Chapter 14 Aqueous Solutions. the comparison of solution processes in water for molecular substances and ionic compounds

Saturated Unsaturated and Supersaturated The solubility of a substance refers to the maximum amount of that substance that can be dissolved in a given quantity of solvent at a certain temperature. Saturated solution - a solution that contains precisely the maximum mass of solute. Unsaturated solution – a solution that contains less than the maximum mass of solute. Supersaturated solution – a solution that contains more than the maximum amount of solute.

Solubility is measured in grams per 100 grams of water (g/100g) Most salts (ionic substances) are soluble in water to some extent Solubility is influenced by the temperature of the solution If a substance does not dissolve in water it is insoluble

Solubility Curve Used to represent the solubility of a particular substance at a given temperature. The graph can be used to measure the amount of solute present in a solution of a particular temperature Any point on the curve indicates a saturated solution. Any point below the curve indicates an unsaturated solution. Any point above the curve indicates a supersaturated solution.

Questions - Solubility Curves What is the solubility of KNO3 at 80C? Which substance has the highest solubility at 100C? At what temperature is NaNO3 solubility 100g/100g of water? At 40C, what would be the solubility of a Super, saturated and unsaturated solution of NaCl?

Crystallisation When something cools the solute may no longer stay dissolved in the solvent and will come out as crystals This is called crystallisation Kidney stones are formed when minerals in urine come out of the solution and form crystals.

Crystal growth can be effected by… Nucleation Nucleation refers to having a nucleus to form around, this is achieved by “seeding”; where a single crystal is added and crystals are able to form around. Impurities such as dust or even a scratch on the glassware can act as a nucleus. The rate the solvent evaporates The faster the rate the smaller the crystals The rate the solution or compound cools Slow cooling = large crystals. Rapid cooling = small crystals.

Calculations An 80g sample of NaNO3 is added to 200g of H2O at 20 C. Use the solubility curve to calculate how much more NaNO3 needs to be added to the solution saturated with NaNO3 at 20C Solution 85g of NaNO3 at 20C to 100g to make saturated 85 X 2 = 170g of NaNO3 in 200g of water 170 (needed – 80(already added) = 90g need to be added

Calculations What happens in a solution containing 50g of KNO3 in 100g of water that is allowed to cool from 40C to 20C? Solution At 40C normally solubility would be 60g/100g of water BUT we have 50g/100g of water At 20C solubililty is 35g/100g of water 50 – 35 = 15g will crystallise out of solution

Solubility of liquids and gases in water It is generally true that the amount of solid that can dissolve in water increases as you increase the temperature of water. When looking at liquids dissolving in water there is not a clear trend so you can not assume that increasing the temperature of water will increase the amount of liquid dissolved. Gases generally become less soluble as the temperature of water increases…..which can be a big issue for fish in the ocean when the temperature of the ocean increases due to global warming!!!! Solubility curves for some gases

Concentration Concentration is the amount of solute dissolved in a given solvent. High concentration – high ratio of solute:solvent. This is called a concentrated solution Low concentration – low ratio of solute:solvent A dilute solution is one which has a low concentration

Unit Conversion microlitre millilitre Litre kiloLitre uL mL L kL /1000 x1000 x1000 x1000

Measuring Concentration- Concentration can be measured in several ways 1. Grams per Litre Concentration = mass of solute in grams (g) volume of solution in litres (L) Concentration = g/L or gL-1 2. Parts per Million (ppm) Concentration = mass of solute in grams (mg) mass of solution (kg) 3. Molarity is defined as the number moles of solute particles per litre of solution The symbol for molarity is M 1M = One Mole of solute per 1 litre. Concentration = mol or mol L-1 L

Molar Concentration (molL-1) or Molarity(M) The number of moles per Litre of a solution is known as the molar concentration.  Concentration (molL-1) = mol / volume in (L) n = mol n c = Concentration V = volume in Litres n=cV C V  

Calculations Calculate the molar concentration of a solution that contains 0.105mol of KNO3 dissolved in 200mL of solution solution 1. Convert 200mL to L = 200/1000 = 0.2L 2. 0.105mol = 0.525 mol/L or 0.525 M 0.2L

Calculations Calculate the amount, in moles, of ammonia(NH3) in 25.0mL of 0.3277M (mol/L) Solution Convert 25 /1000 = 0.025L 0.3277 mol x 0.025L = 0.00819 mol L

Dilution The process of adding more solvent to a solution is called dilution You are not changing the amount of solute but the amount of solvent Therefore the amount (grams) of solute stays the same When a substance is diluted the solute particles have more space between them

Formula When we dilute something we can calculate either the new concentration or volume. This results in a formula called the dilution formula. c1V1 = c2V2 The same amount of solute is in each flask but they have different concentrations and volumes

Calculations The concentration of a seaweed extract in a bottle of fertiliser is 9.0g/L. When used to fertilise plants, the seaweed fertiliser must be diluted, if 10mL of seaweed is diluted to fill 2L what is the new concentration? 10mL /1000= 0.01L c1 = 9.0g/L V1 = 0.010L c2? V2 = 2L 9.0g/L x 0.01L = c2 x 2L 9.0g/L x 0.01L = c2 = 0.045g/L 2L

Calculations How much water must be added to 30mL of 2.5 mol/L solution of NaOH to dilute it to 1 mol/L The number of moles of solute does not change dilution c1 = 2.5 mol/L V1 = 30mL c2 = 1 mol/L V2 = ? c1V1 = c2V2 V2 = c1V1 c2 2.5mol/L x 0.030L = 0.075L 1 mol/L 75mL – 30mL = 45mL is added