Chemistry 3.1.2 Amount of Substance

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3.1.2 - Amount of Substance

Relative atomic mass and relative molecular mass
Be able to define relative atomic mass (Ar).
The mass of atoms and other small particles is measured on the atomic mass scale as a relative value compared to a carbon atom. A carbon-12 atom is assigned a value of exactly 12.000 on this scale and everything else is measure with respect to it. It is the measure of the mass of one atom of the element. The relative atomic mass is a measure of the one atom of the element.

Be able to define relative molecular mass (Mr) in terms of carbon-12.
Molecules are just groups of atoms and as such they are also measured on the relative mass scaled with carbon again being the reference atom. The relative molecular mass is calculated by adding up the relative masses of all the atoms in a molecule of that substance.

The mole and the Avogadro constant (L)
Understand the concept of a mole as applied to electrons, atoms, molecules, ions, formulae and equations.

The mole is a unit of measurement used in chemistry to express amounts of a chemical susbstance, defined as the amount of any substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12. 
The formulas to help work out moles, or work out other information from moles are:
Number of particles = number of moles x Avogadro's constant
Mass = molar mass x number of moles

Understand the concept of the Avogadro constant.
Avogadro's constant = 6.02 10^23.
Avogadro's law states: "equal volumes of different gases contain equal numbers particles at the same temperature and pressure."
 It is the number to which the mass of an atom must be multiplied to give a mass in gram numerically equal to its relative atomic mass. The two important definitions are: the amount of any substance containing an Avogadro number of particles of that substance is called a mole; 1 mole of any substance has a mass equal to its relative mass expressed in grams.
The realtionship between moles, mass


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