A-level Chemistry/OCR/5.1.1 Atoms, Molecules and Stoichiometry

Masses

In chemistry all atomic masses are measured in atomic mass units, the abbreviation is amu, the unit symbol is u. The international agreed standard is that 12 amu's is the exact weight of pure isotopic Carbon-12, so all masses are relative to ${}^{12}C$. This system of relative masses was originally devised by John Dalton.

Key Definitions

Relative Atomic Mass: The atomic mass is the weighted average of all natural occurring isotopes based on the mass of 1/12th of carbon 12. The symbol is

$A_r$

Isotopic Mass: The mass of an isotope of an element.

Molecular Mass: The sum of all the individual masses in a molecular formula. The symbol is $M_r$.

Formula Mass: The sum of all the individual masses in an ionic formula.

Mass Spectrometry

A mass spectrometer is an instrument that chemists use to determine the composition of a molecule. It can also determine the percent composition of an element. It works by a sample of a element is vaporized. Then the element is ionized afterwards it passes through an acceleration chamber. There a magnetic or electrical field is applied causing the ions to be deflected from there trajectory. Since the relative atomic mass is different between isotopes and between elements, the amount deflected will be different. Afterwards a detector records electronically the mass and amount present of the components present. The Mass Spectrometer can be used to detect the relative isotope mass and abundance of each isotope of an element. It can also identify elements in a substance.

Interpreting Mass Spectrum Data

Mass spectrum data can be used to determine the percent composition of an element. If you asked to analyse a mass spectrum data remember that the y-axis is the percentage and x-axis is the mass. The rest is like reading a standard bar graph.

Calculating Relative Atomic Mass

Calculating the relative mass is done by multiplying each isotopic mass by its percent composition and adding them all together. In example:

Calculate the relative atomic mass of Silicon from this table of data:

Isotope	Relative Isotopic Mass	Percent Composition
Silicon-28	27.97693u	92.21%
Silicon-29	28.97649u	4.70%
Silicon-30	29.97376u	3.09%

$\begin{array}{cc}relativeatomicmass& =\frac{27.97693\times 92.21\mathrm{\%}}{100\mathrm{\%}}+\frac{28.97649\times 4.70\mathrm{\%}}{100\mathrm{\%}}+\frac{29.97376\times 3.09\mathrm{\%}}{100\mathrm{\%}}\\ & =25.7975u+1.3618um+.9262u\\ & =28.0855u\end{array}$

Indeed if you check the periodic table of elements the relative atomic mass of silicon is 28.0855u. Also you may be asked to find the percent composition of two isotopes, given there relative isotopic mass and the relative atomic mass of the element. The key to this problem is that the total composition is 100%.