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Unit X : Electrolytes and Colloids

Unit X : Electrolytes and Colloids. … Chapter 14…. Colligative Properties and Molar Mass Determination. A solution prepared from 1.25g of oil of wintergreen (methyl salicylate) in 99.0g of benzene has a boiling point of 80.31°C. Determine the molar mass of this compound.

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Unit X : Electrolytes and Colloids

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  1. Unit X : Electrolytes and Colloids … Chapter 14…

  2. Colligative Properties and Molar Mass Determination • A solution prepared from 1.25g of oil of wintergreen (methyl salicylate) in 99.0g of benzene has a boiling point of 80.31°C. Determine the molar mass of this compound. ΔTbp = 80.31°C – 80.10°C = 0.21°C m = ΔTbp /Kbp = 0.21°C/2.53°C/m = 0.083m amount of solute = (0.083mol/1.00kg)(0.099kg)= 0.0082 mol solute 1.25g/0.0082mol = 150 g/mol

  3. Colligative Properties and Molar Mass Determination • Beta-carotene is the most important of the A vitamins. Its molar mass can be determined by measuring the osmotic pressure generated by a given mass of β-carotene dissolved in the solvent chloroform. Calculate the molar mass of β-carotene if 10.0 mL of a solution containing 7.68 mg of β-carotene has an osmotic pressure of 26.57 mm Hg at 25.0 °C.

  4. Colligative Properties and Molar Mass Determination •  = cRT • c = /RT • = 26.57 mm Hg x (1 atm /760 mm Hg) = 0.03496 atm c = 0.03496atm/(0.082057L·atm/mol·K x298.15K) = 1.0429 x 10-3 mol/L (1.0429 x 10-3 mol/L)(0.0100L) = 1.43 x 10-5 mol 0.00769g/ 1.43 x 10-5 mol = 538 g/mol

  5. Colligative Properties of Solutions Containing Ions (Electrolytes) • Since these properties depend on the number of particles dissolved, solutions of electrolytes (which dissociate in solution) should show greater changes than those of nonelectrolytes.

  6. Colligative Properties of Solutions Containing Ions (Electrolytes) • However, a 1M solution of NaCl does not show twice the change in freezing point that a 1M solution of methanol does.

  7. Colligative Properties of Solutions Containing Ions (Electrolytes) • van’t Hoff factor: • One mole of NaCl in water does not really give rise to two moles of ions. • Some Na+ and Cl- reassociate for a short time, so the true concentration of particles is somewhat less than two times the concentration of NaCl.

  8. Colligative Properties of Solutions Containing Ions (Electrolytes) • van’t Hoff factor cont’d: • Reassociation is more likely at higher concentration. • Therefore, the number of particles present is concentration-dependent. • We modify the previous equations by multiplying by the van’t Hoff factor, i. Tf = Kf  m  i i = ΔTfp,measured / ΔTfp,calculated i = ΔTfp,measured / Kfp,m

  9. Colligative Properties of Solutions Containing Ions (Electrolytes) • A 0.00200m aqueous solution of an ionic compound, Co(NH3)5(NO2)Cl, freezes at -0.00732°C. How many moles of ions does 1.0 mol of the salt produce on being dissolved in water? Kfp = -1.86°C/m ΔTfp = -7.32 x 10-3°C ΔTfp = Kfpm = (-1.86°C/m)(0.00200m) = -3.72 x 10-3°C i = -7.32 x 10-3°C/ -3.72 x 10-3°C = 1.97 ~ 2

  10. Colloids • Suspensions of particles larger than individual ions or molecules, but too small to be settled out by gravity are calledcolloids.

  11. Colloids • Two distinguishing characteristics: • Generally have high molar masses • Ex. Hemoglobin w/ a molar mass in the thousands • The particles are relatively large (ex, 1000nm in diameter) • Tyndall Effect – scatter visible light when dispersed in a solvent, making the mixture appear cloudy

  12. Types of Colloids • Some molecules have a polar,hydrophilic(water-loving) end and a non-polar, hydrophobic(water-hating) end. • Sodium stearate is one example of such a molecule.

  13. Types of Colloids • These molecules can aid in the emulsification of fats and oils in aqueous solutions.

  14. Emulsions • Colloid dispersions of one liquid in another • Ex. Oil in Water • Emulsifying Agent • Stabilizes the colloidal dispersion • Soaps and detergents • Surface-Active agents • Surfactants • Substance that affects the properties of surfaces, therefore affect the interaction between two phases

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