|Stoichiometry||IB Syllabus > > Moles|
Mole concept & Avogadro's constant
1.1.1: Describe the mole concept and apply it to substances. The mole concept applies to all kinds of particles: atoms, molecules, ions, formula units etc. The amount of substance is measured in units of moles. The approximate value of Avogadro's constant (L), 6.02 x 1023 mol-1, should be known.
The structure of matter
It is now accepted that matter in all its forms is made up of indivisible particles that themselves have mass. These particles are called atoms, molecules and ions. The nature of the substance is dictated by the atoms elements that have bonded together to make the bulk substance. This may be an ionic structure, a covalent structre or a metallic structure.
Molecules are made up of two or more atoms chemically bonded together.
Ions are specialised atoms or groups of atoms chemically combined together that have lost or gained electrons and posess an overall electrical charge.
The fundamental particle that is the building block of matter is therefore the atom. There are about 90 naturally occuring types of atoms each with a different arrangement of sub-atomic particles (protons, neutrons and electrons) and consequently different masses.
The structure of matter is one of the following:
The masses that are measured in the laboratory are masses corresponding to vast numbers of tiny atoms or molecules. Logically atoms that are heavier will register larger masses for equal numbers of atoms.
Relative atomic mass
If one carbon atom has a mass of 12 atomic mass units and one magnesium atom has a mass of 24 atomic mass units, then as a magnesium atom is twice as heavy as a carbon atom it follows that this ratio will be maintained for any number of atoms.
On the atomic mass scale the carbon 12 isotope is designated a value of 12 atomic mass units and all other masses are measured relative to this (relative atomic mass)
The mole concept
It is convenient to consider the number of atoms needed to make 12g of carbon and for this number to be given a name - one mole of carbon atoms. This allows us to talk about relative quantities of substances in the macroscopic world and to know the relative number of atoms (or smallest particles) in each bulk substance.
The actual number of atoms that is needed to give the relative atomic mass expressed in grams is called Avogadro's number (symbol L)
Avogadro's number = 6,02 x 1023
Definition of a mole
There are two useful definitions.
1.1.2: Calculate the number of particles and the amount of substance (in moles). Convert between the amount of substance (in moles) and the number of atoms, molecules or formula units
1 mole = 6.02 x 1023 formula units of that substance.
We can also talk about the atoms within molecules.
For example 1 mole of water contains 2 moles of hydrogen atoms and 1 mole of oxyten atoms. It is a simple matter of multiplying the moles of the compound by the atoms or ions that make it up.
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