CH3: Atoms, Molecules, And Stoichiometry

Unified Atomic Mass Unit Definition

One-Twelfth of the mass of a Carbon-12 Atom

A_R and M_R Formula

Weight Average Mass
(1.66 ×10^-27)

Relative atomic mass definition

The average weighted mass of atoms of an element compared to the unified atomic mass unit.

Relative molecular mass definition

The weighted average mass of a molecule compared to the unified atomic mass unit.

Relative isotopic mass definition

The weighted average mass of an atom of an isotope compared to the unified atomic mass unit.

Relative formula mass

The weighted average mass of one formula unit compared to the unified atomic mass.

What is a spectrometer used for?

A mass spectrometer is used to measure isotopic abundance and mass of an element.

1. Atoms are converted into vapor

2. Vaporized atoms are converted into +1 ions with an electron gun.

3. Strong magnetic field deflects the ions towards the detector

Explain the mass/charge ratio

Since the charge is only 1, the x-axis of the graph essentially gives us the mass of the deflected ions.

How to calculate relative atomic mass from a mass spectrometer 

∑ (Abundance x M/e)
  Total Abundance 

The total abundance may not always be 100 

How to identify organic compounds from mass spectrometry

Fragmentation occurs where the organic compound is broken down into smaller molecules.

[M⁺] ion: Gives the total mass of the compound

[M + 1]: Gives the number of carbon atoms

[M + 2]: If Br/Cl is present in the sample

[M + 4]: If 2 Br/Cl atoms is present in the sample

Formula to find the number of carbon atoms

n = (100/1.1) x (abundance [M + 1])/abundance of [M+] 

Using the [M + 2] Peak

• Chlorine
  If the M⁺ peak and the [M+2] peak is in the ratio of 3:1
  Cl³⁵ = 75%
  Cl³⁷ = 25% 

• Bromine
  If the M⁺ peak and the [M+2] peak is in the ratio of 1:1 
  Br⁷⁹ = 50%
  Br⁸¹ = 50%