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Solutions Notes

Solutions Notes. Words to Know. Solution – homogenous mixture Solvent – substance present in the largest amount Solutes – substance present in the smallest amount Aqueous solution – solutions with water as the solvent Concentration – the amount of solute in a given volume of solution

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Solutions Notes

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  1. Solutions Notes

  2. Words to Know • Solution – homogenous mixture • Solvent – substance present in the largest amount • Solutes – substance present in the smallest amount • Aqueous solution – solutions with water as the solvent • Concentration – the amount of solute in a given volume of solution • Concentrated – large amount of solute dissolved in solvent • Dilute – small amount of solute dissolved in solvent

  3. Saturated – a solution that contains as much solute as will dissolve at that temperature • Unsaturated – a solution that hasn’t reached that limit of solute that will dissolve • Supersaturated - a solution that contains more solute than should dissolve at that temperature

  4. Effect of Temperature on Solubility • Increasing the temperature of a solution, increases the amount of solute that can be dissolved • Decreasing the temperature of a solution, causes the solute to recrystallize

  5. Learning Check • How many grams of NaCl will dissolve in 100 g of H2O at 90°C? • 50 g of KCl is dissolved in 100 g of water at 50°C. Is the solution saturated, unsaturated or supersaturated?

  6. Effect of Pressure on Solubility • Pressure has a major effect on the solubility of gas-liquid systems • An increase in pressure increases the solubility of a gas in the liquid

  7. non polar “Like dissolves like” – a solvent usually dissolves solutes that have polarities similar to itself polar alcohol ionic non polar • Polar molecules dissolve other polar molecules and ionic compounds. • Nonpolar molecules dissolve other nonpolar molecules. • Alcohols, which have characteristics of both polar & nonpolar, tend to dissolve in both types of solvents, but will not dissolve ionic solids. alcohol and alcohols and alcohols non polar ionic and other alcohols ionic alcohol Alcohols are organic, covalent molecules with an –OH group. Alcohol names end with “-ol.” polar non polar

  8. Colligative properties Colligative properties - the physical changes that result from adding solute to a solvent. Colligative Properties depend on how many solute particles are present as well as the solvent amount, but they do NOT depend on the type of solute particles. • Boiling Point Elevation • Freezing Point Depression • Osmotic Pressure Increasing • Vapor Pressure Lowering • Conductivity Increasing More particles/ions = greater change

  9. Learning Check 1. Which substance will provide the greatest change in freezing point of water? • NaCl B. CaCl2 C. C6H12O6 D. H2O 2. Which of the following reflect colligative properties? (I) A 0.5 m NaBr solution has a higher vapor pressure than a 0.5 m BaCl2 solution. (II) A 0.5 m NaOH solution freezes at a lower temperature than pure water (III) Pure water freezes at a higher temperature than pure methanol. A. only I B. only II C. only III D. I and II E. I and III 2 ions 1 particle no change in H2O 3 ions 3 ions 2 ions = vapor pressure lowering 2 ions = freezing point depression 0 ions no solutions – colligative properties compare impact of solute on property of a solvent in a solution

  10. 3. A student measured the conductivity in water, of unlabeled liquids, after each added drop. The following graph was produced... a. Identify the line that represents: • aluminum chloride • water • magnesium chloride • sugar • sodium chloride • Which line could also represent potassium iodide? AlCl3, 4 ions H2O, no change in H2O AlCl3 MgCl2, 3 ions C6H12O6, 1 particle MgCl2 NaCl, 2 ions Conductivity (µs/cm) NaCl C6H12O6 H2O # of Drops

  11. Mass % = 1.00 g x 100 % 101.0 g Solution Composition - Mass Percent Mass percent – describes a solution’s composition expresses the mass of solute present in a given mass of solution Mass Percent = mass of solute x 100% mass of solution* * mass of solution = mass of solute + mass of solvent Example – A solution is prepared by mixing 1.00g of C2H5OH, with 100.0g of H2O. Calculate the mass percent of ethanol. Mass Percent = mass of solute x 100% mass of solution Given mass of solute = 1.00 g mass of solution = 100.0 g + 1.00 g = 101.0 g Mass % = 0.990 %

  12. Solution Composition – Molarity Molarity – measure of concentration - number of moles of solute per volume of solution in liters Molarity = moles of solute = mol = M L of solution L Example – Calculate the molarity of a solution prepared by dissolving 11.5 g NaOH in enough water to make 1.50L solution. 1 1 0.192 M mol NaOH 11.5 g NaOH x ___________ x __________ = g NaOH 40.00 1.50 L NaOH

  13. 1.50 L 0.100 M Ex: Calculate the mass of solid AgCl formed when 1.50L of a 0.100M AgNO3 solution is reacted with excess NaCl. NaCl + AgNO3 AgCl + NaNO3 ? g 0.100 mol AgNO3 143.32 1 mol AgCl g AgCl 1.50 L AgNO3 x ______________ x ____________ = x ____________ 21.5 g L AgNO3 1 1 mol AgNO3 1 mol AgCl no grams? start with liters use M as conversion factor to conver to mol # M = # mol 1 L mole ratio convert to desired unit

  14. Example – How many moles of Ag+ ions are present in 25mL of a 0.75M Ag2SO4 solution? Ag2SO4 2 Ag+1 + SO4-2 1 L Ag2SO4 0.75 mol Ag2SO4 2 mol Ag+1 25 mL Ag2SO4 x _____________ x ______________ x ______________ = 1000 1 L Ag2SO4 1 mol Ag2SO4 mL Ag2SO4 0.038 mol Ag+1

  15. Learning check Calculate the molarity of a solution prepared by dissolving 25.6 g NaC2H3O2 in enough water to make 200.0 mL solution.

