# Atoms, Molecules, Relative Atomic and Molecular Masses

## Contents

## 1.1 The Particulate Nature of Matter

Diffusion and Brownian motion as evidence for particles.

Simple kinetic theory (only a qualitative treatment is required).

Home-made smoke cells are a very useful illustration of Brownian motion.

## 1.2 The Mole and Relative Atomic Masses

The Mole, the Avogadro constant and Relative Atomic masses (defined relative to the C-12 isotope).

The determination of Relative Atomic masses and the discover of isotopes by mass spectrometry.

- A mole is a measure of an amount of substance. It can refer to any particle or group of particles of that substance, but the type of particle must always be specified. In Section 6 the Avogadro constant (L) is related to the Faraday's constant (F), but no other method of determining the Avogadro constant is required.

## 1.3 Relative Formula Masses

Relative Formula Mass replaces terminology like molecular weight, relative molecular mass, etc.

The experimental determination using the depression of freezing point.

A knowledge of the elevation of the boiling point method and the use of osmosis is expected, but not the detailed experimental techniques.

Only one particular technique for carrying out the depression of freezing point is required. The use of osmosis for study compounds of high Relative Formula mass, e.g. Polymers, should be pointed out.

An outline of their determination by mass spectrometry, including interpretation of the results for a simple organic compound. (Interpretation of structure from a mass spectrum is required.

A brief survey of the work of Gay-Lussac and Avogadro: Molar Volume of gases; the use of gas density measurements to determine relative formula masses. The ideal gas equation and qualitative treatment of deviations from it.

The direct weighing method and Victor Meyer's method for determining relative formula masses are not required. A modification of the Victor Meyer experiment, e.g. using a glass syringe and stream jacket, could be demonstrated. Experimental details are not required fo rthis sub-section, but students should be familiar with the calculations involved.

## Stoichiometry Calculations:

(1) The calculation of empirical and molecular formulae using composition by mass.

(2) Mass-mass and mass-volume calculations.

(3) Calculations involving solutions (Concentrations measured in moles per litre).