CHAPTER 13: SOLUTIONS

Transcription

1 CHAPTER 13: SOLUTIONS Problems: 1-8, 11-15, 20-30, 37-88, , SOLUTIONS: HOMOGENEOUS MIXTURES solution: homogeneous mixture of substances present as atoms, ions, and/or molecules solute: component present in smaller amount solvent: component present in greater amount Example: Identify the solute and solvent in each of the following: a. saline solution (NaCl water): solute= solvent= b. carbonated water (water CO 2 gas): solute= solvent= Note: Unless otherwise stated, the solvent for most solutions considered in this class will almost always be water! A concentrated solution has a large quantity of solute present for a given amount of solution. A dilute solution has a small quantity of solute present for a given amount of solution SOLUTIONS OF GASES IN WATER: HOW SODA POP GETS ITS FIZZ TEMPERATURE EFFECTS and the SOLUBILITY OF GASES in solution Example: What happens if soda is left outside in the summer? At higher temperatures, gas molecules move than molecules at lower temperatures. faster or slower These gas molecules encounter the surface (between liquid and air) more quickly. They escape from the liquid more quickly. Fewer gas molecules in the liquid! Thus, as temperature increases, the solubility of a gas in a liquid. increases decreases CHEM 139: Tro Chapter 13 F2012 page 1 of 16

2 PRESSURE EFFECTS and the SOLUBILITY OF GASES in solution Henry's Law: The solubility of a gas is proportional to partial pressure of the gas above liquid The greater the partial pressure of a gas above a liquid the greater the solubility of the gas in the liquid. The greater number of gas molecules above a liquid More of the molecules will go into the liquid The higher the solubility of the gas in the liquid lower gas pressure higher gas pressure Ex. 1 Why does a can or a 2-L bottle of soda hiss when it is first opened? At the molecular-level, the bubbles that form in carbonated water or soda are CO 2 molecules coming out of solution as the partial pressure of CO 2 gas above the liquid decreases. CHEM 139: Tro Chapter 13 F2012 page 2 of 16

3 13.3 SOLUTIONS OF SOLIDS DISSOLVED IN WATER: HOW TO MAKE ROCK CANDY THE FORMATION OF A SOLUTION As a solute crystal is dropped into water, the water (solvent) molecules pull apart the solute. Water molecules surround the solute (e.g. NaCl), forming a solvent cage. The Na and Cl ions are now hydrated (surrounded by water molecules). The solute is now dissolved in the solvent and cannot be seen because the ions are far apart, like the particles in a gas. The NaCl crystals above dissolve in water and exist as hydrated Na and Cl ions. In a chemical equation, NaCl is shown as NaCl(aq), meaning it exists as Na and Cl - ions. The RATE OF DISSOLVING can be increased by: 1. heating the solution: solvent molecules move faster break up the solute more frequently break apart solute more quickly 2. stirring the solution: solvent encounters solute molecules more often solvent breaks apart solute more quickly 3. grinding the solute into smaller particles: increases the surface area of the solute more solute surface for solvent to break it up solvent breaks apart solute more quickly CHEM 139: Tro Chapter 13 F2012 page 3 of 16

4 "Like dissolves like" rule: miscible: two liquids mix immiscible: two liquid don t mix soluble: a solid will dissolve in a liquid insoluble: a solid won t dissolve in a liquid polar liquid/solvent nonpolar liquid/solvent polar liquid/solvent miscible immiscible nonpolar liquid/solvent immiscible miscible polar solid/solute soluble insoluble nonpolar solid/solute insoluble soluble ionic compound Check Solubility Rules insoluble Ex. 1: Indicate whether the following is soluble/insoluble or miscible/immiscible in the substances indicated: a. NaCl is in ethanol (C 2 H 5 OH). b. NaCl is in olive oil. Ex. 1: True or False? a. Stirring will increase the rate of dissolving of lithium chromate in water. Explain why. b. Sugar will dissolve faster in hot tea than in cold tea. Explain why. c. A sample of water will mix faster in hot olive oil than cold olive oil. Explain why. CHEM 139: Tro Chapter 13 F2012 page 4 of 16

