Let’s do an example calculation. Imagine for
the sake of argument, we have a one litre
sample of polluted water that was analyzed
for the lead(II) ion, Pb2+. By adding an excess
of sodium sulphate to it, the massive lead
sulphate that’s precipitated out is 229.8
milligrams. What therefore is the mass of
lead in a litre of water? Express the answer
as milligrams of lead per litre of solution.
So, let’s work through this.
The reaction that we’re seeing, as I mentioned
before, is sodium sulphate in solution plus
Pb2+ in solution to give rise to 2Na+ and
lead sulphate. I have left the counter anions
for the lead out of the equation. But, of
course, they exist either, as say, nitrates
or, for example, another soluble counter anion.
First we must obtain the mass percentage of
lead sulphate by dividing the molar mass of
lead by the molar mass of lead sulphate and
then multiply by 100. We then calculate the
amount of lead in the lead sulphate that’s
So, let’s start off. What percentage is lead
as a proportion of lead sulphate? So, what
we need to do is we take the overall formula
mass of lead sulphate, which is the denominator
in the case of this equation, and we take
the mass of lead as the nominator, in this
equation. Multiply by 100, this gives us the
value of 68.32. This tells us that in the
amount of lead sulphate that you have, 68.32%
of that mass must be lead. And therefore,
the amount of lead in the sample of 229.8
milligrams would involve taking 0.6832 which
is the decimal of that percentage and multiplying
it by the mass of lead sulphate that we precipitated
This tells us that in one litre of our suspect
solution, we have 157 milligrams of lead,
which pretty much is rather unpleasant.
So, in summary, we can represent ionic RedOx
equations and so forth as molecular equations,
complete ionic equations or as net ionic equations
where spectator ions which don’t precipitate
in the reaction have been cancelled or removed.
Most reactions are either precipitation reactions,
acid-based reactions or oxidation reduction
reactions. Acid-based reactions involve proton
transfer and oxidation reactions or RedOx
reactions involve the movement of electrons
from one species to another.
Oxidation reactions are the most important
type that you’ll come across and they’re using
a wide variety of different approaches not
just for analytical chemistry, but also from
a biological perspective as well in, for example,
things like heme, in hemoglobin and also,
for example, in prostaglandin synthesis.
Oxidation reduction reactions usually fall
into the following categories - combination
reactions, decomposition reactions, displacement
reactions and combustion reactions.
Molarity, as we’ve seen, is defined as the
number of moles of solute per litre of solution.
And knowing the molarity would then allow
you to calculate the amount of solute in a
given volume of solution.
Quantitative analysis involves the determination
of the amount of a species in a material.
And there are a wide variety of methods for