Post under construction
"Concentration of a species" is not the same as "Concentration of the solution"
When we refer to the concentrations of aqueous solutions, it is important to distinguish between
Why is it important to distinguish? See below (at end).
APOLOGY: The editor used to create these pages does not allow superscripts and subscripts. So in the text, superscripts and subscripts are in-line.
For example: The solution concentration is 0.100 mol L-1
Subscripts are made a little more obvious by using a smaller font: eg c(C12H22O11) = 0.100 mol L-1
Symbols and formulas that have both superscripts and subscripts can be clumsy: eg, CO32-
On occasions, I have retained superscripts and subscripts by posting images of equations or text.
Solution concentration, or labelled concentration, refers to the amount of solute that is put into (and dissolved in) water, per defined volume of the solution. It is the value that you see on the labels of solution bottles.
Perhaps this would better be called the solute concentration.
The symbol used is c(solute). See Module 0907 Solution concentration.
Solution concentrations can be specified in various units, but let’s just focus here on the unit "mol L-1" (for which the abbreviation M is used) – that is, moles (of solute dissolved) per litre of solution.
The concentration of a sample of solution does not depend on its volume: so concentration is an intensive property.
Some examples: Solutions of sucrose (C12H22O11), potassium sulfate (K2SO4), hydrogen chloride (HCl), and methanoic acid (HCOOH).
In all of the following solutions, the solution concentration is 0.100 mol L-1 : ie, c(solute) = 0.100 mol L-1
What is actually present in the solutions described above? What species (molecules or ions) are present, and what is the concentration of each of them? The symbol used is [species].
Let’s consider some examples to make sense of this:
A 0.100 M sucrose solution, ie, a solution with
Sucrose is a non-electrolyte: the molecules of sucrose retain their identity when they dissolve in water. So the concentration of the sucrose molecules (the species present) is also 0.100 mol L-1, and we write
Yes, in this solution, c(C12H22O11) = 0.100 mol L-1 and [C12H22O11] = 0.100 mol L-1.
That might seem trivial, but this is not always the case: in fact, this is the case only for solutions of non-electrolytes.
For example ……
A 0.100 mol L-1 potassium sulfate solution, ie, a solution with c(K2SO4) = 0.100 mol L-1
Like all soluble ionic salts, the ions in the solid lattice separate from each other on dissolving in water, and the ions are aquated (or hydrated). See Module 0905 Dissolution of ionic salts in water. This ionisation process can be represented by the following chemical equation:
So, what species are in solution? There is none of a species with composition K2SO4, because all of the K+ and SO42- ions have separated from each other. So .....
But we can see from the equation that for every mole of the substance K2SO4 dissolved, twice as many moles of K+(aq) ions are formed (and are present in solution). So the concentration of the K+(aq) ion species is 0.200 mol L-1
And, by corresponding logic ...
More cases ......
A 100 mol L-1 hydrogen chloride solution, ie, a solution which we would label c(HCl) = 0.100 mol L-1
Hydrogen chloride gas is a molecular solute which is also a strong electroyte: all of the HCl molecules ionise when it is dissolved in water (See Module 0910 Electrolytes - strong or weak?). This can be represented by the chemical equation:
This process is alternatively, and better, represented as a competition between water molecules and choride ions for H+ ions (See module on weak acids, not yet published):