1. Chemistry of Solutions Chapter 7

2. Types of Solutions Although there are many examples of solutions in different phases – gases in gases; gases, liquids, or solids in liquids; and liquids or solids in solids – the most frequent situation in chemistry is working with something dissolved in a liquid. A solution is a homogeneous mixture – i.e., no separation of solute and solvent, concentration the same everywhere.

3. Water Water is the most common solvent in a chemistry laboratory. Dissolves many materials because of its ability to form hydrogen bonds or because of its polarity. However, water has trouble dissolving many non-polar substances, particularly organic compounds.

4. Like Dissolves Like Polar solvents like to dissolve polar or ionic solutes – salt in water, acetic acid in water, methanol in water, acetic acid in methanol Nonpolar solvents like to dissolve nonpolar solutes – toluene in hexane, hexane in carbon tetrachloride Note that surfactants work by having a nonpolar end that is attracted to nonpolar grease and an opposite polar end attracted to water to carry the grease away. Also a model for cell walls (lipid chemistry).

5. Electrolytes and Nonelectrolytes An electrolyte is a solute that separates into ions in water. Differentiated by labels “strong” and “weak”. Strong – dissociate 100% into ions. (NaCl) Weak – stays mostly as intact molecules. Only a small portion dissociates into ions – (acetic acid, phenol, ammonia) A nonelectrolyte does not dissociate at all (sugar, ethanol). Stays as intact molecules. Conductivity of a solution is a good measure of strength of an electrolyte.

6. Equivalents Used to describe electrolyte concentrations – examples in book are taken from medical applications. Def: an equivalent is the number of moles of an ion providing one mole of positive or negative charge. # equivalents of ion = # moles of ion * Absolute value of charge of the ion e.g., 0.4 moles Ca +2 = 0.8 equivalents Ca +2

7. Example on Equivalents A solution contains 40 mEq/L Cl - and 15 mEq/L of HPO 4 2-. If Na + is the only cation in the solution, what is the sodium ion concentration in milliequivalents per liter? What are the molar concentrations of each component of the solution?

9. Solubility Not every solution system is completely miscible. It is possible to saturate a solution. A saturated solution has the maximum amount of solute dissolved in a solvent at a given temperature. We see this all the time with the solubility of, for example, sugar in water. Solubility usually increases with temperature. Hence, more sugar dissolves in hot tea than in iced tea. This is because most solution processes are endothermic – they absorb heat to make them go.

10. Solubility Example The solubility of KCl in water: At 20 deg C, 34 g KCl will dissolve in 100 g water At 50 deg C, 43 g KCl will dissolve in 100 g water A solution containing 80. g of KCl in 200. g of water at 50 deg C is cooled to 20 deg C. How many grams of KCl remain in solution at 20 deg C? How many grams of KCl crystallized from solution after cooling?

12. Concentrations Defined in the form

13. Percent concentrations Mass Percent – most common, except in medical applications Volume Percent (volumes not strictly additive) Mass / Volume Percent (using grams of solute, ml of solution) – seems to be commonly used in medical applications

14. Example A patient needs 100. g of glucose in the next 12 hours. How many liters of a 5% (m/v) glucose solution must be given?

16. Molarity Most common in the chemistry laboratory Gives the number of moles of solute present in a given volume. Easy to relate back to chemical equations which operate based on moles.

17. Example 1 Calculate the molarity of 5.85 g of sodium chloride in 400. ml of solution.

18. Example 2 Calculate the number of grams of solute needed to make 175 ml of 3.00 M sodium nitrate?

19. Example 3 How many milliliters of 0.800 M calcium nitrate contain 0.0500 moles of this solute?

20. Example 4 What is the final concentration in molarity of a solution in which water is added to 25 ml of a 25% (m/v) solution of sulfuric acid until the final volume is 100.0 ml?

21. Example 5 How many liters of 0.50 M phosphoric acid can be made from 0.500 liter of a 6.0 M phosphoric acid stock solution?

22. Example 6 Lead(II) nitrate reacts with potassium chloride to produce lead(II) chloride and potassium nitrate. The lead(II) chloride precipitates as a solid and is removed from the reaction as it is formed. Write a balanced equation for this reaction. How many grams of lead(II) chloride will be formed from 50.0 ml of 1.50 M potassium chloride and excess lead(II) nitrate? How many milliliters of 2.00 M lead(II) nitrate are needed to react completely with 50.0 ml of 1.50 M potassium chloride?