TOF maths q

A molecule Q is ionised by electron impact in a TOF mass spectrometer.
The Q+ ion has a kinetic energy of 2.09 x 10–15 J
This ion takes 1.23 x 10–5 s to reach the detector.
The length of the flight tube is 1.50 m
Calculate the relative molecular mass of Q.
KE =1/2mv2
where m = mass (kg) and v = speed (m s –1)
The Avogadro constant, L = 6.022 x 1023 mol–1

Mark scheme
M1 V = d/t or = 1.22 x 10^5 ms-1
M2 m = 2KE or 2 x 2.09 x 10–15
v2 (1.22 x 10^5)2
or
M2 m = 2KE x t2 or 2 x 2.09 x 10–15 x (1.23 x 10 –5)
2
d2
1.502
M3 m = 2.8(1) x 10-25 (kg)
M4 = 2.81 x 10-25 x L = 0.169
M5 0.169 x 1000 = 169.(2)

Hi, Could you explain Why you times the mass by avogrados constant to find the Mr? I cant seem to find the equation linking these. @TypicalNerd
Thanks

(edited 11 months ago)

Reply 1

DanielK1456

10

Not sure, but if the number of ions is moles x avogadro's constant and there is only one ion then moles x avogadro's constant is 1. Moles is Mass/Mr so rearranging gives Mass x Avogadro's constant = 1xMr = Mr. I hope this is right.

Reply 2

TypicalNerd

18

Original post by BCGscience

A molecule Q is ionised by electron impact in a TOF mass spectrometer.
The Q+ ion has a kinetic energy of 2.09 x 10–15 J
This ion takes 1.23 x 10–5 s to reach the detector.
The length of the flight tube is 1.50 m
Calculate the relative molecular mass of Q.
KE =1/2mv2
where m = mass (kg) and v = speed (m s –1)
The Avogadro constant, L = 6.022 x 1023 mol–1

Mark scheme
M1 V = d/t or = 1.22 x 10^5 ms-1
M2 m = 2KE or 2 x 2.09 x 10–15
v2 (1.22 x 10^5)2
or
M2 m = 2KE x t2 or 2 x 2.09 x 10–15 x (1.23 x 10 –5)
2
d2
1.502
M3 m = 2.8(1) x 10-25 (kg)
M4 = 2.81 x 10-25 x L = 0.169
M5 0.169 x 1000 = 169.(2)

Hi, Could you explain Why you times the mass by avogrados constant to find the Mr? I cant seem to find the equation linking these. @TypicalNerd
Thanks

You could infer it dimensionally by looking at the units of each quantity if you don’t know of an equation:

-Avogadro’s constant has the units mol^-1

-Masses are given in kg or g.

-Relative masses usually have the units g mol^-1, but in this case, they’ve calculated it in kg mol^-1 and converted it in the next step.

To get a quantity measured in kg mol^-1, you have to multiply a quantity in kg by a quantity in mol^-1. As such, mass x L gives a correct unit, so you can be reasonably sure it works.

Reply 3

TypicalNerd

18

Original post by DanielK1456

Not sure, but if the number of ions is moles x avogadro's constant and there is only one ion then moles x avogadro's constant is 1. Moles is Mass/Mr so rearranging gives Mass x Avogadro's constant = 1xMr = Mr. I hope this is right.

You can also use the equation N = mol x L, where N = 1. This is in fact how the equation mass x Na = Mr is formally derived.

TOF maths q

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you