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Solutions: Concentration

Solutions: Concentration. Chapter 14. Solution. Homogenous mixture of 2 or more substances in single phase = 1 layer Component present in largest amt = solvent Other component(s) = solute Alloys, air are all solutions (solns). Colligative properties.

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Solutions: Concentration

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  1. Solutions:Concentration Chapter 14

  2. Solution • Homogenous mixture of 2 or more substances in single phase • = 1 layer • Component present in largest amt = solvent • Other component(s) = solute • Alloys, air are all solutions (solns)

  3. Colligative properties • = change of mp, bp, osmotic pressure of soln, change in vapor pressure • Irrespective of solute identity • Dependent on concentration

  4. Concentration • Molarity (M) = moles solute/L of solution = mol/L • We’re all familiar with this • Drawback  won’t give you proper amount of solvent used to make soln • Molality (m) better

  5. Problem • Give the concentration (in M) of 0.0012 grams of NaCl in 545 mL of water • MW of NaCl = 58.442 g/mol

  6. Molality • Molality (m) = moles solute/kg of solvent • Let’s look to the right •  molality  molarity

  7. Problem • Give the concentration (in m) of 0.0012 grams of NaCl in 545 mL of water • Density of water @ 25°C = 0.9970 g/mL • MW of NaCl = 58.442 g/mol

  8. Mole fraction • Mole fraction of A (XA) = nA/ntot • Amt of component A/total components • Soln contains 1.00 mol ethanol and 9.00 mol water

  9. Weight percent • Ex: 46.1 g ethanol & 162 g water • Commonly used in household products like vinegar & bleach

  10. Problem • Concentrated sulfuric acid has a density of 1.84 g/cm3 and is 95.0% (w/w) H2SO4. MW H2SO4 = 98.079 g/mol. • Calculate the molarity and the molality of this solution.

  11. Solution: molarity

  12. Solution: molality

  13. Problem • A 10.7 molal solution of NaOH has a density of 1.33 g/cm3 at 20°C. MW NaOH = 39.996 g/mol & MW H2O = 18.0153 g/mol. • Calculate the mole fraction of NaOH, the weight percentage of NaOH and the molarity of the solution.

  14. Solution

  15. More practice • An aqueous soln of NaCl is created using 133 g of NaCl diluted to a total soln volume of 1.00 L. • Calculate the molarity, molality, and mass percent of the soln, given a density of 1.08 g/mL and MW of NaCl = 58.442 g/mol.

  16. Solution

  17. Part per million • = PPM (in grams) • Ex: 1.0 ppm = 1.0 g of substance in system w/ 1.0 million g total mass • @ STP water density  1.0 g/mL • So, mg/L and ppm are  • Used predominately by environmental and analytical chemists

  18. Solution process • One can add only so much solute to solvent • Since no more dissolves  soln said to be saturated • NaCl = 35.9 g/100 mL water (25°C) • Albeit, nothing changes visually, soln is constantly dissolving and re-solidifying ions

  19. Solution process • Essentially, solubility = solute concentration in equilibrium w/undissolved solute in saturated soln • Unsaturated soln = soln w/less than saturated amt of solute • NaCl < 35.9 g/100 mL water (25°C) • Supersaturated soln = soln w/more solute than sat. soln • NaCl > 35.9 g/100 mL water (25°C)

  20. Making supersaturated solutions • Pour in excessive amount of solute • Heat up the soln • Stir until all solute dissolves • Cool it slowly • No shaking, no jarring of soln • Gives lower freezing point • Once disturbed (energy in), causes crystallization to occur  excess crystallized out of soln • Exothermic • Heat packs of sodium acetate (can reach 50°C!) • http://www.npr.org/programs/wesun/features/2001/dec/heatpack/011229.heatpack.html • Your second lab deals with this

  21. Liquids as solutes • Miscible = mixable • Immiscible = unmixable • Used in language too: • Mestizo, mischling • Ability to dissolve based on similar polarities (or lack thereof) of solute/solvent • Like dissolves in like

  22. Let’s try these • Considering intermolecular forces, give reasons for the following observations: • a) Octane, C8H18, is very miscible with CCl4. • b) Methanol, CH3OH, mixes in all proportions with water. • c) Sodium bromide is not very soluble in diethyl ether (CH3CH2—O—CH2CH3). • d) Octanol, C8H17OH, is not very soluble in water.

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