Solution Concentration Chapter 4 Reactions in Aqueous Solutions (part 2)

Copyright © Cengage Learning. All rights reserved. 4 | 2 Molar Concentration, Molarity, (M) Moles of solute per liter of solution

Copyright © Cengage Learning. All rights reserved. 4 | 3 To prepare a solution, add the measured amount of solute to a volumetric flask, then add water to bring the solution to the mark on the flask.

4 Solution Stoichiometry The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. M = molarity = moles of solute liters of solution What mass of KI is required to make 500. mL of a 2.80 M KI solution? volume of KI solutionmoles KIgrams KI M KI 500. mL= 232 g KI 166 g KI 1 mol KI x 2.80 mol KI 1 L soln x 1 L 1000 mL x

5 Preparing a Solution of Known Concentration

6 Dilution is the procedure for preparing a less concentrated solution from a more concentrated solution. Dilution Add Solvent Moles of solute before dilution (i) Moles of solute after dilution (f) = MiViMiVi MfVfMfVf =

Copyright © Cengage Learning. All rights reserved. 4 | 7

Copyright © Cengage Learning. All rights reserved. 4 | 8 You place a 1.52−g of potassium dichromate, K 2 Cr 2 O 7, into a 50.0−mL volumetric flask. You then add water to bring the solution up to the mark on the neck of the flask. What is the molarity of K 2 Cr 2 O 7 in the solution? Molar mass of K 2 Cr 2 O 7 is 294 g M

Copyright © Cengage Learning. All rights reserved. 4 | 9 A solution of sodium chloride used for intravenous transfusion (physiological saline solution) has a concentration of M NaCl. How many moles of NaCl are contained in 500.−mL of physiological saline? How many grams of NaCl are in the 500.−mL of solution?

10 How would you prepare 60.0 mL of M HNO 3 from a stock solution of 4.00 M HNO 3 ? M i V i = M f V f M i = 4.00 M M f = MV f = L V i = ? L V i = MfVfMfVf MiMi = M x L 4.00 M = L = 3.00 mL Dilute 3.00 mL of acid with water to a total volume of 60.0 mL.

Copyright © Cengage Learning. All rights reserved. 4 | 11 A saturated stock solution of NaCl is 6.00 M. How much of this stock solution is needed to prepare 1.00−L of physiological saline soluiton (0.154 M)?

Copyright © Cengage Learning. All rights reserved. 4 | 12 Quantitative Analysis The determination of the amount of a substance or species present in a material.

Copyright © Cengage Learning. All rights reserved. 4 | 13 Gravimetric Analysis A type of quantitative analysis in which the amount of a species in a material is determined by converting the species to a product that can be isolated completely and weighed.

14 Gravimetric Analysis 1.Dissolve unknown substance in water 2.React unknown with known substance to form a precipitate 3.Filter and dry precipitate 4.Weigh precipitate 5.Use chemical formula and mass of precipitate to determine amount of unknown ion

Copyright © Cengage Learning. All rights reserved. 4 | 15 The figure on the right shows the reaction of Ba(NO 3 ) 2 with K 2 CrO 4 forming the yellow BaCrO 4 precipitate.

Copyright © Cengage Learning. All rights reserved. 4 | 16 The BaCrO 4 precipitate is being filtered in the figure on the right. It can then be dried and weighed.

Copyright © Cengage Learning. All rights reserved. 4 | 17 A soluble silver compound was analyzed for the percentage of silver by adding sodium chloride solution to precipitate the silver ion as silver chloride. If g of silver compound gave g of silver chloride, what is the mass percent of silver in the compound?

Copyright © Cengage Learning. All rights reserved. 4 | 18 = g Ag in the compound = 85.03% Ag Molar mass of silver chloride (AgCl) = g

Copyright © Cengage Learning. All rights reserved. 4 | 19 Titration A procedure for determining the amount of substance A by adding a carefully measured volume with a known concentration of B until the reaction of A and B is just complete.

20 Titrations In a titration a solution of accurately known concentration is added gradually added to another solution of unknown concentration until the chemical reaction between the two solutions is complete. Equivalence point – the point at which the reaction is complete Indicator – substance that changes color at (or near) the equivalence point Slowly add base to unknown acid UNTIL the indicator changes color

Copyright © Cengage Learning. All rights reserved. 4 | 21 In the titration above, the indicator changes color to indicate when the reaction is just complete.

Copyright © Cengage Learning. All rights reserved. 4 | 22 Volumetric Analysis A type of quantitative analysis based on titration.

23 Titrations can be used in the analysis of Acid-base reactions Redox reactions H 2 SO 4 + 2NaOH 2H 2 O + Na 2 SO 4 5Fe 2+ + MnO H + Mn Fe H 2 O

24 What volume of a M NaOH solution is required to titrate mL of a 4.50 M H 2 SO 4 solution? WRITE THE CHEMICAL EQUATION! volume acidmoles redmoles basevolume base H 2 SO 4 + 2NaOH 2H 2 O + Na 2 SO mol H 2 SO mL soln x 2 mol NaOH 1 mol H 2 SO 4 x 1000 ml soln mol NaOH x mL = 158 mL M acid rxn coef. M base

Copyright © Cengage Learning. All rights reserved. 4 | 25 Zinc sulfide reacts with hydrochloric acid to produce hydrogen sulfide gas: ZnS(s) + 2HCl(aq)  ZnCl 2 (aq) + H 2 S(g) How many milliliters of M HCl are required to react with g ZnS?

Copyright © Cengage Learning. All rights reserved. 4 | 26 Molar mass of ZnS = g = L = 157 mL HCl solution

Copyright © Cengage Learning. All rights reserved. 4 | 27 A dilute solution of hydrogen peroxide is sold in drugstores as a mild antiseptic. A typical solution was analyzed for the percentage of hydrogen peroxide by titrating it with potassium permanganate: 5H 2 O 2 (aq) + 2KMnO 4 (aq) + 6H + (aq)  8H 2 O(l) + 5O 2 (g) + 2K + (aq) + 2Mn 2+ (aq) What is the mass percent of H 2 O 2 in a solution if 57.5 g of solution required 38.9 mL of M KMnO 4 for its titration?

Copyright © Cengage Learning. All rights reserved. 4 | 28 Molar mass of H 2 O 2 = g = 1.77 g H 2 O 2 = 3.07% H 2 O 2