1. Moles in Solution A solution consists of a solvent with a solute dissolved in it The concentration of a solutions tells us how much solute is present in a known volume of solution.

2. Mass concentrations The mass concentration of a solute is the mass dissolved in 1 dm 3 of solutions. Mass concentration = Mass. Volume (dm 3 ) What is the concentration, in g dm -3, a solution that contains 2.805g of dissolved KOH in 250 cm3 of water? Mass concentration = 2.805 =. 11.220 g dm -3 0.250

3. From p17 of textbook, Answers p240 Find the mass concentration, in g dm -3, for the following reactions. (a)0.042 moles of HNO 3 dissolved in 250cm 3 of solutions. (b)0.500 moles of HCl dissolved in 4 dm 3 of solution (c)3.56 x 10 -3 moles of H 2 SO 4 dissolved in 24 cm 3 of solution. 10.584 g dm -3 4.5625 g dm -3 13.969 dm -3

4. Molar concentrations Concentrations of solutions are measured in the unit mole per cubic decimetre, mol dm -3. 1 mol dm -3 means there is a mole of solute per cubic decimetre of solutions, 2 mol dm -3 means there are 2 moles of solute per cubic decimetre and so on.

5. Finding the concentration in mol dm -3. For example, 1.17g of sodium chloride was dissolved in water to make 500 cm 3 of solution. What is the concentrations in mol dm -3 ? The mass of one mole (Molar mass) of sodium chloride, NaCl, is 23.0 + 35.5 = 58.5g Number of moles = mass in g Molar Mass So 1.17g of NaCl contains 1.17 mol = 0.0200 mol 58.5 Molar concentrations Concentration = number of moles volume in dm 3 Concentration = 0.0200 = 0.0400 mol dm -3 0.500

6. Using cm 3 Number of moles in solution = conc x vol 1000 How many moles are present in 25 cm 3 of a solution of concentration 0.100 mol dm -3 Number of moles in solution = 0.10 x 25 1000 Number of moles in solution = 0.00025 moles

7. Molar solutions You will often see ‘M’ used on solution in chemistry ‘M’ means Molar and refers to a solution with concentration in moles per cubic decimetre, mol dm -3. Thus 2 mol dm -3 and 2M mean the same thing i.e. 2 moles of solute in 1 dm 3 of solution. Standard solutions A standard solution has a known concentration If you need to make up a standard solution, you will need to weigh out a substance To do this, you will need to: Consider the volume of solution required; work the amount, in moles, of solute convert this amount of solute into a mass, in g, so that you know how much to weigh out.

8. What mass of sodium hydroxide is needed to make 250 cm 3 of a 0.500M solution? Number of moles in solution = conc x vol 1000 = 0.500 x 250 1000 = 0.125 mol Molar mass of NaOH = 23.0 + 16.0 + 1.0 = 40.0 Mass = molar mass x number of moles Mass = 40.0 x 0.125 Mass = 5.00g

9. 0.50 mol dm -3 8.0 mol dm -3 2.0 mol dm -3 1.25 moles 0.0125 moles 2.65 g In mol dm -3