Dilution Review

• Take 5 minutes with your partner to solve the scenario below.

• Oops! I spilled 100mL of water into my 300mL of 6 M HCl. Is my solution too weak to use if my lab requires a 1M to 1.5 M HCl solution?

• How do you know?

Colligative Properties

Defining Colligative Properties

• PHYSICAL properties of the solvent that change when a solute is added– The degree of change is dependent upon

the number of solute particles added to the solvent

– The more solute particles = the greater the change in colligative property

Number of Particles of Solute• Think back to what you’ve learned about ionic

and covalent compounds dissolving in water.– Ionic compounds DISSOCIATE as they dissolve.– Covalent compounds do NOT dissociate as they

dissolve.

• Apply this idea.– How many solute particles will 1 piece of NaCl

add to a solvent?• Answer: 2 (Na+ and Cl-)

Applying the Concept Further• How many solute particles will 1 piece of

magnesium phosphate add to a solvent?– Mg3(PO4)2 breaks into 3 Mg2+ ions and 2 PO4

3- ions. Answer: 5 solute particles

• How many solute particles will 1 piece of sugar add to a solvent?– Sugar is covalent. Answer: 1 solute particle

• Finally, which of the two above will affect a colligative property the most?

Two Colligative Properties

1. Boiling point

2. Freezing point

Boiling point is changed when solute is added to the solvent.• Boiling point elevates when a solute is

added to a solvent.

• The solution requires more energy to reach boiling.

• Example: Salt water will not boil at 100°C. It will boil at a HIGHER temp.

Calculating the NEW Boiling Pt.

∆Tb = Kb m i

• ∆Tb = change in boiling pt.• Kb = boiling point constant for

the solvent (will be given)• m = molality• i = number of ions present (USE

ONLY WITH IONIC SOLUTES!)

Example of Boiling Pt. Calculation

• What is the boiling point when 15.0g NaCl is dissolved into 200 mL of water? (Kb of water is 0.52 °C/m)

– ∆Tb = Kb m i

• You are solving for ∆Tb , and you have the Kb to use.

• m =You’ll need to calculate the molality from the info in the question. (change 15.0 g of NaCl to moles and 200 mL to kg and plug in)

• i = Finally, is the solute ionic? YES…NaCl is ionic and will give TWO ions when dissolved.

• What is the boiling point when 15.0g NaCl is dissolved into 200 mL of water?• You’ll need to calculate molality first.

– 15.0g NaCl x 1 mol NaCl = 0.256 moles NaCl

58.5 g NaCl

– 200 mL = 200 g = 0.2 kg of water

– Molality = 0.256 moles NaCl/ 0.2 kg water = 1.28m

• ∆Tb = Kb m i

• ∆Tb = (0.52 C/m)(1.28m)(2)

• ∆Tb = 1.33 C (note: This is the CHANGE in boiling point. It does NOT answer our question.)

• New Boiling Point: 100C + 1.33C = 101.33 • We took the regular boiling point of water and added the

change since boiling point ELEVATES with a solute added.

Ten Minute PartnersBoiling Point Calculations

1. What is the boiling point when 30.0g CaCl2 is dissolved into 200 mL of water?

2. The molality of the above example is similar to the problem that we worked together a moment ago. Explain the difference in boiling temperature.

3. CHALLENGING: How many grams of NaCl would need to be added to the water to change the boiling temperature of 200 mL to 110°C?

Kb of water = 0.52°C/m

Answers to Boiling Point Calculations

1. ∆Tb= (1.36m)(0.52C/m)(3) = 2.12C100C + 2.12C = 102.12C

2. The difference is due to the difference in the number of solute particles (ions). 3.10C = m(0.52)(2)

m = 9.629.62m = mole/0.2kg

mole = 1.92 mole NaCl 112.2 grams NaCl

Freezing point is changed when solute is added to the solvent.

• Freezing point depresses when a solute is added to a solvent.

• The solution requires a lower temp to reach freezing.

• Example: Salt water will not freeze at 0°C. It will freeze at a LOWER temp.

Calculating a NEW Freezing Pt.

∆Tf = Kf m i

• ∆Tf = change in freezing pt.• Kf = freezing point constant for the

solvent (will be given)• m = molality• i = number of ions present (USE ONLY

WITH IONIC SOLUTES)

Example of Freezing Pt. Calculation

• What is the freezing point when 15.0g NaCl is dissolved into 200 mL of water?

• (Kf of water is 1.86 °C/m)

– ∆Tf = Kf m i

• You are solving for ∆Tf , and you have the Kf to use.

• m =You’ll need to calculate the molality from the info in the question. (change 15.0 g of NaCl to moles and 200 mL to kg and plug in)

• i = Finally, is the solute ionic? YES…NaCl is ionic and will give TWO ions when dissolved.

• What is the freezing point when 15.0g NaCl is dissolved into 200 mL of water?• You’ll need to calculate molality first.

– 15.0g NaCl x 1 mol NaCl = 0.256 moles NaCl

58.5 g NaCl

– 200 mL = 200 g = 0.2 kg of water

– Molality = 0.256 moles NaCl/ 0.2 kg water = 1.28m

• ∆Tf = Kf m i

• ∆Tf = (1.86 C/m)(1.28m)(2)

• ∆Tf = 4.76 C (note: This is the CHANGE in freezing point. It does NOT answer our question.)

• New Freezing Point: 0C – 4.76C = -4.76C• We took the regular freezing point of water and subtracted

the change since freezing point DEPRESSES with a solute added.

Ten Minute PartnersFreezing Point Calculations

1. What is the freezing point when 30.0g CaCl2 is dissolved into 200 mL of water?

2. The molality of the above example is similar to the problem that we solved together. Explain the difference in freezing temperature.

3. CHALLENGING: How many grams of NaCl would need to be added to the water to change the freezing temperature of 200 mL to -8°C?

Kf of water = 1.86°C/m

Answers to Freezing Point Calculations

1. ∆Tf= (0.271m)(1.86C/m)(3) =1.51C0C - 1.51C = -1.51C

2. The difference is due to the difference in the number of solute particles (ions).

3. 8C = m(1.86C/m)(2)molality = 2.15

2.15 = mole/0.2kgmole = 0.43 mole NaCl

25.14 grams NaCl

Practice Practice Practice!

• The 15-4 Practice Problems in your practice packet are boiling point/freezing point calculations.

• The key is posted at the front of the classroom.

• PLEASE SOLVE MANY OF THESE PROBLEMS! You’ll see several on Wednesday’s test.