Atomic Structure-Exam Qs

Question 1

A sample of titanium contains four isotopes, 46Ti, 47Ti, 48Ti and 49Ti
This sample has a relative atomic mass of 47.8
In this sample the ratio of abundance of isotopes 46Ti, 47Ti and 49Ti is 2:2:1
Calculate the percentage abundance of 46Ti in this sample.

Answer 1

47.8 =(46x 2X)+(47x2X)+(48[100-5x])+(49xX)/100
235y+4800-240y=4780
5y=20
y=4
4x2=8

Question 2

In a TOF mass spectrometer, ions are accelerated to the same kinetic
energy (KE).
The kinetic energy of an ion is given by the equation
Where:
KE = kinetic energy / J
m = mass / kg
v = speed / m s–1
In a TOF mass spectrometer, each 84Kr+
ion is accelerated to a kinetic
energy of 4.83 × 10−16 J and the time of flight is 1.72 × 10−5 s

KE=o.5xMxV^2

Answer 2

m = (84/1000)/6.02 x 10^23 (= 1.395 x 10-25 kg)
4.83 × 10−16) /0.5 x (1.395 x 10-25)
v = √(6924731183) = 83214.97
d2 = 2 x (4.83 × 10−16) x (1.73 x 10–5)
= v x t = 83214.97 x 1.72 × 10−5 = 1.43 (m)
M4 = 1.44 (m)

Question 3

A sample of chromium containing the isotopes 50Cr, 52Cr and 53Cr has a
relative atomic mass of 52.1
The sample contains 86.1% of the 52Cr isotope.
Calculate the percentage abundance of each of the other two isotopes.

Answer 3

% of 50Cr and 53Cr = 13.9%
Let % of 53Cr = x% and Let % of 50Cr = (13.9 - x)%
52.1 =(50 x 13.9-X)+(52 x 86.1)+(53 x X)

3x = 37.8
M2 52.1 =
OR
3x = 3.9
1
M3 x = % of 53Cr = 12.6%
AQA Chemistry A-Level - Atomic Structure MS PhysicsAndMathsTutor.com
Page 5 of 15
1
M4 % of 50Cr = 1.3%

Question 4

Give two reasons why it is necessary to ionise the isotopes of chromium
before they can be analysed in a TOF mass spectrometer

Answer 4

(Ions will interact with and) be accelerated (by an electric field)
Ions create a current when hitting the detector