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Solutions & Solubility

Solutions & Solubility. Definitions. A solution is a homogeneous mixture A solute is dissolved in a solvent . solute is the substance being dissolved solvent is the liquid in which the solute is dissolved an aqueous solution has water as solvent

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Solutions & Solubility

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  1. Solutions &Solubility

  2. Definitions • A solution is a homogeneous mixture • A solute is dissolved in a solvent. • solute is the substance being dissolved • solvent is the liquid in which the solute is dissolved • an aqueous solution has water as solvent • A saturated solution is one where the concentration is at a maximum - no more solute is able to dissolve. • A saturated solution represents an equilibrium: the rate of dissolving is equal to the rate of crystallization. The salt continues to dissolve, but crystallizes at the same rate so that there “appears” to be nothing happening.

  3. Definitions • Solution - homogeneous mixture Solute - substance being dissolved Solvent - present in greater amount

  4. SUPERSATURATED SOLUTION becomes unstable, crystals form UNSATURATED SOLUTION more solute dissolves SATURATED SOLUTION no more solute dissolves Solubility concentration

  5. Dissolution of Solid Solute What are the driving forces which cause solutes to dissolve to form solutions? 1. Covalent solutes that are polar dissolve by H-bonding to water and covalent solutes that are non polar dissolve by London Dispersion Forces (LDF ) 2. Ionic solutes dissolve by dissociation into their ions by ion dipole

  6. 3 Stages of Solution Process • Separation of Solute • must overcome IMF or ion-ion attractions in solute • requires energy, ENDOTHERMIC ( + DH) • Separation of Solvent • must overcome IMF of solvent particles • requires energy, ENDOTHERMIC (+ DH) • Interaction of Solute & Solvent • attractive bonds form between solute particles and solvent particles • “Solvation” or “Hydration” (where water = solvent) • releases energy, EXOTHERMIC (-DH)

  7. Factors Affecting Solubility 1. Nature of Solute / Solvent. - Like dissolves like intermolecular forces (IMF) 2. Temperature - i) Solids/Liquids- Solubility increases with Temperature Increase kinetic energy (K.E.) increases motion and collision between solute / solvent. ii) gas - Solubility decreases with Temperature Increase K.E. result in gas escaping to atmosphere. 3. Pressure Factor - i) Solids/Liquids - Very little effect Solids and Liquids are already close together, extra pressure will not increase solubility. ii) gas - Solubility increases with Pressure. Increase pressure squeezes gas solute into solvent.

  8. Solubilities of Solids vs Temperature Solubilities of several ionic solid as a function of temperature. MOST salts have greater solubility in hot water. A few salts have negative heat of solution, (exothermic process) and they become less soluble with increasing temperature. • 1.KNO3at 30°C  • 2.NaCl at 50°C  • 3.NH4Cl at 40°C 

  9. Solubility curve learning check • What mass of solute will dissolve in 100mL of water at the following temperatures. Also determine which of the three substances is most soluble in water at 15°C. • 1.KNO3at 70°C  • 2.NaCl at 100°C  • 3.NH4Cl at 90°C  Solution:  Solubility 1.KNO3at 70°C is 138g / 100mL 2.NaCl at 100°C is 38g / 100mL 3.NH4Cl at 90°Cis 72g / 100mL 4.NaCl is the most soluble at 15°C

  10. Solubility curve learning check cont’d 1. What term - saturated, unsaturated, or supersaturated - best describes: • a solution that contains 70g of NaNO3 per 100 mL H2O at 30°C • a solution that contains 60g of dissolved KCl per 100 mL H2O at 80°C Solution • The NaNO3 solution is unsaturated. At 30°C a saturated solution would be able to dissolve approximately 95 g of NaNO3 . Since there are only 70g in the solution, 25 more grams of NaNO3 could be added and it would all dissolve. • The KCl solution is supersaturated. At 80°C a saturated KCl solution contains 50 g KCl per 100 mL H2O. This solution is holding 10 g of excess KCl. • If a small crystal of KCl is added to this supersaturated solution, the excess KCl will immediately come out of solution. 

  11. Temperature & the Solubility of GasesThe solubility of gases DECREASES at higher temperatures

  12. Solution and Concentration How can we express the amount of solute to solvent in a solution? • This is called the concentration • Concentration of a solution is a measure ofthe number of particlesof the solute in the solvent • A concentrated solution will havea large number of particles of the solute in the solvent. • A dilute solution will have a small number of particles of the solute in the solvent.

