# Amounts of Substance

Formulae:
Empirical -> Simplest ratio of atoms of each element in a molecule

Molecular -> Actual number of atoms of each element in a molecule.
Virtually all molecules, except for simple molecular, use the empirical formula as it is
impossible to determine the exact number of atoms in a molecule.
Avogadros Constant is a scaling factor that we use when working with very small or very large numbers. It
is the number of particles/molecules in one mole of a substance, or exactly 12.00g or solid 12C. The value
of avogadros constant at A-Level is 6.02×10^23
N = Number of atoms/molecules/particles etc…
n = Number of moles
Standard Index Units:
We often have to convert between grams, cm3
and other such values into other units;
Weight (SI = Grams) Volume (SI = cm3
)
Kg -> Grams = Divide by 1000 M3
-> cm3
= Divide by 1003
Grams -> Milligrams (mg) = Divide by 1000 Dm3
-> cm3
= Divide by 103
Reacting Mass Calculations:
 Identify the substances involved in the reaction
 Find the relative mass (Mr) of both substances
 Calculate moles of known substance
 Use molar ratio (in the given equation) to find the moles of the required substance
 Calculate the required mass using Mass = Mr * Mole
You may have to use and ICE table to find the equilibrium moles. You will mostly be told the initial moles of
1 substance. Use the molar ratio to deduce the final equilibrium moles (note, product always start with 0
moles). Sometimes however, you may have to use algebra in these tables, just use x as the initial moles
and subtract from this value.
Percentage Yield:
You have to ensure you have found your
equilibrium moles to find maximum
mass of product
It is possible for the yield to be over 100%, and this is due to impurities in the product. However, the yield
will usually be significantly lower than 100%. This is due to loss of product through purification, impurities
in reactants leading to the production of by-products, if the reaction is reversible or not so some product
may be converted back into reactants, or sometimes some of the product will dissolve especially if the
product is a salt.
Atom Economy:

Ensure you use the Mr of products/reactants!
You can increase the atom economy of a reaction by finding uses for a by-product
Ideal Gas Equation:
The gas pressure is the sum of the forces exerted on the side of a container by gas particles colliding with
the side of the container. Gas pressure is affected by many factors;
 Temperature -> increase in temperature means particles collide with container with more Ek, so
more energetic collisions, increasing gas pressure
 Mass of Particles -> increase in mass means particles collide with container with more Ek, so more
energetic collisions, increasing gas pressure
 Volume of container -> increase in volume means particles collide with container with less often, so
gas pressure decreases
 Number of particles -> increase in # of particles means particles collide with container more often,
so gas pressure increases
Real gasses do obey the ideal gas equation
fairly well, so long as they are not near 0K. However, the ideal gas equation
does make a number of important assumptions;
 Particles are point masses
 Perfectly elastic collisions between particles
 No IMF between particles
Concentration:
Concentration is measured in mol/dm3
, so to calculate the concentration divide the moles by the volume
(in dm3
). If concentration asked for in g/dm3
, you have to use Mass = Mr * Mole to calculate the mass in
grams from the moles.
Back Titrations:
These are the most difficult calculation you could be asked to perform. Follow these steps when
performing these titrations;
 Calculate the moles of the acid/base that you know the concentration of

Calculate the moles of the other acid/base using the molar ratio
 Multiply these moles of unknown acid/base by a scaling factor to find moles in the standard
solution
 Find the initial moles of the acid/base in the standard solution
 Moles used up are the moles of the substance you require
 Find whatever they want you to with these moles (purity is actual moles/expected moles *100)