3.1.2.1  Relative atomic mass and relative molecular mass

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The relative atomic mass (Ar) of an element is the weighted average mass of an atom, which takes into consideration all isotopes, relative to 1/12th the mass of a carbon-12 atom.  This can be expressed as follows:

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\(A_r =\) \(\frac{average \ mass \ of \ 1 \ atom \ of \ an \ element}{1/12th \ mass \ of \ an \ atom \ of  \ ^{12}C}\)

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The relative molecular mass \(A_r\) is the average mass of 1 molecule relative to 1/12th the mass of a carbon-12 atom. This can be expressed as follows:

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\(M_r =\) \(\frac{average \ mass \ of \ 1 \ molecule}{1/12th \ mass \ of \ an \ atom \ of  \ ^{12}C}\)

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Relative formula mass is used for ionic substances as they have giant lattice structures and do not exist as molecules such as NaCl or MgCl\(_2\).  Relative formula mass is based on the chemical formula and is the total mass of all atoms in the formula.

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3.1.2.2 The mole and the Avogadro constant

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The Avogadro constant is the number of particles that is equivalent to 1 mole of substance.  This number is represented by L as given below:

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\(L = 6.023\times 10^{23}\)

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In terms of mass of a substance if the relative atomic mass of an element is expressed in grams then this will be one mole of a substance and will be equivalent to the Avogadro constant.

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So if you have 12g of \(^{12}C\) it will be 1 mole of carbon having \(6.023\times 10^{23}\) particles, in this case atoms.

2 moles of \(^{12}C\) will have a mass of 24g.

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1 mole of methane \(CH_4\) would be it's relative molecular mass expressed in grams so 12 + 4 = 16g.

0.5 mole of methane will have a mass of 8g.

0.25 mole of methane will have a mass of 4g.

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