Chapter 8 - Solutions

Chapter Eight: Solutions Mixtures pure substances: elements, covalent compounds, and ionic compounds most matter we come into contact with is a mixture of two or more pure substances heterogenous mixtures: does not have a uniform composition throughout the sample homogenous mixture: has a uniform composition throughout the sample Solutions a solution is a homogenous mixture that contains small particles (< 1 nm). liquid solutions are transparent. they consist of two parts: • solute: the substance present in a lesser amount • solvent: the substance present in a larger amount an aqueous solution has water as the solvent. Colloids and Suspensions a colloid is a homogenous mixture with larger particles (1 nm - 1 μm), often having an opaque appearance. • particles in a colloid cannot be filtered from its other components • they do not settle out a suspension is a heterogenous mixture that contains large particles (> μm), suspended in liquid. • particles are so large that they do not dissolve in a liquid • they can be filtered from the liquid or separated using a centrifuge. • ex: blood is a suspension containing blood cells that can be separated from liquid blood plasma by centrifugation Electrolytes and Nonelectrolyes electrolyte: a substance that conducts an electric current in water • NaCl (aq) dissociates into Na+ (aq) and Cl- (aq) nonelectrolyte: a substance that does not conduct an electric current in water • H2O does not dissociate Classification a strong electrolyte dissociates completely in water to form ions a weak electrolyte dissociates partially in water to form some ions, leaving mostly uncharged molecules General Features of Solubility solubility: the amount of solute that dissolves in a given amount of solvent • usually reported in grams of solute per 100 mL of solution (g/100 mL) saturated solution: contains the maximum number of grams of solute that can dissolve unsaturated solution: contains less than the maximum number of grams of solute that can dissolve Basic Principles solubility can be summed up as “like dissolves like”. most ionic and polar covalent compounds are soluble in water, a polar solvent. small neutral molecules with O or N atoms that can hydrogen bond to water are water soluble. nonpolar compounds are soluble in nonpolar solvents. octane (C8H18) dissolves in CCl4 because both are nonpolar liquids that exhibit only london dispersion forces. when solvation releases more energy than required to separate particles, the overall process is exothermic (heat is released). when the separation of particles requires more energy than is released during solvation, the process is endothermic (heat is absorbed). Ionic Compounds - Additional Principles many ionic compounds are soluble in water. however, when the attraction of the cation and anion in a solid is stronger than the attraction between the ions and water, the ionic compound will not be soluble in water. General Rules for the Solubility of Ionic Compounds 1. a compound is soluble if it contains one or more of the following cations: + + + • group 1A cations: Li , Na + , K , Rb+ , Cs • ammonium, NH + 4 1. a compound is soluble if it contains one of the following anions: - 2+ 2+ • halides: Cl - , Br , I - (except for salts with Ag+, Hg2 , and Pb ) • nitrate, NO3 • acetate, CH3CO2 • sulfate, SO4 (except for salts with Ba , Hg2 , and Pb - 2- 2+ 2+ 2+ Effects of Temperature for most ionic and molecular solids, solubility generally increases as temperature increases. by dissolving a solid in a solvent at high temperature and allowing it to cool slowly, a supersaturated solution can be made. • supersaturated solution: contains more than the predicted maximum amount of solute at a given temperature the solubility of a gas decreases with increasing temperature Effects of Pressure henry’s law: the solubility of a gas in a liquid is proportional to the partial pressure of the gas above the liquid. the higher the pressure, the higher the solubility of a gas in a solvent. Concentration Units - Weight/Volume Percent concentration: how much solute is dissolved in a given amount of solution weight/volume percent concentration (w/v)%: the number of grams of solute dissolved in 100 mL of solution example: vinegar contains 5g of acetic acid in 100 mL of solution, what is the (w/v) % concentration? Volume/Volume Percent example: if a bottle of rubbing alcohol contains 70 mL of 2-propanol in 100 mL of solution, then what is the (v/v)% concentration? Using Percent Concentration as a Conversion Factor example: a saline solution used in intravenous drips contains 0.92% (w/v) NaCl in water. how many grams of NaCl