 ev, not 1 kev.

what did everyone else get?
What was your first step?
9. (Original post by derpz)
What was your first step?

KE=4 x1.602x10^-19x1000
=6.408x10-16 J

mass=(18x4)/6.022x10^23
This mass is in grams so divide mass by 1000.
1.1956x10^-25 kg

Sub in equation and rearrange.
v^2=1.0719x10^10

v=103533

time=0.1m/103533

time=9.66x10^-7 seconds.

Whoops I must have miscalculated in my previous answer.
10. (Original post by EvilScientist)
KE=4 x1.602x10^-19x1000
=6.408x10-16 J

mass=(18x4)/6.022x10^23
This mass is in grams so divide mass by 1000.
1.1956x10^-25 kg

Sub in equation and rearrange.
v^2=1.0719x10^10

v=103533

time=0.1m/103533

time=9.66x10^-7 seconds.

Whoops I must have miscalculated in my previous answer.
Didnt read the question properly and thought it was 18O4 (with the 18 not in superscript).

Your working looks correct though
11. (Original post by derpz)
Didnt read the question properly and thought it was 18O4 (with the 18 not in superscript).

Your working looks correct though
Yeah hahaha i thought it was 18 moles of O4 as well, I didn't realise it was the Mass number at first
12. (Original post by sambradley)
What exam board are you with? This is filed under AS level, and I know my AS was nowhere near as difficult as this 😂
Its the new reformed AS so i think its a lot harder
13. (Original post by Teukjae707)
Its the new reformed AS so i think its a lot harder
And theres me thinking chemistry is hard currently. My younger siblings are gonna have such a fun time. My sister is 10 and has an english revision guide.
14. (Original post by sambradley)
What exam board are you with? This is filed under AS level, and I know my AS was nowhere near as difficult as this 😂
This is a very basic AS level question.
15. (Original post by alow)
This is a very basic AS level question.
Are you doing AS levels now too? Or have you already done them?
Our teachers said this would be one of the hardest questions in the exam.
The rest are hard but are doable with practice and some thinking
16. (Original post by Teukjae707)
Are you doing AS levels now too? Or have you already done them?
Our teachers said this would be one of the hardest questions in the exam.
The rest are hard but are doable with practice and some thinking
I did ASs a few years ago. This is simply a question where you are given the energy of an object, work out the velocity using and then it's just to work out the time taken.
17. (Original post by alow)
I did ASs a few years ago. This is simply a question where you are given the energy of an object, work out the velocity using and then it's just to work out the time taken.
The only part I get stuck on is the first part where you need to find the mass, I cant remember the equation with Avogadro's constant (that is the one you use right?)
18. (Original post by derpz)
The only part I get stuck on is the first part where you need to find the mass, I cant remember the equation with Avogadros constant (that i the one you use right?)
The mass of one mole of 18O4 is 18g.
19. (Original post by alow)
The mass of one mole of 18O4 is 18g.
the mass of one mole of an atom of O is 16g so that cant be right
im sure it means the mass of one mole of O4 is 72g.
20. (Original post by Lola1244)
the mass of one mole of an atom of O is 16g so that cant be right
im sure it means the mass of one mole of O4 is 72g.
Yes that's correct.

This question is really trivial, I'm not sure what everyone's getting their knickers in a twist about.
Just calculate the velocity and use v=d/t

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