AS Unit 1 Lesson 3. Mass Spec.
Q1. A gaseous sample of chromium can be analysed in a mass spectrometer. Before deflection, the
chromium atoms are ionised and then accelerated.
(a) Describe briefly how positive ions are formed from gaseous chromium atoms in a mass
spectrometer.
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(2)
(b) What is used in a mass spectrometer to accelerate the positive ions?
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(1)
(c) What is used in a mass spectrometer to deflect the positive ions?
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(1)
(d) The mass spectrum of a sample of chromium shows four peaks. Use the data below to
calculate the relative atomic mass of chromium in the sample. Give your answer to two
decimal places.
m/z
50
52
53
54
Relative abundance / %
4.3
83.8
9.5
2.4
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(2)
(Total 6 marks)
Q2.The mass spectrum of a sample of krypton taken from a meteorite is shown below.
(a) Use this spectrum to calculate the relative atomic mass of this sample of krypton.
Give your answer to one decimal place.
Explain why the value you have calculated is slightly different from the relative atomic
mass given in the Periodic Table.
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(Extra space) .................................................................................................
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(4)
M1. (a) High speed electrons OR electrons from an electron gun (1)
Knocks out an (outer-shell) electron (on the chromium atom) (1)
Accept Cr(g) + e– → Cr+(g) + 2e–
NOT e– gun alone / beam of e– / bombardment with e–
2
(b) Electric field OR (attraction to) -ve plate OR electrostatic attraction (1)
OR (repelled by) +ve plate OR charged plate
NOT high p.d. / electromagnetic field / electric plates
1
(c) Magnet OR magnetic field OR electromagnet(1)
1
(d) Ar = (50 × 0.043) + (52 × 0.838) + (53 × 0.095) + (54 × 0.024) (1)
52.06 OR 52.05 (1)
Mark consequentially on transcription, or addition of %, error
2
[6]
M2.(a)
M1 for the top line
M2 is for division by 17
1
1
= 84.0
Not 84
No consequential marking from M1 or M2
Ignore units
1
The Ar in the Periodic table takes account of the other isotopes /
different amounts of isotopes (or words to that effect regarding isotopes)
Award independently
Comparison implied
Isotope(s) alone, M4 = 0
1
E1. This question was, on the whole, well answered, although it was not uncommon to see vague
references to electrons being removed by an electron gun in part (a). Parts (b) and (c) were
generally well done, with the odd incorrect reference in (b) to magnets. Most candidates were
familiar with the technique for calculating the Ar value of chromium but a significant number
failed to quote their answer to 2 decimal places, as required by the question. A few arithmetical
errors were also seen, and a small number of candidates omitted to divide the percentages by
100.
E2.The majority of students calculated the relative atomic mass of krypton but then failed to quote
their answer to the precision stated in the question. The last mark in part (a) proved more
difficult with many students suggesting errors in measurements or precision. Part (b) gave a
good spread of marks. The first three marks were gained by most students but some failed to
give state symbols in their equation. The last two marks were more difficult and many
references to the atom being split in two were seen rather than being ionised twice.