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

Transcription

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

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 common 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, i.e. partially or completely dissociate in solution 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 Strong electrolyte

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 CH 3 COOH(aq) CH 3 COO (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 PbI 2 : Pb(NO 3 ) 2 (aq) + 2KI(aq) PbI 2 (s) + 2KNO 3 (aq)

13 Precipitate

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) Ag 2 SO 4 (2) CaCO 3 (3) Na 3 PO 4

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) + H 2 O(l) H 3 O + (aq) + Cl (aq) NH 3 (aq) + H 2 O(l) NH 4 + (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 NO 2 HCO 3

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 Oxidation is loss electrons and Reduction is gain 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.

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 (L) 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/ 12.0 g/mol 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 /mol 2.0 moles x 10.81g Boron = grams Grams = There are 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 / g/mol = 0.25 mol HCl M = n/v M = 0.25 mol HCl/ 1L M = 0.25 The concentration of the solution is 0.25M.

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, K 2 Cr 2 O 7, are required to prepare a 250 ml solution with a concentration of 2.16 M? 250 ml x 1 L/ 1000 ml = L M= n/v n= M x v n= 2.16 M x.250 L 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.

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 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.

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 dh 2 0 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- 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.

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. M i V i = M f V f

39 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. 800mL x 1L/ 1000mL = 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.

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

43 Class Problems 1 1. Calculate the molarity of a solution prepared by dissolving 11.5 g of solid NaOH in enough water to make 1.50 L of solution. Ans: M 2. Calculate the molarity of a solution prepared by dissolving 1.56 g of gaseous HCl in water to make 26.8 ml of solution. Ans: 1.60 M HCl 3. Give the concentration of each type of ion in the following solutions: (a) 0.50 M Co(NO 3 ) 2 (b) 1 M Fe(ClO 4 ) 3 4. Calculate the number of moles of Cl - ions in 1.75 L of 1.0 x 10-3 M ZnCl 2. Ans: 3.5 x 10-3 mol Cl - 5. Typical blood serum is about 0.14 M NaCl. What volume of lood contains 1.0 mg NaCl? Ans: 0.12 ml 6. To analyse the alcohol content of a certain wine, a chemist needs 1.00 L of an aqueous M K 2 Cr 2 O 7 (potassium dichromate) solution. How much solid K 2 Cr 2 O 7 must be weighed out to make this solution? Ans: 58.8 g 7. What volume of 16 M sulphuric acid must be used to prepare 1.5 L of a 0.10 M H 2 SO 4 solution? Ans: 9.4 ml 8. Concentrated sulphuric acid is 97% w/w and 1.84 g/ml density and hydrochloric acid is 37% w/w and 1.18 g/ml. Calculate their concentration in molarity. Ans: 18.2M, 12.0M 9. Describe how you prepare 2.00 L of each of the following solutions: (a) M NaOH from solid NaOH (b) M NaOH from 1.00 M NaOH stock solution. (c) 0.50 M H 2 SO 4 from concentrated (18 M ) sulphuric acid (d) 0.5 M NiCl 2 from the salt NiCl 2 6H 2 O (e) 0.5 M HNO 3 from concentrated (16 M ) reagent

44 Class Problems 2 1. How many atoms are in 75.0 g of nitrogen gas? 2. How many grams of Mg contain the same number of atoms as 20.0 of Ca? 3. How many ml of water should be added to 25.0 ml of a stock 12.0 M HCl solution to make 1.50 M HCl? Assume that the volumes are additive. 4. How many ml of M NaOH solution contain 2.00 g NaOH (Mw = 40.0 g/mol)? 5. What weight of KClO 3 (s) (Mw = ) contains 5.0 g of oxygen atoms? 6. If ml of 0.10 M H 2 SO 4 is neutralised by ml of NaOH solution, determine the molarity of NaOH. 7. A bottle contains x carbon atoms that weigh 3.00 g. Determine the mass containing an equal number of nickel atoms in another bottle. 8. How many grams of Cd 3 P 2 can be made from 375 g of Cd and 61.7 g of P? 9. To what volume of water should 25.0 g BaCl 2 to give a solution that is 0.3 M in chloride ion? 10. What is the total number of atoms present in 25.0 mg C 10 H 16 O? 11. What is the number of oxygen atoms in a 20 g Al 2 (SO 4 ) 3 sample?