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10-7-08[lecture10]ec

# 10-7-08[lecture10]ec - Chem 116 Student Notes(EC Lecture 10...

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Unformatted text preview: Chem 116 Student Notes September 23, 2008 (EC) Lecture 10: Concentrations and Colligative Properties 4 Ways to Express Mixture Density (aka Concentration) • 3 kinds of “particle density” o Molarity (n/V) o o • Useful for stoichiometry calculations Molality (n/m) Useful for boiling/freezing point calculations Mole Fraction (decimal # between 0‐1) Useful for partial pressure calculations 1 kind of “mass density” o Weight Percent (0 – 100%) Useful for chemical analysis of the unknowns “solution” = “mixture” in liquid phase Clicker Question: 1. 2. 3. 4. Solute Na+ and Cl‐ ions Water NaCl (s) Water Solvent Water + ‐ Na and Cl ions Water NaCl (s) (correct) the Na and Cl ions separate (wrong) water = solvent since there is more of it (wrong) NaCl is not in solid form in the solution (wrong) water = solvent since there is more of it In a Solution: 1. BPE: Boiling Point = Higher a. Solute increases the BP of the solvent i. ∆Tbp = i msoluteKbp = (effective molality) (solvent factor) ) (solvent factor given or looked up) =(# of ions) ( =change in boiling temperature caused by the solute 1. Add this number to the Tobp(solvent) b. Solute increase the BP by lowering the vapor pressure (Raoult’s Law) i. Psolvent = Xsolvent Posolvent fraction of surface taken up by solvent = (fraction of surface taken up by solute) (fraction taken up by pure solvent‐1.0) 1. Xsolvent = mole fraction of the solvent 2. Xsolvent + Xsolute1 (+ Xsolute2 + Xsolute3)= 1 ii. This is done by blocking some of the solvent from escaping on the surface iii. For this we have to assume that the solute has no vapor pressure iv. A higher temperature is therefore needed to reach the vapor pressure of the atmosphere (boiling) 2. FPD: Freezing Point = Lower a. Solute decreases the FP of the solvent i. ∆Tfp = i msoluteKfp = (effective molality) (solvent factor) ) (solvent factor given or looked up) =(# of ions) ( =change in freezing temperature caused by the solute 1. Subtract this number to the Tofp(solvent) b. Solute increase the FP by lowering the vapor pressure (Raoult’s Law) i. Psolvent = Xsolvent Posolvent fraction of surface taken up by solvent = (fraction of surface taken up by solute) (fraction taken up by pure solvent‐1.0) 1. Xsolvent = mole fraction of the solvent 2. Xsolvent + Xsolute1 (+ Xsolute2 + Xsolute3)= 1 3. Osmotic Pressure a. (osmotic pressure, π)V = nRT b. c. ...
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