are contained in 250 mL of this solution? step one: identify the known quantities and the desired quantity. known quantities: 0.92% (w/v) NaCl solution, 250 mL desired quantity: ? g NaCl step two: write out the conversion factors step three: solve the problem Parts Per Million when a solution contains a very small concentration of solute, it is often expressed in parts per million. Molarity molarity: the number of moles of solute per liter of solution, abbreviated as M. example: calculate the molarity of a solution made from 20.0 g of NaOH in 250 mL of solution. step one: identify the known quantities and the desired quantity known quantities: 20.0 g NaOH, 250 mL solution desired quantity: ? M (mol/L) step two: convert the number of grams of solute to the number of moles. convert the volume to liters if necessary. • use the molar mass to convert grams of NaOH to moles of NaOH (molar mass 40.0 g/mol). • convert milliliters of solution to liters of solution using a mL-L conversion factor step three: divide the number of moles of solute by the number of liters of solution to obtain the molarity molarity is a conversion factor that relates the number of moles of solute to the volume of solution it occupies. to calculate the moles of solute, rearrange the equation: to calculate the volume of the solution, rearrange the equation: example: what volume in milliliters of a 0.30 M solution of glucose contains 0.025 mol of glucose? step one: identify the known quantities and the desired quantity known quantities: 0.30 M, 0.025 mol glucose desired quantity: ? V (L) step two: divide the number of moles by molarity to obtain the volume in liters step three: use a mL-L conversion factor to convert liters to milliliters Dilution dilution: the addition of solvent to decrease the concentration of solute. the solution volume changes, but the amount of solute is constant. example: what is the concentration of a solution formed by diluting 5.0 mL of a 3.2 M glucose solution to 40.0 mL? step one: identify the known quantities and the desired quantity known quantities: M1 = 3.2 M, V1 = 5.0 mL, V2 = 40.0 mL desired quantity: M2 = ? step two: write the equation and isolate M2 on one side step three: solve the problem; substitute the three known quantities into the equation and solve for M2 Colligative Properties colligative properties: properties of a solution that depend on the concentration of the solute but not its identity the number of dissolved solute particles affects the properties, but the identity of the solute does not. colligative properties include: • elevation of the boiling point • lowering of the melting point • pressure due to osmosis Boiling Point Elevation volatile solute: readily escapes into the vapor phase nonvolatile solute: does not readily escape, so it has a negligible vapor pressure at a given temperature • the vapor pressure above a solution of a nonvolatile solute is lower than the vapor pressure of the pure solvent boiling point elevation: a liquid solution that contains a nonvolatile solute has a higher boiling point than the solvent alone the amount that the boiling point increases depends only on the number of dissolved particles. one mole of any nonvolatile solute raises the boiling point of 1 kg of H2O the same amount, 0.51 °C. Freezing Point Depression freezing point depression: a liquid solution that contains a nonvolatile solute has a lower freezing point than the solvent alone since the presence of the solute makes it harder for the solvent to form an organized solid the amount of freezing point depression depends only on the number of dissolved particles. one mole of any nonvolatile solute lowers the freezing point of 1 kg of H2O the same amount, 1.86 °C. Osmosis and Dialysis the membrane that surrounds living cells is a semipermeable membrane. semipermeable membranes allow water and small molecules to pass through, but ions and large molecules cannot. osmosis: the passage of a solvent, usually water, across a semipermeable membrane from a solution of low concentration to one of higher solute concentration osmotic pressure: the pressure that prevents the flow of additional solvent into a solution on one side of a semipermeable membrane Focus on Health and Medicine: Dialysis in the human body, blood is filtered through the kidneys by the process of dialysis, which is similar to osmosis in dialysis, water, small molecules, and ions can pass through the semipermeable (dialyzing) membrane (nephrons); only large biological molecules like proteins and starch molecules cannot. this allows waste products like urea to be removed from the bloodstream and eliminated in the urine.