Species in Solutions Showing relative concentrations of species present in aqueous solutions.

The composition of aqueous solutions The physical properties of a solution (conductivity, freezing point and pH) help us determine what species are present in an aqueous solution of a given concentration.

The composition of aqueous solutions All aqueous solutions contain: The substance dissolved (either molecular form or as ions) Liquid water H 3 O + (aq) and OH - (aq) – from the ionisation of water.

The composition of aqueous solutions The concentration of water molecules: There are 1000 g of water in 1 L of water, so the concentration of H 2 O(l) in moles per litre for dilute solutions is: c(H 2 O) = c(gL -1 )/M(g mol -1 ) = 1000 /18 = 55.5 mol L -1

The dissolving process Click here to see an animation of salt dissolving in water.here

Sodium Chloride A 0.1 mol L -1 solution of sodium chloride is a good conductor and has a pH of 7. The freezing point indicates that there are 2 moles of ions present in a litre of solution. The solution therefore contains: –55.5 mol L -1 H 2 O(l) –0.1 mol L -1 Na + (aq) –0.1 mol L -1 Cl - (aq) –10 -7 mol L -1 H 3 O + (aq) –10 -7 mol L -1 OH - (aq) H 2 O Na + Cl - H 3 O + OH -

Ammonium Chloride A 0.1 mol L -1 solution of ammonium chloride is a good conductor and has a pH of 5. The freezing point indicates that there are 2 moles of ions present in a litre of solution. The solution therefore contains: –55.5 mol L -1 H 2 O(l) –0.1 mol L -1 NH 4 + (aq) –0.1 mol L -1 Cl - (aq) –10 -5 mol L -1 H 3 O + (aq) –10 -5 mol L -1 NH 3 (aq) –10 -9 mol L -1 OH - (aq) H 2 O NH 4 + Cl - H 3 O + OH - NH 3

Ammonium Chloride (continued) The ammonium ion hydrolyses with water to form ammonia and hydronium ions: NH 4 + (aq) + H 2 O(l)  NH 3 (aq) + H 3 O + (aq) This is because it is the conjugate acid of the weak base, ammonia.

Calcium Chloride A 0.1 mol L -1 solution of calcium chloride is a good conductor and has a pH of 7. The freezing point indicates that there are 3 moles of ions present in a litre of solution. The solution therefore contains: –55.5 mol L -1 H 2 O(l) –0.1 mol L -1 Ca 2+ (aq) –0.2 mol L -1 Cl - (aq) –10 -7 mol L -1 H 3 O + (aq) –10 -7 mol L -1 OH - (aq) H 2 O Ca 2+ Cl - H 3 O + OH -

Your turn now For each of the following substances –Work out what species are present in solution –Work out the approximate concentrations of each species present –Draw bar graphs for their composition (include water) 0.1 mol L -1 Ethanoic acid (pH = 3, conductivity poor, solute particle count = 2) 0.1 mol L -1 Aluminium sulfate (pH = 3, conductivity excellent, solute particle count = 5)