Module 2: Solutions The Science of Mixing : Have you ever been in a wrong mix?

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1 PART 1 Name: All matter around us exists in a mixed state. Chemists look at the atomic level and try to explain why certain matters mix homogeneously (uniformly) and certain types of matters (or compounds) stay separated or heterogenous. In chemistry, we develop our understanding of matter and its behavior in two (2) ways: 1) We observe matter and try to see if we can generalize something about what we see in a way that applies to all situations. This is how laws are discovered like the ideal gas law, etc. This is the outside-in view. You view bulk properties from the outside looking into matter s behavior without trying to explain why matter behaves the way it does. For example, the gas law relationships and the ideal gas model expresses the law mathematically as: P.V=n.R.T or (P1.V1)/T1=(P2.V2/.T2) These equations describe gas behavior in different conditions of P, V, T, n. These are mathematical or engineering models and these models do not explain why. They just predict behavior under different or varying conditions. 1) Can you come up with 2 more bulk models related to this chapter based on your reading and homework assignment? Name these models and explain what they predict. Be short and concise (in one sentence!) Model: Describes: Model: Describes: 2) Sometimes it is extremely helpful to look within matter (molecular or nano scale) and try to see if our nano level understanding of matter can help explain explain bulk properties. Name 3 aspects of matter at the molecular nano level that govern bulk properties: Name at least two bulk properties that are governed by these molecular aspects that you identified in step 2. Energetics of mixing (is it a bulk or nano scale property? ) 3) What does it mean when two (or more) compounds have a net positive (endothermic) energy of mixing? 4) What does it mean when two (or more) compounds have a net zero energy of mixing? 5) What does it mean when two (or more) compounds have a net negative (exothermic) energy of mixing? 6) What does enthalpy of a solution describe (in one line)? and what are the symbols used for the 3 enthalpy terms that make up enthalpy of solution? 1 P a g e Solutions Group Work 2018 K. Golestaneh

2 7) Draw the Lewis structure of acetone, CH3COCH3 and chloroform, CHCl3 using correct bond angle relationships; include partial charges on each atom (if any). Show how these two molecules position themselves when next to each other and show the attraction using a dotted line. Answer questions a) List the primary and secondary intermolecular forces (in the order of significance or strength) in acetone:, b) Is acetone a polar or nonpolar molecule?, c) Which is more polar, acetone or chloroform?. d) Do you predict the Hsln for mixing acetone with chloroform to be positive (>0), negative (<0) or close to zero (ideal solution)? Explain why. 8) Air contains 21% oxygen, O2 gas (which means that at 1.0 atm air pressure oxygen gas has a partial pressure of 0.21 atm. Review your gas laws). Calculate mass percent of oxygen gas in water of a mountain lake saturated with air at 20 C. The value of Henry s law constant for oxygen gas at this temperature is 1.38x10-3 M/atm. Answer: 9.3E-4 % 2 P a g e Solutions Group Work 2018 K. Golestaneh

3 PART 2: 1. Calculate the vapor pressure of the mixture when 25 g of acetone, CH3COCH3, is added to 25 g of chloroform (CHCl3) at 35 C. Pure acetone has a vapor pressure of 363 torr and pure chloroform has a vapor pressure of 303 torr at 35 C. a. Calculate the total vapor pressure (torr) of the mixture (solution). b. Calculate mole fraction of acetone in the vapor phase. 2. A student weighs 0.25 g of CaCl 2 and dissolves it in 250 ml of water. Assume a solution density of 1.00 g/ml (same as pure water) and a solution volume of 250 ml. Determine: a. Molarity of CaCl 2 b. Molality of CaCl 2 c. Molarity of chloride ion. d. Total ion molarity e. Mass of chloride ion in the solution 3 P a g e Solutions Group Work 2018 K. Golestaneh

4 f. ppm calcium ion g. ppm chloride ion h. % by mass CaCl 2 i. Volume of solution (L) which contains 125 mg of chloride ion 3. What intermolecular forces (IMF) are present in iodine? a. Do you expect iodine to be soluble in water? b. Do you expect iodine to be solid, liquid or gas at normal conditions? c. Comment on the extent of solubility of iodine water compared to methyl alcohol (CH 3OH)? 4. A student wants to make some seawater to maintain her salt-water aquarium as it is much cheaper to make your own sea water than to purchase it prepared. After a search she finds that she can mix the following salts according to the stated concentration in molarity. Salt NaCl CaCl 2 KCl MgCl 2 MgSO 4 Molarity (M) a. Calculate molarity of each ion listed below based on preparing 1.00 L of solution (complete the following table). Pay attention to summing up molarity of common ions between compounds! Ion Cl - 2- SO 4 Na + Ca 2+ Mg 2+ K + Molarity (M) 4 P a g e Solutions Group Work 2018 K. Golestaneh

5 Next, the student makes 1.00 L of the solution according to the compound molarities shown on the first table and determines the density of her artificial seawater (in the laboratory) to be 1.03 g/ml. b. What is the expected freezing point of this artificial seawater? Assume that the solution mass is the same as water (solvent) mass since the solution is mostly water. 5. Reverse osmosis water purification using high pressure pumps is a practical method of removing salts from seawater, in order to make it suitable for drinking. Calculate the minimum pumping pressure (psi) necessary (at 25 C) in order to purify a seawater with a composition stated in the previous problem (involving artificial seawater preparation). Part 2 answers: torr, 0.71 (mol frac) 2. a) 9.0E-3M b) 9.0E-3m c) 1.8E-2M d)0.027 M e)0.16 g f) 360 ppm g) 640 ppm h) 0.10% i) 0.20 L 3. a) Dispersion b) more soluble in alcohol since the methyl group of alcohol promotes solubility through dispersion IMF bonding. 4. a) C M M ion M M Cation Anion Na NaCl Cl CaCl Ca KCl K MgCl Mg MgSO SO Sum T f, C sln, kg Need to apply a minimum of 402 psi just to overcome osmotic pressure. (In practice we need more pressure to obtain appreciable purified water flow.) 5 P a g e Solutions Group Work 2018 K. Golestaneh