5 SOLUBILITY AND SATURATION: Unsaturated, Saturated, and Supersaturated Solutions If a solid is soluble in a solvent, more solid dissolves in the solvent at higher temperatures. unsaturated: contains less than the maximum amount of solute that a solvent can hold at a specific temperature saturated: contains the maximum amount of solute that a solvent can hold at a specific temperature supersaturated: contains more than the maximum amount of solute that a solvent should be able to hold at specific temperature A supersaturated NaC 2 H 3 O 2 solution recrystallizing after addition of more solute: Preparing a supersaturated solution: How can a solution hold more solute than it should be able to hold? If a given amount of solute is dissolved in a solvent at a higher temperature, and the solution is allowed to cool without being disturbed, the solute will remain in solution. But the solution is unstable, and the solute will come out of solution (i.e. recrystallize) if the solution is disturbed (e.g. by adding more solute, scratching the glass, etc.) For some substances, the recrystallization process is exothermic. The solution releases heat when it recrystallizes. Hot packs used to warm hands and feet in winter For some substances, the recrystallization process is endothermic The solution absorbs heat when it recrystallizes and becomes colder. Cold packs used for sports injuries CHEM 139: Tro Chapter 13 F2012 page 5 of 16

6 Ex. 1: Check all of the statements below that are true: a. Both stirring and heating will increase the rate of dissolving NaCl in water. b. A cup of hot tea can be made sweeter than a cup of cold tea. c. A cup of cold water will contain more dissolved O 2 gas than a cup of boiling water. d. Adding a few granules of sugar to a cup of coffee produces a saturated solution. e. More sugar will dissolve in vegetable oil if the container is shaken. Ex. 2: A student dislikes the taste of the tap water in her apartment. She had read that dissolved calcium and magnesium salts are usually the cause of bad tasting tap water. She also heard that one can purify water by boiling purify it, so she decides to boil a pot of her tap water to improve the taste. Explain if and why the student will be happy with the results of her tap water after boiling. What types of solute(s) can or cannot be removed by boiling water? SOLUTION CONCENTRATIONS solution: homogeneous mixture of substances present as atoms, ions, and/or molecules solute: component present in smaller amount solvent: component present in greater amount A concentrated solution has a large quantity of solute present for a given amount of solution. A dilute solution has a small quantity of solute present for a given amount of solution. SOLUTION CONCENTRATION: concentrat ion = amount of solute amount of solution The more solute in a given amount of solution the more concentrated the solution CHEM 139: Tro Chapter 13 F2012 page 6 of 16

7 13.6 SPECIFYING SOLUTION CONCENTRATION: MOLARITY More often in chemistry, solution concentrations are given as a molarity: moles of solute Molarity = (in units of M=molar) liters (L) of solution Note: The molarity of a substance is indicated when it s shown in square brackets: [KCl] = the molarity or molar concentration of KCl SOLVING MOLAR CONCENTRATION PROBLEMS: Ex. 1 Find the molarity of a solution prepared by dissolving moles of NaBr in ml of solution. Ex. 2 Find the molarity of a solution prepared by dissolving 25.0 g of KCl in ml of solution. Get in the habit of writing molarity explicitly as a unit factor: a. 6.00M hydrochloric acid b M NaCl solution CHEM 139: Tro Chapter 13 F2012 page 7 of 16

8 SOLVING MOLAR CONCENTRATION PROBLEMS: If molarity and volume are both given, you can calculate # of moles since: moles of solute volume molarity = volume (in L) liters (L) of solution so volume units will cancels # of moles Thus, when given volume and molarity for a solution, multiply them to get # of moles! Ex. 1 Calculate the number of moles of H 2 SO 4 present in 50.0 ml of 6.00 M H 2 SO 4. Ex. 2 What mass of sodium chloride is required to make 1.00 L of a 1.00M sodium chloride solution? Ex. 3 What mass of magnesium nitrate required to prepare ml of a 1.50 M magnesium nitrate solution? CHEM 139: Tro Chapter 13 F2012 page 8 of 16

