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

Aqueous Solutions In Chemistry, many reactions take place in water. This is also true for Biological processes. 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. 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. Three types of reactions take place in aqueous solutions: Precipitation, Acid- Base and Redox.

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

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

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

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

Electrolytic Properties of Aqueous Solutions

Hydrated Ions

Electrolytic Properties of Aqueous Solutions What about weak electrolytes? What about weak electrolytes? What makes them weak? What makes them weak? Ionization of acetic acid Ionization of acetic acid CH 3 COOH (aq) ↔ CH 3 COO – (aq) + H + (aq) CH 3 COOH (aq) ↔ CH 3 COO – (aq) + H + (aq)

Electrolytic Solutions

Precipitation Reactions Precipitation Reaction- a reaction that results in the formation of an insoluble product. Precipitation Reaction- a reaction that results in the formation of an insoluble product. These reactions usually involve ionic compounds. These reactions usually involve ionic compounds. Formation of PbI 2 : Formation of PbI 2 : Pb(NO 3 ) 2 (aq) + 2KI (aq) → PbI 2 (s) + 2KNO 3 (aq) Pb(NO 3 ) 2 (aq) + 2KI (aq) → PbI 2 (s) + 2KNO 3 (aq)

Preciptate

Precipitate

Precipitation Reactions How do you know whether or not a precipitate will form when a compound is added to a solution? 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! 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. 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. Three levels of solubility: Soluble, slightly soluble or insoluble.

Precipitation Reactions

Determining Solubility Determine the level of solubility for the following: Determine the level of solubility for the following: (1) Ag 2 SO 4 (2) CaCO 3 (3) Na 3 PO 4

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. 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. Bases- generally have a bitter taste, change litmus from red to blue, feel slippery, conduct electricity. Br Ø nstead Acid- proton donor Br Ø nstead Acid- proton donor Br Ø nstead Base- proton acceptor Br Ø nstead Base- proton acceptor

Acid-Base Reactions Acid or Base? Acid or Base? HCl (aq) + H 2 O (l) → H 3 O + (aq) + Cl – (aq) HCl (aq) + H 2 O (l) → H 3 O + (aq) + Cl – (aq) NH 3 (aq) + H 2 O (l) → NH 4 + (aq) + OH – (aq) NH 3 (aq) + H 2 O (l) → NH 4 + (aq) + OH – (aq)

Acid-Base Reactions Look at the following compounds and decide whether they are a Br Ø nstead Acid or a Br Ø nstead Base. Look at the following compounds and decide whether they are a Br Ø nstead Acid or a Br Ø nstead Base. HBr HBr NO 2 – NO 2 – HCO 3 – HCO 3 –

Acid-Base Reactions

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

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

Oxidation-Reduction Reactions

Concentration of Solutions Concentration of a Solution- amount of solute present in a given quantity of solvent or solution. 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. 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– the number of moles of solute per liter of solution. Molarity- moles of solutes/ liters of solution Molarity- moles of solutes/ liters of solution Or n/v Or n/v Moles- grams of sample/ molecular weight of sample Moles- grams of sample/ molecular weight of sample Or g/ mw Or g/ mw

Concentration of Solutions How many moles are there in 24.0g of C? 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.

Concentration of Solutions How many grams are in 2.0 moles of Boron? 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 = There are g of Boron in 2.0 moles of Boron. There are g of Boron in 2.0 moles of Boron.

Concentration of Solutions What is the Molarity of a 1L solution containing 9.0g HCl? 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.

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

Preparation of Solutions

Concentration of Solutions How many grams of Potassium Dichromate, K 2 Cr 2 O 7, are required to prepare a 250mL solution with a concentration of 2.16M? How many grams of Potassium Dichromate, K 2 Cr 2 O 7, 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 K 2 Cr 2 O 7 x g K 2 Cr 2 O 7 Grams = grams of K 2 Cr 2 O 7 are needed to prepare the requested solution.

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. 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 C 6 H 12 O 6 / 180.2g C 6 H 12 O 6 moles = x 10 –2 mol C 6 H 12 O 6 M = n/v v = n/M v = x 10 –2 mol C 6 H 12 O 6 / 2.53M C 6 H 12 O 6 v = 8.36mL She should use 8.36mL of the 2.53M glucose solution.

Preparation of Solutions Explain the process of creating 1L of 3.0M KCl. 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 dH 2 0 to dissolve KCl. Fill flask to 1L meniscus.

Dilution of Solutions Dilution- the procedure for preparing a less concentrated solution from a more concentrated one. 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. Dilutions can be made in increments of 10, 20, 50 or any other value. Serial Dilution- the process 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. Serial Dilution- the process 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.

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

Dilution of Solutions

Dilution of solutions Since moles are constant before and after dilution, we can use the following formula for calculations. Since moles are constant before and after dilution, we can use the following formula for calculations. M i V i = M f V f M i V i = M f V f

Dilution of Solutions Describe how you would prepare 800mL of a 2.0M H 2 SO 4 solution, starting with a 6.0M stock solution of. Describe how you would prepare 800mL of a 2.0M H 2 SO 4 solution, starting with a 6.0M stock solution of. 800mL x 1L/ 1000mL = 0.800L = 0.800L M i V i = M f V f 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 H 2 SO 4 solution should be diluted to give a final volume of 800mL.

Concentration of Solutions There are several ways to determine the concentration of a solution. 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. 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: Spectroscopy is helpful because it gives us: Amount of light transmitted through a solution Amount of light transmitted through a solution Amount of light absorbed by 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. Beer’s Law- a relationship between proportionality constant, path length of radiation going through solution and concentration of the solution. A = abc A = abc A = -log(T) A = -log(T)

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

Beer’s Law