Review of Basic Concepts, Molarity, Solutions, Dilutions and Beer’s Law

1. Review of Basic Concepts, Molarity, Solutions, Dilutions and Beer’s Law Chapter 4 4.5

2. Aqueous Solutions • In Chemistry, many reactions take place in water. This is also true for Biological processes. • Reactions that take place in water are said to occur in an aqueous solution. • Three types of reactions take place in aqueous solutions: Precipitation, Acid-Base and Redox.

3. Properties of Aqueous Solutions • Solution-a homogeneous mixture of two or more substances. • Solute-a substance in a solution that is present in the smallest amount. • Solvent-a substance in a solution that is present in the largest amount. • In an aqueous solution, the solute is a liquid or solid and the solvent is always water.

4. Properties of Aqueous Solutions • All solutes that dissolve in water fit into one of two categories: electrolyte or non-electrolyte. • Electrolyte-a substance that when dissolved in water conducts electricity • Non-electrolyte-a substance that when dissolved in water does not conduct electricity. • To have an electrolyte, ions must be present in water.

5. Electrolytic Properties of Aqueous Solutions • NaCl in water. • What happens? • NaCl(s)→ Na+(aq) + Cl–(aq) • Completely dissociates

6. Strong vs. Weak Electrolytes • How do you know when an electrolyte is strong or weak? • Take a look at how HCl dissociates in water. • HCl(s)→ H+(aq) + Cl–(aq)

7. Electrolytic Properties of Aqueous Solutions

8. Electrolytic Properties of Aqueous Solutions

9. Hydrated Ions

10. Electrolytic Properties of Aqueous Solutions • What about weak electrolytes? • What makes them weak? • Ionization of acetic acid • CH3COOH(aq)↔ CH3COO–(aq) + H+(aq)

11. Electrolytic Solutions

12. Precipitation Reactions • Precipitation Reaction-a reaction that results in the formation of an insoluble product. • These reactions usually involve ionic compounds. • Formation of PbI2: • Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)

13. Preciptate

14. Precipitate

15. Precipitation Reactions • How do you know whether or not a precipitate will form when a compound is added to a solution? • By knowing the solubility of the solute! • Solubility-The maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature. • Three levels of solubility: Soluble, slightly soluble or insoluble.

16. Precipitation Reactions

17. Determining Solubility • Determine the level of solubility for the following: (1) Ag2SO4 (2) CaCO3 (3) Na3PO4

18. Acid-Base Reactions • Acids-generally have a sour taste, change litmus from blue to red, can react with certain metals to produce gas, conduct electricity. • Bases-generally have a bitter taste, change litmus from red to blue, feel slippery, conduct electricity. • BrØnstead Acid-proton donor • BrØnstead Base-proton acceptor

19. Acid-Base Reactions • Acid or Base? • HCl(aq) + H2O(l)→ H3O+(aq) + Cl–(aq) • NH3(aq) + H2O(l)→ NH4+(aq) + OH–(aq)

20. Acid-Base Reactions • Look at the following compounds and decide whether they are a BrØnstead Acid or a BrØnstead Base. • HBr • NO2– • HCO3–

21. Acid-Base Reactions

22. Oxidation-Reduction Reactions • Can also be called Redox reactions. • Considered electron-transfer reactions. • Occur in steps called half-reactions. • Half-Reactions- Two parts to a redox reaction that explicitly show the electrons involved and where they are transferred.

23. Oxidation Reduction Reactions • Oxidation Reaction-refers to the half-reaction that involves the loss of electrons. • Reduction Reaction-refers to the half-reaction that involves the gain of electrons. OILRIG • Oxidizing agent-the compound or ion in a redox reaction that donates electrons. • Reducing agent-the compound or ion in a redox reaction that accepts electrons.

24. Oxidation-Reduction Reactions

25. Concentration of Solutions • Concentration of a Solution-amount of solute present in a given quantity of solvent or solution. • We will use Molarity, M for measurement. Molarity can also be called Molar Concentration. • Molarity–the number of moles of solute per liter of solution. • Molarity- moles of solutes/ liters of solution • Or n/v • Moles- grams of sample/ molecular weight of sample • Or g/ mw

26. Concentration of Solutions • How many moles are there in 24.0g of C? moles = g/mw moles = 24.0g C/ 12.0g C moles = 2.0 There are 2.0 moles of C in 24.0g of C.

