Solutions and Units of Concentration. May 25, 2015

Transcription

1 Solutions and Units of Concentration May 25, 2015

2 What is Solubility? Solubility: the maximum amount of solute that will dissolve in a certain amount of solvent at a given temperature Example: grams of salt (NaCl) in 100 g of water at C.

3 Factors Affecting Solubility 1. Nature of Solute / Solvent: Like dissolves like 2. Temperature: a. Solids/Liquids- Solubility increases with Temperature - Increase K.E. increases motion and collision between solute / solvent b. Gases - Solubility decreases with Temperature - Increase K.E. result in gas escaping to atmosphere

4 Factors Affecting Solubility 3. Pressure Factor: a. Solids/Liquids - Very little effect (esp. solids in liquids) - Solids and Liquids are already close together, extra pressure will not increase solubility b. Gases - Solubility increases with Pressure - Increase pressure squeezes gas solute into solvent. - HENRY S LAW: S = Solubility P = Press

5 Temperature & the Solubility of Gases The solubility of gases DECREASES at higher temperatures

6 SOLUBILITY FORMULAS amount of solute amount of solvent Given solubility = amount of solute amount of solvent unknown

7 SOLUBILITY PROBLEMS 1. The solubility of a solid is 15g / 100g of water. How many grams of the solid must be dissolved in 1 kg of water to make a saturated solution? (1 kg = 1000 g)

8 SOLUBILITY PROBLEMS amount of solute amount of solvent = amount of solute amount of solvent 15 g X = X=150g 100g 1000g

9 SOLUBILITY EXAMPLES 2. If you have 50g in 500g of water, using solubility from problem #1 (15 g / 100 g), is the solution saturated? 15g g = X g X = 75g is saturated No, 50 g is not saturated!

10 To read the graph: 1. Find the line for the substance. 2. The amount that dissolves at a given temp. is on the y- axis.

11 How much KNO 3 dissolves in 100g H 2 O at 50 o C? 1. Find the line (red) 2. Find the temperature and follow up to the line. (green) 3. Read across to the y- axis and this is the answer. (blue) 4. Since it is above the ½-way between 80 and 90, 87g KNO 3 will dissolve.

12 Types of Solutions: Saturated solution: point on the line Contains maximum amount of solute at given temp Contains what it should hold Supersaturated: above the line Contains more solute than a saturated solution Contains more than it should hold Unsaturated: below the line is Contains less solute than saturated solution Contains less solute than it could hold

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14 To do Calculations: To calculate how much extra has been dissolved: Extra = Dissolved amt - saturated in soln (given value) line that temp. To calculate how much more can be dissolved:? Much more = saturated - given line temp value

15 Example 1: How much less KCl is dissolved at 20 o C than at 60 o C in 100g H 2 O? Read the line value: 32g at 20 o C Subtract it from the given value: 45g 32g = 13 g

16 Example 2: How much more KCl is required to saturate the solution if 25g are dissolved at 40 o C? Read the line value: 40g at 40 o C Subtract the given value: 40g -25 g = 15 g

17 Your turn! Use your graph 1) What is the solubility of potassium nitrate at 30 0 C? 46 g/100 g 2) How many grams of ammonia can I dissolve in 100 grams of water at a temperature of 45 0 C? 32 g 3) At what temperature is the solubility of sodium chloride the same as the solubility of potassium chloride? 35 deg C 4) How many grams of ammonium chloride would I need to make 100 grams of a saturated solution at 70 0 C? 60 g 5) What do all of the compounds that decreased in solubility over the temperature range in the graph have in common? All gases 6) What compound is least soluble at 40 0 C? SO 2

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19 Colligative Properties When dissolving a solute in a solvent, the properties of the solvent are modified. Vapor pressure decreases Melting point Boiling point decreases increases Osmosis is possible (osmotic pressure) Colligative Properties are properties of a liquid that change when a solute is added The magnitude of the change depends on the number of solute particles in the solution, NOT on the identity of the solute particles.

20 Vapor Pressure Lowering for a Solution The diagram below shows how a phase diagram is affected by dissolving a solute in a solvent.

21 Adding a solute increases the boiling point.

22 Freezing Point Depression and Boiling Point Elevation Boiling Point Elevation T b =mk b (for water k b =0.51 o C/m) Freezing Point Depression T f =mk f (for water k f =1.86 o C/m) Note: m is the molality of the particles, so if the solute is ionic, multiply by the #of particles it dissociates to.

23 Example 1: Find the new freezing point of 3m NaCl in water. Freezing Point Depression T f =mk f (for water k f =1.86 o C/m) T f =(3m)(1.86 o C/m) 5.58 o C

24 Example 2: How many moles of ethylene glycol (C 2 H 6 O 2 ) should be added to 25 kg of water to lower the freezing point by 45 C? T f =mk f (for water k f =1.86 o C/m) 45=(m)(1.86 o C/m) = 24.2 m m = moles 24.2 = moles kg solvent 25 = 604 moles