9 This is how we actually make these solutions in the lab: 13.5 SPECIFYING SOLUTION CONCENTRATION: MASS PERCENT MASS PERCENT CONCENTRATION (M/M%) mass of solute mass of solute M/M% = 100% = 100% mass of solution mass of solute mass of solvent Ex. 1 A solution is prepared by dissolving 5.0 g of NaCl in g of water. a. The solute is, and the solvent is. b. Calculate the mass percent concentration for the solution. Ex. 2 What is the mass percent concentration of a solution made by dissolving 1.50 g of KCl in g of water? CHEM 139: Tro Chapter 13 F2012 page 9 of 16

10 Ex. 3 A person accused of a DUI violation submitted a 6.00 g sample of blood for alcohol content analysis. The analysis determined the presence of 5.73 mg of alcohol in the blood. If a person with a blood alcohol content (mass percent of alcohol in the blood) of 0.08% is legally impaired, was this person impaired? (Note that the masses must be in the same units when solving for a percentage.) Writing Mass Percent Concentration Unit Factors For the following, indicate the mass of solute and mass of solvent present in 100 g of solution. a. 5.00% solution of NaF b. 25.0% glucose solution Solving Mass Percent Concentration Problems Ex. 1 Dextrose is a naturally occurring form of glucose (C 6 H 12 O 6 ) more commonly known as corn sugar or grape sugar. Patients with low blood sugar who are unable to take glucose gels or tablets are often given intravenous injections of dextrose. What mass of a 5.0% dextrose solution can be prepared using 1.50 g of dextrose? Ex. 2 What mass of solute is required to prepare g of a 2.50% KCl solution? CHEM 139: Tro Chapter 13 F2012 page 10 of 16

11 Ex. 3 What mass of water is required to dissolve 5.00 g of sodium hydroxide to prepare a 10.0% sodium hydroxide solution? WEIGHT/VOLUME PERCENT (% w/v), also called MASS/VOLUME PERCENT (% m/v) mass of solute % m/v = 100% ml of solution Ex. 1 Calculate the mass/volume percent for a solution that is prepared by dissolving 5.0 g of KI in enough water to give 250 ml of solution. Ex. 2 If a physiological saline solution is 0.900% (w/v) NaCl. What mass of NaCl is present in ml of the solution? Ex. 3 A topical antibiotic is 1.0% (m/v) Clindamycin. What mass of Clindamycin is present in 60.0 ml of the 1.0% (w/v) Clindamycin solution? CHEM 139: Tro Chapter 13 F2012 page 11 of 16

12 13.7 SOLUTION DILUTION We can often dilute more concentrated solutions with water to get a solution with a specific concentration, as shown below. We can calculate the concentration of a new solution using the dilution equation below: M 1 V 1 = M 2 V 2 where M 1 =initial/higher concentration and M 2 =final concentration V 1 =volume of original solution and V 2 =volume of final solution Ex. 1: Calculate the new concentration of a sulfuric acid solution prepared by diluting 5.00 ml of 2.00M H 2 SO 4 (aq) solution with enough deionized water to prepare ml of solution. Ex. 2: What volume of a 1.25M sodium chloride solution is required to prepare 50.0 ml of a 0.500M sodium chloride solution? CHEM 139: Tro Chapter 13 F2012 page 12 of 16