  13. Ways to express concentration 3 ways of expressing concentration • Percentages • PPM or PPB concentrations • molar concentrations and mass concentrations

  14. % Concentration • % (w/w) = • W is weight same as mass • % (w/v) = • % (v/v) =

  15. %m = 3.5 g CoCl2 100g H2O = 3.5% (m/m) % Concentration: % Mass Example 3.5 g of CoCl2 is dissolved in 100mL solution. Assuming the density of the solution is 1.0 g/mL, what is concentration of the solution in % mass?

  16. Examples • % V/V = 4.1 L / 55 L x 100 = 7.5% V/V • 7.5% V/V is also 7.5 mL / 100 mL • % W/V = 16 g / 50 mL x 100 = 32% W/V • 32% W/V is also 32 g / 100 mL • % W/W = 1.7 g / 35.0 g x 100 = 4.9% W/W • 4.9 % is also 4.9g/100 g More practice (remember mass of solution!!!!) • What is the % W/W of copper in an alloy when 50.0 g of Cu is mixed with 200 g of Zn? 20.0% • What is approximate % V/V if 30 mL of pure ethanol is added to 250 mL of water? 10.7% • What is the %W/W if 8.0g copper is added to enough zinc to produce 100 g of an alloy. 8%

  17. Learning check cont’d • Glucose is a sugar that is found abundantly in nature. What is the percent by mass of a solution made by dissolving 163 g of glucose in 755 g of water? Do you need to know the formula of glucose? Why or why not? 17.8 W/W% glucose • What is the mass percent sucrose in a solution obtained by mixing 225 g of an aqueous solution that is 6.25% sucrose by mass with 135 g of an aqueous solution that is 8.20% sucrose by mass? (6.98 W/W%) • The volume percent of toluene is 38.0% . What is the volume of the solvent if a solution is made by mixing 40.0 mL benzene in toluene. (Solvent, Toluene =65mL)

  18. Low concentrations • For very dilute solutions, weight/weight (w/w) and weight/volume (w/v)concentrations are sometimes expressed in parts per million. • Parts per million also can be expressed • as milligrams per liter (mg/L) (w/v) • As milligram per kilogram (mg/Kg) (w/w)

  19. PPM learning check 1. Question: A solution has a concentration of 1.25g/L. What is its concentration in ppm? • Convert the mass in grams to a mass in milligrams: 1.25g = 1.25 x 1000mg = 1250mg • Re-write the concentration in mg/L = 1250mg/L = 1250ppm 2. Question: 150mL of an aqueous sodium chloride solution contains 0.0045g NaCl. Calculate the concentration of NaCl in parts per million (ppm). ppm = mass solute (mg) ÷ volume solution (L) mass NaCl = 0.0045g = 0.0045 x 1000mg = 4.5mg  volume solution = 150mL = 150 ÷ 1000 = 0.150L concentration of NaCl = 4.5mg ÷ 0.150L = 30mg/L = 30ppm

  20. Molar concentration • is able to compare the amount of solute (moles) dissolved in a certain volume of solution. • Molar concentration (mol/L) or also called Molarity (M) = Moles of solute (n)/Volume of solution (V MUST be in litres) • is the number of moles of solute in one litre of a solution. We use "M" to denote molar concentration and it has the units of"moles/L". • C = n / V • n = C x V

  21. Concentration: Molarity Example If 0.435 g of KMnO4 is dissolved in enough water to give 250. mL of solution, what is the molarity of KMnO4? As is almost always the case, the first step is to convert the mass of material to moles. MM = 158.0 g/mol(39+55+(4X16) n= m/MM 0.435 g KMnO4 / 158.0 g/ mol = 0.00275 mol KMnO4 Now that the number of moles of substance is known, this can be combined with the volume of solution — which must be in liters — to give the molarity. Because 250. mL is equivalent to 0.250 L . Molarity KMnO4 = 0.00275 mol KMnO4 = 0.0110 Mol/L = M 0.250 L solution

  22. Molarity learning check • How many moles of H2SO4 are there in 250mL  of a 0.8M sulphuric acid solution? • If 20g of NaOH is dissolved in sufficient water to produce 500 mL of solution, calculate the molar concentration in molarity. Questions • What is the molarity of the solution formed by dissolving 80 g of sodium hydroxide (NaOH) in 500 mLs of water? (Na=23, O=16, H=1) Ans = 4.0 M • What is the molarity of the solution formed by dissolving 9.8 g of sulphuric acid (H2SO4) in 1000 cm3 of water? (H=1, S=32, O=16) Ans = 0.10 M 3. What mass (g) of hydrogen chloride (HCl) is needed to make up 500cm3 of a solution of concentration 0.2mol/L? (H=101, Cl=35.5) Ans = 3.7g HW – pg 103-104 in workbook

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