### concentration, molarity

All mixtures are composed of two or more components. If the components cannot be distinguished from each other, we call it homogeneous. All particles in the mixture are divided very equally amongst the space.
This is always the case if the mixture is composed of gases, for example: air.
If one of the components is (l) and this mixture is transparent (not trouble), then you've got a homogeneous solution, for example sugar water. The liquid component fills the whole space and is the biggest part of the mixture; then this is the solvent. The other component(s) is(are) the solute(s) that apart can be (s), (l) or (g).

If the particles of a component are not equally divided in the space, then we have a heterogeneous mixture.
examples: chalk in water, smoke, oil + water, foam.

Only if the particles of a component are equally divided in the space (homogeneous), we can calculate the concentration of it: every liter of the solvent contains an equal amount of molecules of solute, no matter where you take that liter out of the mixture.

For hetergeneous divided substances we cannot calculate any concentration.

In chemistry we take as a unit of concentration mostly: mol/liter = mol.l-1
If we want to indicate the concentration of a substance in a mathematical formula, we do that with square brackets: [NaCl]; [O2]

Another frequently used word for mol/liter (concentration) is molarity, often indicated with .....M.
Suppose that you have a salt solution with a concentration of 0,4 mol HCl per liter water, then you say: this salt solution is 0.4 molar;
The molarity is 0,4 or, in writing: 0.4M HCl.

If the concentration of a solution is known, and also the amount of solute, then you can calculate the total amount of present solute.
10 ml 0.1M HCl contains 0.01 x 0.1 mol HCl (10 ml=0.01 l)
5 liter 0.02M NaCl contains 5 x 0.02 mol NaCl

At concentration calculations you always must give attention to at least two substances:
1. the solute is given in mols.
2. The solvent is given in liters.

Apart from that, you must know how to transform mol to mmol, from liter to ml, and v.v.:
1 ml = 10-3 l
1 l = 103 ml
1 mol/l = 10-3 mol/ml

When something is analysed in chemistry, often the concentrations of many investigated substances seemt to be far below 1M, for example: 10-3M 3.10-9M etc.
To calculate fast and easy with such low concentration, chemistry uses the so called p-values. In a mathematical way this means: -log; Just leave out the sign and the the basic numver 10:

p..... = 3 means: [.....] = 10-3 mol/liter

It also happens that the substance itself is abbreviated; charges are left out, sometimes even more:

• pH = 5 means: [H3O+] = 10-5 mol/l
• pOH = 7 means: [OH-] = 10-7 mol/liter

Now try (if possible) to execute action task A, to be found at the end of this module.