This question is about the isotopes of chromium - AQA - A-Level Chemistry - Question 2 - 2020 - Paper 1
Question 2
This question is about the isotopes of chromium.
0 2.1 Give the meaning of the term relative atomic mass.
0 2.2 A sample of chromium containing the isotopes 54Cr, ... show full transcript
Worked Solution & Example Answer:This question is about the isotopes of chromium - AQA - A-Level Chemistry - Question 2 - 2020 - Paper 1
Step 1
Give the meaning of the term relative atomic mass.
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Answer
Relative atomic mass is the average mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom.
Step 2
Calculate the percentage abundance of each of the other two isotopes.
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Answer
Let the abundance of 54Cr be x% and the abundance of 53Cr be (100 - 86.1 - x)%.
Using the equation for average atomic mass:
52.1=100(x×54)+(86.1×52)+((100−86.1−x)×53)
Set the equation:
52.1×100=x×54+86.1×52+(100−86.1−x)×53
Solve for x:
x = 12.6 ext{%} for 54Cr and 100 - 86.1 - 12.6 = 1.3 ext{%} for 53Cr.
Step 3
State, in terms of the numbers of fundamental particles, one similarity and one difference between atoms of 54Cr and 53Cr.
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Answer
Similarity: Both 54Cr and 53Cr have the same number of protons (24).
Difference: 54Cr has 30 neutrons, while 53Cr has 29 neutrons.
Step 4
Give two reasons why it is necessary to ionise the isotopes of chromium before they can be analysed in a TOF mass spectrometer.
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Answer
Ions are accelerated by an electric field, allowing them to gain kinetic energy.
Ions create a current when hitting the detector, essential for measured analysis.
Step 5
Calculate the time, in s, for the 52Cr ion to travel down the flight tube to reach the detector.
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Answer
Using the formula for kinetic energy:
KE=2mv2
Rearranging it gives
v=m2×KE
Where:
KE = 1.102 \times 10^{-3} J
m = 52 / 1000 kg (as 52Cr is approximately 52 g/mol and using the molar mass)
After calculating speed, use:
d=vt
Where d is the distance (1.25 m) to find time:
t=vd
