# 3.4 Expressing concentration in moles

In the last section concentration was described in terms of a mass of solute dissolved in a defined volume. However, it is often more practical to express concentrations as the number of moles of solute is dissolved in a specific volume (1 dm3) at a given temperature.

Thus, if you have 10.0 g of sodium chloride dissolved in 1.00 dm3 of water, you can calculate the concentration of that solution in moles per dm3 (mol dm−3 ).

First, you need to calculate the molar mass of sodium chloride (NaCl).

- So….what is the molar mass of sodium chloride?

- The relative atomic masses of Na and Cl are 23.0 and 35.5, respectively, so: relative mass of 1 mole of NaCl = (23.0 + 35.5) g = 58.5 g

  Thus the molar mass of NaCl is 58.5 g mol−1.

Second, you need to calculate the number of moles of NaCl.

How many moles are there in 10.0 g of NaCl?

Number of moles of NaCl = mass of NaCl/molar mass of NaCl

= 10.0/58.5 = 0.171 mol

Therefore, 0.171 mol of NaCl is dissolved in 1.00 dm 3 of water, so the concentration of the solution is 0.171 mol dm−3 .

- What happens to a crystalline structure of sodium chloride when it is added to water?

- You know sodium chloride dissolves to form a solution, the crystalline structure which comprises an array of equal numbers of sodium cations (Na+) and chloride anions (Cl-), breaks up and the ions become surrounded by water molecules.

So when sodium chloride dissolves in water, the resulting solution will contain equal numbers of sodium and chloride ions:

- Write a chemical equation for this process.

- NaCl(s) = Na+(aq) + Cl−(aq)

Thus, if 1.0 mole of sodium chloride is dissolved in water to form 1 dm 3, giving a 1 mol dm−3 solution, the solution will contain 1.0 mol dm−3 of Na+(aq) ions and 1.0 mol dm−3 Cl−(aq) ions.

Next consider what would happen if you dissolved calcium chloride (CaCl2) in water.

- Write a chemical equation for this process.

- CaCl 2 (s) = Ca 2+(aq) + 2Cl−(aq)

The resulting solution has two chloride ions for every calcium ion. So if you have a 1 mol dm−3 solution of calcium chloride, this means there is a concentration of 1 mol dm−3 of Ca2+(aq) ions and 2 mol dm−3 of Cl −(aq) ions. In this example, you would multiply the concentration by 2 for the chloride ions as there are twice as many.

Finally, having been introduced to the mole, and how it can be used to “count” atoms, molecules and ions; in the next section you will see how you can take this concept and answer a question posed at the very start of the course.

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