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Chemistry tough question

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

Scroll to see replies

Reply 1

alow

How much velocity will 4keV give the molecule?

Reply 2

Teukjae707

Original post by alow

How much velocity will 4keV give the molecule?

I dont get what you mean by that?... :frown:

Reply 3

chemting

Original post by Teukjae707

I dont get what you mean by that?... :frown:

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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Reply 4

derpz

Original post by Teukjae707

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.

Reply 5

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 😂

Reply 6

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?

(edited 8 years ago)

Reply 7

derpz

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?

Reply 8

EvilScientist

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.

Reply 9

derpz

Original post by EvilScientist

Didnt read the question properly and thought it was 18O4 (with the 18 not in superscript).

Your working looks correct though

Reply 10

EvilScientist

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 :laugh:

Reply 11

Teukjae707

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 :frown:

Reply 12

sambradley

Original post by Teukjae707

Its the new reformed AS so i think its a lot harder :frown:

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.

Reply 13

alow

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.

Reply 14

Teukjae707

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

Reply 15

alow

Original post by Teukjae707

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

E_k=\frac12 mv^2

and then it's just

\text{time}=\frac{\text{distance}}{\text{speed}}

to work out the time taken.

Reply 16

derpz

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

E_k=\frac12 mv^2

and then it's just

\text{time}=\frac{\text{distance}}{\text{speed}}

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?)

(edited 8 years ago)

Reply 17

alow

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.

Reply 18

Lola1244

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.

Reply 19

langlitz

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