Fig. 1 shows the output from a mass spectrometer used to check a cylinder of neon gas before it is used in a laboratory reaction. The sample was vaporised, ...
Question 1 Report
Fig. 1 shows the output from a mass spectrometer used to check a cylinder of neon gas before it is used in a laboratory reaction. The sample was vaporised, ionised and accelerated. Each atom formed an ion with a charge of +1. The peaks are caused by atoms with different numbers of neutrons, not by different elements. The horizontal scale gives mass-to-charge ratio, m/z. The diagram is not to scale. A student also has a separate sodium chloride solution for another investigation, but it must not be mixed with the gas sample.
(a) State the number of neutrons in ion A. [2]
(b) Give the meaning of the term isotope. [2]
(c) Use Fig. 1 to state which ion, A, B or C, is least abundant and explain your choice. [3]
(d) Calculate the relative atomic mass of the neon sample, using relative abundances of 30 for A, 50 for B and 5 for C. Show your working. [4]
Answer Details
(a) Neon has atomic number 10, so every neon atom has 10 protons. For ion A, the mass number is 20. \(20-10=10\), so ion A has 10 neutrons. [2]
(b) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. The differing neutrons give different mass numbers. [2]
(c) Ion C is least abundant. Its peak is the shortest, and peak height represents relative abundance: fewer atoms of that isotope produce a lower peak. [3]
(d) Use a weighted mean, because each isotope contributes according to its abundance.
\[\frac{(20\times30)+(21\times50)+(22\times5)}{30+50+5}=\frac{1760}{85}=20.7\]
The relative atomic mass of this neon sample is 20.7 (or 20.71). [4]
Exam reminder: For a \(+1\) ion, \(m/z\) is equal to its mass number.
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