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Discovering chemistry
Discovering chemistry

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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 dm3 ).

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 mol1.

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 dm3 .

  • 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 dm3 solution, the solution will contain 1.0 mol dm3 of Na+(aq) ions and 1.0 mol dm3 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 dm3 solution of calcium chloride, this means there is a concentration of 1 mol dm3 of Ca2+(aq) ions and 2 mol dm3 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.