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OCR A-level Chemistry Relative atomic mass

Relative atomic mass

This page covers the following topics:

1. Isotopes
2. Relative atomic mass
3. TOF mass spectrometry
4. Analysis of mass spectra

Isotopes are atoms of the same element, which have the same number of protons but a different number of neutrons. They are present in different levels of abundance. Some isotopes are radioactive.

Many isotopes are denoted by using the chemical symbol of an element and two additional numbers. The top number in such notation is the mass of an isotope in atomic units and the lower number is the charge of the nucleus, which matches the atomic number of an element.

Isotopes

Relative atomic mass is a weighted average mass of all the isotopes of an element. Weighted means that it takes into account the natural abundance of each isotope.

To find the relative atomic mass from given atomic masses of isotopes and their abundances, multiply corresponding atomic masses and their abundances in decimals and add the multiplication products together. If you use abundances as percentages, divide your final result by 100%.

Relative atomic mass

Mass spectrometry is an analytical technique which separates gaseous ions by their mass-to-charge ratio (m/z). It is primarily used to analyse organic compounds by comparing known spectra and can be applied in environmental tests, forensics, detection of drugs. It can also be applied in determining the abundances of isotopes and calculating the relative atomic mass.

In mass spectrometry a sample is vaporised and ionised. After ionisation particles are accelerated towards the detector and each are given an equal kinetic energy. The latter stage is called ion-drift. During time-of-flight (TOF) mass spectrometry ions are fired into a vacuum column. The absence of air molecules in vacuum ensures that the particles are not slowed down by other molecules. It is then measured how long it takes for the ions to reach the detector at the other end. Lighter ions, with a smaller m/z (mass to charge ratio), travel faster.

TOF mass spectrometry

A mass spectra shows the proportion of sample ions with each m/z value. Thus, this peak height is proportional to the abundance of compounds or isotopes when determining the relative atomic mass. When analysing m/z graphs, the peak with the highest m/z value in it is the molecular peak. Sometimes there might another small peak after molecular one arising due to various isotopes present in a molecule. This small peak may be ignored for the determination of the molecular peak value due to its small abundance.

In mass spectrometry a sample is vaporised and ionised by bombarding a sample with electrons (electron impact, loss of 1 electron) or adding protons (electrospray ionisation, gain of 1 proton). The electrons break some of the weaker covalent bonds, while the protons attach themselves to the molecules without much splitting. Since the charge of mass spectrometry ions is usually +, the mass of the molecule analysed is usually the same as the m/z value at the molecular peak when using electron impact. If the electronspray ionisation is used instead, because of the additional attached proton the mass of the molecular peak is bigger than the molecular mass by 1 unit. The base peak in an m/z graph is the highest peak.

Analysis of mass spectra

1

What is the heaviest hydrogen isotope?

What is the heaviest hydrogen isotope?

2

Magnesium has three stable isotopes, ²⁴Mg (79%), ²⁡Mg (10%) and ²⁢Mg (11%). What is the relative atomic mass of magnesium?

Magnesium has three stable isotopes, ²⁴Mg (79%), ²⁡Mg (10%) and ²⁢Mg (11%). What is the relative atomic mass of magnesium?

3

How many neutrons does ³⁷₁₇Cl have?

How many neutrons does ³⁷₁₇Cl have?

4

In a TOF mass spectrometer a ³⁡Cl⁺ ion has travelled through a vacuum tube in 4.57 seconds. Find the time taken for a ³⁷Cl⁺ ion to travel through the same vacuum tube.

In a TOF mass spectrometer a ³⁡Cl⁺ ion has travelled through a vacuum tube in 4.57 seconds. Find the time taken for a ³⁷Cl⁺ ion to travel through the same vacuum tube.

5

Chlorine has two stable isotopes, ³⁡Cl (75%) and ³⁷Cl (25%). What is the relative atomic mass of chlorine?

Chlorine has two stable isotopes, ³⁡Cl (75%) and ³⁷Cl (25%). What is the relative atomic mass of chlorine?

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