Amount of substance

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Definitions

Relative isotopic mass: The mass of a single atom of an isotope compared to 1/12th the mass of an atom of carbon-12

however, most elements are composed of a mixture of naturally occuring isotopes in varying proportions, so we must caculate the relative atomic mass. 

Relative atomic mass: The weighted mean mass of an atom of an element (taking into account the relative abundence of isotopes) compared with 1/12th of the mass of an atom of carbon-12.

to calculate the relative atomic mass of an element you must use this method - 

(http://study.com/cimages/multimages/16/averageatomicmassformula.png)

Relative molecular mass: The weighted mean mass of a molecule compared with 1/12th of the mass of an atom of carbon-12. 

Mr - is calculated by adding up the individual relative atomic masses of each atom present in a molecule. But this is only used for simple molecular structures such as CO2. 

Relative formula mass on the other hand is the same concept but it relates to giant structures such as graphite and NaCl. 

The molar gas volume 

Molar volume is the volume that a mole of gas molecules occupies. 

At standard condition, 1 mole of any gas takes up a volume of 24dm3 

(http://www.chemistryrules.me.uk/middle/molarvoltriangle.gif)

This means that the number of particles present in a gas at STP is the amount of moles x 24

standard temperature and pressure (STP) is at -273 degrees celcius (0 kelvin) and has 1 bar of atmostphereic pressure. At these conditions there is no

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