Topic 2: Amount of Substance Notes

Notes for topic 2, amount of substance

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Topic 2: Amount of Substance
2.1: Relative atomic and molecular masses, the Avogadro
constant and the mole.
Relative atomic mass (Ar)
Definition: Relative atomic mass, Ar, is the weighted average mass of an atom of an
element, taking into account its naturally occurring isotopes, relative to 1/12th the relative
atomic mass of an atom of carbon12.
Calculation:
Ar = average mass of 1 atom of an element / 1/12th mass of an atom of carbon12
= average mass of 1 atom of an element x 12 / mass of an atom of carbon12
Used to compare masses of atoms
Carbon12 used as the baseline element because the mass spectrometer allows us
to measure the masses of individual isotopes extremely accurately.
1/12 of the Ar of carbon12 = exactly 1
Relative molecular mass (Mr)
Definition: Relative molecular mass, Mr, of a molecule is the mass of that molecule
compared to 1/12th the relative atomic mass of an atom of carbon12.
Calculation:
Mr = average mass of 1 molecule / 1/12th mass of an atom of carbon12
= average mass of 1 molecule x 12 / mass of an atom of carbon12
Used to compare masses of molecules
By comparing mass of the molecule to that of a carbon12 atom
Find the relative molecular mass by adding up the relative atomic masses of all the
atoms present and we find this from the formula
Relative formula mass
Use relative formula mass for ionic compounds because they do not exist as
molecules
Use the state symbol Mr
The Avogadro Constant
Definition: The Avogadro constant (or number) is the number of atoms in 12g of carbon12.
Actual number of atoms in 1g of hydrogen atoms is 6.022 x 10^23.

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Difference between this scale (based on H=1) and scale used today (based on
carbon12) is miniscule
The Mole
Definition: A mole is an amount of substance containing 6.022 x 10^23 particles.
Calculation: We must first know the substance's formula to find out how many moles are
present in the particular mass of the substance. From the formula we can then work out the
mass of one mole of the substance.…read more

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The Empirical Formula
Definition: The formula that represents the simplest ratio of the atoms of each element
present in a compound.
Calculation:
1.
Find the masses of each of the elements present in the compound.
2.
Work out the number of moles of atoms of each element.
3.
Use: Number of moles = mass in g / mass of 1 mole in g
4.
Convert the number of moles of each element into a whole number ratio.…read more

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Number of moles in solution, n = M x V / 1000
2.5: Balanced equations and related calculations
Balanced symbol equations
Ionic equations
Strike out the spectator ions
Finding concentrations using titrations
Use: We carry out titrations to find the concentration of a solution, for example, an alkali. We
react the acid with the alkali using a suitable indicator.
Need to know:
The concentration of the acid
The equation for the reaction between the acid and the alkali
Steps in a titration:
1.…read more

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