How to calculate the relative atomic mass of an element from a mass spectrum

We know that relative atomic mass (A_r) is the average of (% abundance of an isotope x isotopic mass) for each of the isotopes in a sample of an element …

e.g. calculate the relative atomic mass for chlorine given that it has 2 isotopes, ³⁵Cl with an abundance of 75% and ³⁷Cl with an abundance of 25%.

A_r = (35 x 75) + (37 x 25) / 100 = 35.5

We can also use the mass spectrum to determine relative atomic mass, which is explained in the video below:

Practice questions

Use the mass spectrum below to calculate the relative atomic mass of lithium.

2. The mass spectrum of neon consists of three lines corresponding to m/z ratios of 20, 21 and 22 with relative intensities of 0.901, 0.0026 and 0.088. Explain the significance of these data and calculate the relative atomic mass of neon.

3. The following isotopes and abundances are for the element chromium

Isotope	Relative abundance / %
⁵⁰Cr	4.34
⁵²Cr	83.79
⁵³Cr	9.50
⁵⁴Cr	2.37

(a) Sketch the mass spectrum you would expect for naturally occurring chromium (use a vertical axis of 10cm representing 100% relative intensity) and label the peaks with the corresponding isotopic symbol.

(b) Calculate the relative atomic mass of chromium to an appropriate number of significant figures.

Answers