Concentration Units

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# Concentration Units - PowerPoint PPT Presentation

moles of solute. liters of solution. moles of solute. m =. mass of solvent (kg). M =. Concentration Units. Molarity (M). Molality (m). 12.3. moles of solute. m = molality =. What is the molality of a solution prepared by dissolving 3.4 moles of KI into 500.g of water?.

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moles of solute

liters of solution

moles of solute

m =

mass of solvent (kg)

M =

Concentration Units

Molarity(M)

Molality(m)

12.3

moles of solute

m =molality=

What is the molality of a solution prepared by

dissolving 3.4 moles of KI into 500.g of water?

kg of solvent

Molality

The molal concentrationof a solution is expressed as the moles of solute per kilogramof solvent.

3.4 moles

------------ = 1.7 m

.500 kg

# of moles = 3.4

Mass of water = 500g

or .500kg

moles of solute

m=

mass of solvent (kg)

What is the molality of a solution made from 155g of sodium chloride and 1500g of water?

155g of NaCl

---------------- = 2.65 molNaCl

58.5g/mol

1500g of water = 1.5kg of water

2.65 moles NaCl

____________________

=1.8m

1.5kg water

Colligative Properties

The addition of solutes into a solvent changes the physical properties of the solvent. Two examples of this would be boiling point elevation and freezing point depression.

moles of solute

m=

mass of solvent (kg)

149 moles

=

3.202 kg solvent

What is the freezing point of a solution containing

149 moles of ethylene glycol (antifreeze) in 3202 g of

water?

DTf = Kfm

Kf water = 1.86 0C/m

= 2.41 m

DTf = Kfm

= 1.86 0C/m x 2.41 m = 4.48 0C

= 0.00 0C – 4.48 0C = -4.48 0C

Tf = T f – DTf

actual number of particles in solution

van’t Hoff factor (i) =

number of formula units dissolved in the solution

Colligative Properties of Electrolyte Solutions

Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.

0.1 mNaCl solution

0.1 m Na+ ions & 0.1 mCl- ions

ivalue

Example solute

1

nonelectrolytes

2

NaCl

CaCl2

3

12.7

Change in Freezing Point
• Which chemical would be most effective to de-ice a frozen street and why?
• SiO2
• NaCl
• CaCl2
Freezing Point Depression

At what temperature will a 5.4 molal solution of NaCl freeze?

Solution

∆Tfp= Kf • m • i

∆Tfp= (1.86 oC/molal) • 5.4 m • 2

∆Tfp= 20.oC

FP = 0 – 20. = -20. oC