  16. Standard Solution • Standard Solution – a solution whose concentration is accurately known Example – A chemist needs 1.0 L of a 0.200M K2Cr2O7 solution. How much solid K2Cr2O7 must be weighed out to make this solution? 1.0 L K2Cr2O7 0.200 mol K2Cr2O7 294.20 g K2Cr2O7 x ________________ x ______________ = 1 L K2Cr2O7 1 mol K2Cr2O7 59 g K2Cr2O7

  17. _____ V1 = M2V2 V1 = (0.10 M)(1.5 L) ____________ M1 16 M Dilution Dilution – process of adding more solvent to a solution Moles of solute before dilution = Moles of solute after dilution M1V1 = M2V2 Example: What volume of 16M H2SO4 must be used to prepare 1.5L of a 0.10M H2SO4 solution? Given V1 = ? M1 = 16 M V2 = 1.5 L M2 = 0.10 M V1 = 0.0094 L

  18. V2 = (1.00 M)(500.0 mL) _______________ 17.5 M _____ V2 = M1V1 M2 Learning Check Example: Prepare 500.0mL of 1.00 M HC2H3O2 from a 17.5 M stock solution. What volume of the stock solution is required? Given V1 = 500.0 mL M1 = 1.00 M M2 = 17.5 M V2 = ? V2 = 28.6 mL

  19. Notes-Acids and Bases

  20. Acids and Bases Arrhenius ACIDS – produces hydrogen ions in aqueous solutions, sour taste, low pH, and the fact that they turn litmus paper red HCl (aq) H+(aq) + Cl-(aq) Arrhenius BASES – produces hydroxide ions in aqueous solutions, bitter taste, slippery feel, high pH, and the fact that they turn litmus paper blue NaOH (aq) Na+(aq) + OH-(aq) Arrhenius definition – limits the concept of a base

  21. proton donor proton acceptor Bronsted – Lowry definition – gives a broader definition of a base Bronsted – Lowry ACID – a proton (H+) donor Bronsted – Lowry BASE – a proton (H+) acceptor General Reaction – HA (aq) + H2O (l) H3O+(aq) + A-(aq) Acid Base Conjugate Conjugate Acid Base Conjugate Base – everything that remains of the acid molecule after a proton is lost Conjugate Acid – the base with the transferred proton (H+) Conjugate Acid – Base Pair – two substances related to each other by the donating and accepting of a single proton

  22. Examples: Finish each equation and identify each member of the conjugate acid –base pair. H2SO4(aq) + H2O (l) HSO4-1(aq) + H3O+ (aq) Conjugate Base Conjugate Acid Acid Base CO32-(aq) + H2O (l) HCO3-1(aq) + OH- (aq) Conjugate Acid Conjugate Base Base Acid The hydronium ion, H3O+, forms when water behaves as a base. This happens when the two unshared pairs of electrons on O bond covalently with the H+.

  23. Learning check Write the conjugate ACID • NH3 • HCO3-1 Write the conjugate BASE • H3PO4 • HBr Finish each equation and identify each member of the conjugate acid –base pair. a. H2SO3 (aq) + H2O (l)  b. SO4-2(aq) + H2O (l) 

  24. Water as an Acid and a Base Amphoteric – a substance that can behave as either an acid or a base - water is the most common amphoteric substance Ionization of Water – H2O (l) + H2O (l) H3O+(aq) + OH-(aq) In the shorthand form: H2O (l) H+(aq) + OH-(aq)

  25. [ ] = concentration [H+] = hydrogen ion concentration in M [OH-] = hydroxide ion concentration in M Ion-product constant – Kw refers to the ionization of water Kw = [H+][OH-] At 25C, Kw = [H+][OH-] = [1.0 x 10-7] [1.0 x 10-7] = 1.0 x 10-14 If [H+] increases, the [OH-] decreases, so the products of the two is still 1.0 x 10-14. There are three possible situations – • A neutral solution, where [H+] = [OH-] • An acidic solution, where [H+]  [OH-] • A basic solution, where [H+]  [OH-]

  26. Example: Calculate [H+] or [OH-] as required for each of the following solutions at 25C, for each solution state whether it is neutral, acidic, or basic. a. 1.0 x 10-5 M OH- b. 10.0 M H+ Kw = [H+][OH-] Kw = [H+][OH-] 1 x 10-14 = [H+][1.0 x 10-5 M] 1 x 10-14 = [10.0 M][OH-] [H+] = 1.0 x 10-9 M [OH-] = 1.00 x 10-15 M BASIC ACIDIC

  27. pH scale pH scale – because the [H+] in an aqueous solution is typically small, logarithms are used to express solution acidity pH = -log [H+] pOH = -log [OH-] Graphing calculator Non graphing calculator 1. Press the +/- key 1. Enter the [H+] 2. Press the log key 2. Press the log key 3. Enter the [H+] 3. Press the +/- key Significant Figure Rule – The number of places to the right of the decimal for a log must be equal to the number of significant figures in the original number.