27. Concentration of Solutions • How many grams are in 2.0 moles of Boron? moles= g/MW 2.0 moles = grams/ 10.81g Boron 2.0 moles x 10.81g Boron = grams Grams = 21.62 • There are 21.62 g of Boron in 2.0 moles of Boron.

28. Concentration of Solutions • What is the Molarity of a 1L solution containing 9.0g HCl? 9.00g HCl x 1 mol HCl/ 18.00g HCl = 0.5 mol HCl M = n/v M = 0.5 mol HCl/ 1L M = 0.5 The concentration of the solution is 0.5M.

29. Preparation of Solutions • Now that you know how to calculate M, n and v, what does that mean? • You can make your own solutions! • What are the steps in making a proper solution?

30. Preparation of Solutions

31. Concentration of Solutions • How many grams of Potassium Dichromate, K2Cr2O7, are required to prepare a 250mL solution with a concentration of 2.16M? 250mL x 1L/ 1000mL = .250L M= n/v n= M x v n= 2.16M x .250L n= 0.54 mol moles = g/MW Grams = moles x MW Grams = 0.54 mol K2Cr2O7 x 294.2 g K2Cr2O7 Grams = 159 159 grams of K2Cr2O7 are needed to prepare the requested solution.

32. Concentration of Solutions • In a biochemical assay, a chemist needs to add 0.381g of glucose to a reaction mixture. Calculate the volume in millimeters of a 2.53M glucose solution that she should use for this addition. moles = g/MW moles = 0.381g C6H12O6/ 180.2g C6H12O6 moles = 2.114 x 10 –2 mol C6H12O6 M = n/v v = n/M v = 2.114 x 10 –2 mol C6H12O6 / 2.53M C6H12O6 v = 8.36mL She should use 8.36mL of the 2.53M glucose solution.

33. Preparation of Solutions • Explain the process of creating 1L of 3.0M KCl. M = n/v n = M x v n = 3.0M x 1L n = 4.0 mol of KCl needed moles= g/MW Grams = moles x MW Grams = 4.0 mol KCl x 36.0g KCl Grams = 144g KCl Weigh out 144g of KCl. Put in a 1L flask. Add enough dH20 to dissolve KCl. Fill flask to 1L meniscus.

34. Dilution of Solutions • Dilution-the procedure for preparing a less concentrated solution from a more concentrated one. • Dilutions can be made in increments of 10, 20, 50 or any other value. • Serial Dilution-theprocess of diluting a solution by removing part of it, placing this in a new flask and adding water to a known volume in the new flask.

35. Dilution of Solutions • When you want to dilute a solution, what happens to the number of moles present in the solution? • Do they increase? • Decrease? • Stay the same?

36. Dilution of Solutions

37. Dilution of Solutions

38. Dilution of solutions • Since moles are constant before and after dilution, we can use the following formula for calculations. • MiVi = MfVf

39. Dilution of Solutions • Describe how you would prepare 800mL of a 2.0M H2SO4 solution, starting with a 6.0M stock solution of . 800mL x 1L/ 1000mL = 0.800L MiVi = MfVf 6.0M x Vi = 2.0M x 0.800L 6.0M x Vi = 1.6M x L Vi = 1.6M x L/ 6.0M Vi = 0.26L 0.26L of the 6.0M H2SO4 solution should be diluted to give a final volume of 800mL.

40. Concentration of Solutions • There are several ways to determine the concentration of a solution. • In this week’s lab, we will be using spectroscopy to help us identify the molar concentration of an unknown substance. • Spectroscopy is helpful because it gives us: • Amount of light transmitted through a solution • Amount of light absorbed by a solution • Beer’s Law- a relationship between proportionality constant, path length of radiation going through solution and concentration of the solution. • A = abc • A = -log(T)

41. Beer’s Law • If we know the value of T, we can solve the previous equation and figure out what A is. • Absorbance vs. wavelength • Absorbance vs. concentration • Use excel and be sure to add in your equation for the line. • Calculation of A or C: • A = mc

42. Beer’s Law