Copper is known to have four naturally occurring isotopes that are detected as singly…

CHEMISTRY · Multi-part · A Level · Chemistry · Atomic structure · Atomic structure

Problem

Copper is known to have four naturally occurring isotopes that are detected as singly charged ions in a high-resolution mass spectrometer. The laboratory instrument reports the following uncorrected signal intensities and exact isotopic masses:

Isotope	Exact mass (amu)	Signal intensity (counts)
$^{63}\text{Cu}$	62.9296	$4.50\times10^{5}$
$^{65}\text{Cu}$	64.9278	$1.85\times10^{5}$
$^{66}\text{Cu}$	65.9310	$4.20\times10^{2}$
$^{67}\text{Cu}$	66.9292	$9.80\times10^{1}$

No background subtraction has yet been applied. (a) Normalize the signal intensities so that the total abundance of all four isotopes sums to 100.0\,%. Present the percentage abundance of each isotope to three significant figures. (b) Using your normalized abundances from part (a), calculate the average atomic mass of copper to four significant figures. (c) Determine the percent relative error introduced into the average atomic mass if the two least abundant isotopes,

^{66}\text{Cu}

and

^{67}\text{Cu}

, are omitted from the calculation. Show your work. (d) Explain whether the omission of trace isotopes is justifiable when reporting a periodic-table atomic mass, citing numerical evidence from your calculations.

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