  28. pH Scale

  29. Example – Calculate the pH or pOH a. [H+] = 5.9 x 10-9 M b. [OH-] = 2.4 x 10-6 M pH = - log [H+] pOH = - log [OH-] pH = - log (5.9 x 10-9 M) pOH = - log (2.4 x 10-6 M) pH = 8.23 pOH = 5.62

  30. Since Kw = [H+][OH-] = 1.0 x 10-14, pH + pOH = 14.00 Example - The pH of blood is about 7.4. What is the pOH of blood? pH + pOH =14.00 7.4 + pOH = 14.00 pOH = 6.6

  31. In order to calculate the concentration from the pH or pOH, [H+] = 10-pH [OH-] = 10-pOH Graphing calculator Non-graphing calculator • Press the 2nd 1. Enter the pH function, then log 2. Press the +/- key 2. Press the +/- key 3. Press the inverse 3. Enter the pH log key

  32. Example - The pH of a human blood sample was measured to be 7.41. What is the [H+] in blood? [H+] = 10-pH [H+] = 10-7.41 [H+] = 3.9 x 10-8 M

  33. Example – The pOH of the water in a fish tank is found to be 6.59. What is the [H+] for this water? [OH-] = 10-pOH [OH-] = 10-6.59 [OH-] = 2.6 x 10-7 M Kw = [H+][OH-] 1 x 10-14 = [H+][2.6 x 10-7 M] [H+] = 3.8 x 10-8 M

  34. Learning check • Determine the pH of a solution with a hydrogen ion concentration of 3.2 x10-12 M. • What is the [OH-] concentration of a solution with a hydrogen ion concentration of 8.9x10-4M? • What is the pH of a solution with a hydroxide ion concentration of 5.7x10-10 M?

  35. How Do We Measure pH? • For less accurate measurements, one can use • Litmus paper • “Red” paper turns blue above ~pH = 8 • “Blue” paper turns red below ~pH = 5 • An indicator

  36. How Do We Measure pH? For more accurate measurements, one uses a pH meter, which measures the voltage in the solution.

  37. Strong Acids • seven strong acids are HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4. • These are, by definition, strong electrolytes and exist totally as ions in aqueous solution.

  38. Strong Bases • Strong bases are the soluble hydroxides, which are the alkali metal and heavier alkaline earth metal hydroxides (Ca2+, Sr2+, and Ba2+). • Again, these substances dissociate completely in aqueous solution, strong electrolytes

  39. Strong, Weak, or Nonelectrolyte • Electrolytes are substances which, when dissolved in water, break up into cations (plus-charged ions) and anions (minus-charged ions). We say they ionize. Strong electrolytes ionize completely (100%), while weak electrolytes ionize only partially (usually on the order of 1–10%). The ions in an electrolyte can be used to complete an electric circuit and power a bulb. • Strong electrolytes fall into three categories: strong acids, strong bases, and soluble salts. • The weak electrolytes include weak acids, weak basesand insoluble salts. • Moleculesare nonelectrolytes. strong electrolyte strong base weak acid weak electrolyte soluble salt strong electrolyte weak electrolyte weak acid insoluble salt weak electrolyte molecule nonelectrolyte

  40. Learning check

  41. Titration A known concentration of base (or acid) is slowly added to a solution of acid (or base).

  42. Titration A pH meter or indicators are used to determine when the solution has reached the equivalence point, at which the stoichiometric amount of acid equals that of base.

  43. Titration of a Strong Acid with a Strong Base From the start of the titration to near the equivalence point, the pH goes up slowly.

  44. Titration of a Strong Acid with a Strong Base Just before and after the equivalence point, the pH increases rapidly.

  45. Titration of a Strong Acid with a Strong Base At the equivalence point, moles acid = moles base, and the solution contains only water and the salt from the cation of the base and the anion of the acid.

  46. Titration of a Strong Acid with a Strong Base As more base is added, the increase in pH again levels off.

  47. Neutralization Neutralization Reaction = Acid + Base  Salt + Water Salt – ionic compound containing a positive ion other than H+ and a negative ion other than OH-

  48. Buffered solutions – resists a change in its pH even when a strong acid or base is added to it - A solution is buffered in the presence of a weak acid and its conjugate base

  49. pH and pOH Calculations

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