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solutions2 - Solomons Study Notes General Chemistry CHE 132...

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S olomon’s Study Notes General Chemistry CHE 132 Spring 2009 Midterm 3 Solomon Weiskop PhD [ Solutions (2) ] These Study Notes cover the material of Ch. 15
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Study Notes & Practice Problems are available to print out by registering at www.solomonlinetutor.com Solomon Weiskop PhD Solomon’s CHE 1 32 Tutoring © Copyright 2009
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1 1. Concentration There are quite a few different ways to express the concentration of a solution. So far we’ve only used Molarity ( M ). But we are now going to discuss a few more: molality ? , mole fraction 𝑿 , parts per million ( ppm ) & parts per billion ( ppb ) and mass percent ( mass % ). Molarity (M) Molarity M = moles of solute liters of solution as a formula: 𝐌 = ? 𝐕 L molality ? molality ? = moles of solute ?𝒊????𝒂?? ?? ???𝒗??? as a formula: ? = ? ???? ( ?𝐠 ) (Unfortunately both mass and molality are represented by the letter m. So here I write out the word ‘mass’.) Note that, for molality, in the denominator it is kilograms of solvent (and not liters of solution (as was the case for Molarity). We will use molality ? when discussing Boiling Point Elevation and Freezing Point Depression (later in these Notes).
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2 Ex.1: What is the molality of a solution made by dissolving 72g propanol (CH 3 CH 2 CH 2 OH MM=60g/mol) in 350 mL of water at a temperature where the density of water is 0.982g/mL. Here the solute is propanol. The solvent is water: moles of propanol = 72g 60g/mol = 1.2 mol kilograms of water = 350mL 0.982g mL 1 kg 1000g = 0.3437 kg ∴ ? = 1.2mole 0.3437 kg = 3.49mole kg = 3.49 molal Ex.2: How many grams of CaCl 2 (MM= 111g/mol) must be added to 200mL of water to prepare a 0.45 molal aqueous CaCl 2 solution? Assume the density of liquid water is 1g/mL. ? = n mass kg n = ? mass(kg) moles of CaCl 2 = 0.45 moles CaCl 2 kg H 2 O 200mL 1g mL 1kg 1000g = 0.09 mole CaCl 2 converting to grams: 0.09 mole 111g/mol = 10g CaCl 2
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3 Propanol is a molecular compound that, when dissolved in water, does not dissociate into ions: CH 3 CH 2 CH 2 OH ? H 2 O CH 3 CH 2 CH 2 OH aq Ionic compounds such as salts or metal hydroxides, on the other hand, do dissociate into ions when dissolved in water: CaCl 2 s H 2 O Ca 2+ aq + 2Cl 1 aq NaOH s H 2 O Na 1+ aq + OH 1 aq When we discuss Colligative Properties (Section 5) it will turn out to be important to take this into account. We will do so by using: van’t Hoff ionic factor 𝒊 𝒊 counts the number of “particles” that result when a solute is dissolved in water Looking at the chemical equations written earlier we can see that for propanol CH 3 CH 2 CH 2 OH 𝒊 = 1 for calcium chloride CaCl 2 𝒊 = 3 for sodium hydroxide NaOH 𝒊 = 2
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4 Mole Fraction ( 𝑿 ) 𝑿 solute = moles of solute total moles solute + solvent 𝐗 solute = n solute n tot 𝑿 solvent = moles of solvent total moles solute + solvent 𝑿 solvent = n solvent n tot where n tot = n solute + n solvent Note : 𝑿 solute + 𝑿 solvent = 1 If there is more than one solute, you can be asked to calculate the mole fraction for each of them and also for the solvent.
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