Concentration.

Step 1 of 5

Grams per litre

A glass of salt water can be slightly salty or very salty. The quantity that measures this is the concentration: how much solute there is in each unit of volume of solution. The simple way is in grams per litre,

$c = \frac{m}{V}$⁠

The glass holds 10 g of salt in 100 mL of water. In dilute solutions like these, the volume of the solution is almost the same as the volume of the water, so take it as 100 mL.

Step 2 of 5

Moles per litre

Chemistry counts particles, so the most useful concentration is the molar concentration: the number of moles of solute in each litre of solution,

$c = \frac{n}{V}$⁠

Its unit is $\mathrm{mol/L}$⁠, also written $\mathrm{M}$⁠. To work it out, first go from grams to moles with the molar mass: for $\mathrm{NaCl}$⁠, $M = 58.44\ \mathrm{g/mol}$$M =$$58.44\ \mathrm{g/mol}$⁠. The glass now holds 11.7 g.

Step 3 of 5

Make a solution

The working can be turned round: if you know the concentration you want, find how many moles you need, $n = cV$⁠, then how many grams, $m = nM$⁠. Salt added is now free.

Step 4 of 5

In the lab

In the lab a solution of exact concentration is made up like this: weigh out the solute, dissolve it in a little water in a volumetric flask, and top up with water to the mark. That way the volume of the solution, not of the water, is exactly the volume in the working.

Step 5 of 5

Only what dissolved

A concentration describes what is in the solution. Crystals on the bottom are not in it, so they do not count. The glass now holds 45 g of salt at 20 °C.