13 13.10 OSMOSIS: WHY DRINKING SALT WATER CAUSES DEHYDRATION Diffusion: The gradual mixing of molecules of two substances that mix or dissolve in one another to form a uniform mixture because of their kinetic properties. Solute molecules move from regions of higher concentration to regions of lower concentration to achieve a uniform distribution. OSMOSIS AND MEMBRANES semipermeable membrane: allows solvent molecules to pass through but blocks the passage of solute molecules osmosis: The overall movement of solvent molecules through a semi-permeable membrane from a less concentrated solution to a more concentrated solution. Osmosis occurs when two solutions separated by a semi-permeable membrane have differing solute concentrations The figure at the right shows osmosis at the molecular level: Note that during osmosis only the only the solvent (H 2 O) molecules move through the semi-permeable membrane. CHEM 139: Tro Chapter 13 F2012 page 13 of 16

14 Example: We often hear the phrase "learning by osmosis." Let s consider if a student s head is a semi-permeable membrane, and knowledge is the solute. What would literally happen if the student slept with his head on top of his chemistry textbook in an effort to learn by osmosis? More simply, osmosis occurs because concentrated aqueous solutions are thirsty and can draw water away from less concentrated solutions. Example: Consider the image at the right to explain how osmosis occurs if one drinks salt water, so the stomach has a more concentrated NaCl solution compared to other bodily tissues. Note that the membranes of living cells are semipermeable membranes. A living cell can experience osmosis, as shown in the examples below. (a) A red blood cell has its typical shape in a surrounding fluid has a solute concentration equal to its own. (b) In pure water, H 2 O flows into the red blood cell, so it becomes swollen. (c) In a more concentrated solution, the red blood cell loses water to its environment, losing its shape. CHEM 139: Tro Chapter 13 F2012 page 14 of 16

15 Consider the images below, showing a concentrated solution to the left and pure water to the right of a semipermeable membrane before and after osmosis occurs: The difference in height between the two liquids is a measure of the opposing pressure generated by the concentration differences. This pressure is called the osmotic pressure. isotonic: when two solutions have equal osmotic pressures hypertonic: the more concentrated solution of two solutions with different osmotic pressures hypotonic: the less concentrated solution of two solutions with different osmotic pressures Ex. 1: In the images above, which is the hypertonic solution? Ex. 2: Check all of the following statements that are true: a. If two solutions are isotonic, they must have equal concentrations. b. Osmosis essentially results in the dilution of a more concentrated solution. c. Only the solute particles move through the semi-permeable membrane during osmosis. d. During osmosis water diffuses from the hypotonic solution to the hypertonic solution. e. Osmotic pressure increases with greater differences in solute concentration. CHEM 139: Tro Chapter 13 F2012 page 15 of 16

16 DIALYSIS USED TO ADDRESS KIDNEY MALFUNCTION COLLOIDS: uniform mixtures similar to solutions, but the solute particles and molecules in colloids (called the dispersed phase ) are much larger colloids often appear cloudy. colloidal means gluelike examples of colloids include glues, gels, shaving cream, etc. KIDNEY MALFUNCTION 1 The kidneys maintain the body's internal equilibrium of water and minerals (Na, K, Cl, Ca 2, Mg 2, SO 4 2- ). In humans, uric acid disposal (excreted in urine) also occurs via the kidneys. Those suffering from kidney malfunction (or renal failure) require dialysis until a kidney (renal) transplant occurs. DIALYSIS: The passage of ions and small ions through a dialyzing membrane dialyzing membrane: similar to a semipermeable membrane but with larger pores allows small ions and small molecules to move through it along with the solvent molecules while preventing colloid particles and large molecules from moving through it dialyzing membrane small molecule colloid particle pure water ions dialyzing membrane small molecule colloid particle pure water ions Before dialysis begins After dialysis Dialysis for Kidney Malfunction For some suffering from kidney malfunction or failure, hemodialysis cleans the blood. Hemodialysis: Blood is pumped through a tube made of a dialyzing membrane. The tubing passes through a bath to collect impurities, leaving blood proteins and large molecules in the blood. 1 CHEM 139: Tro Chapter 13 F2012 page 16 of 16