1 © 2006 Brooks/Cole - Thomson Quantitative Aspects of Reactions in Solution Sections

2 © 2006 Brooks/Cole - Thomson TerminologyTerminology In solution we need to define the SOLVENTSOLVENT the component whose physical state is preserved when solution forms; usually the component in the largest proportion SOLUTESOLUTE the other solution component

3 © 2006 Brooks/Cole - Thomson Concentration of Solute The amount of solute in a solution is given by its concentration The amount of solute in a solution is given by its concentration. Concentration (M) = [ …]

4 © 2006 Brooks/Cole - Thomson The Nature of a CuCl 2 Solution: Ion Concentrations CuCl 2 (aq) --> Cu 2+ (aq) + 2 Cl - (aq) If [CuCl 2 ] = 0.30 M, then [Cu 2+ ] = 0.30 M [Cl - ] = 2 x 0.30 M

5 © 2006 Brooks/Cole - Thomson Preparing a Solution

6 © 2006 Brooks/Cole - Thomson 1.0 L of water was used to make 1.0 L of solution. Notice the water left over. CCR, page 206 Do NOT add 1.0 L of water!!!

7 © 2006 Brooks/Cole - Thomson PROBLEM: Dissolve 5.00 g of NiCl 2 6 H 2 O in enough water to make 250 mL of solution. Calculate molarity. Step 1: Calculate moles of NiCl 2 6H 2 O Step 2: Calculate molarity NiCl 2 6 H 2 O [NiCl 2 6 H 2 O ] = M

8 © 2006 Brooks/Cole - Thomson Step 1: Calculate moles of acid required. ( mol/L)(0.250 L) = mol Step 2: Calculate mass of acid required. ( mol )(90.00 g/mol) = 1.13 g USING MOLARITY moles = MV What mass of oxalic acid, H 2 C 2 O 4, is required to make 250. mL of a M solution?

9 © 2006 Brooks/Cole - Thomson Preparing Solutions Weigh out a solid solute and dissolve to a given quantity of solution.Weigh out a solid solute and dissolve to a given quantity of solution.OR Dilute a concentrated solution to give one that is less concentrated.Dilute a concentrated solution to give one that is less concentrated.

10 © 2006 Brooks/Cole - Thomson PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? But how much water do we add? Add water to the 3.0 M solution to lower its concentration to 0.50 M Dilute the solution!

11 © 2006 Brooks/Cole - Thomson PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do ? How much water is added? The important point is that --->

12 © 2006 Brooks/Cole - Thomson PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? Amount of NaOH in original solution = M V = M V = (3.0 mol/L)(0.050 L) = 0.15 mol NaOH Amount of NaOH in final solution must also = 0.15 mol NaOH Volume of final solution = (0.15 mol NaOH)(1 L/0.50 mol) = 0.30 L or 300 mL

13 © 2006 Brooks/Cole - Thomson PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? Conclusion: add enough water to the 50.0 mL of 3.0 M NaOH to make 300 mL of 300 mL of 0.50 M NaOH M NaOH.

14 © 2006 Brooks/Cole - Thomson A shortcut A shortcut C initial V initial = C final V final Preparing Solutions by Dilution

15 © 2006 Brooks/Cole - Thomson pH, a Concentration Scale pH: a way to express acidity -- the concentration of H + in solution. Low pH: high [H + ] High pH: low [H + ] Acidic solutionpH < 7 Neutral pH = 7 Neutral pH = 7 Basic solution pH > 7 Basic solution pH > 7 Acidic solutionpH < 7 Neutral pH = 7 Neutral pH = 7 Basic solution pH > 7 Basic solution pH > 7

16 © 2006 Brooks/Cole - Thomson The pH Scale pH = - log [H + ] In a neutral solution, [H + ] = [OH - ] = 1.00 x M at 25 o C pH = - log [H + ] = -log (1.00 x ) = - [0 + (-7)] = - [0 + (-7)] = 7

17 © 2006 Brooks/Cole - Thomson [H + ] and pH If the [H + ] of soda is 1.6 x M, the pH is ____? Because pH = - log [H + ] then pH= - log (1.6 x ) pH= - log (1.6 x ) pH = -{log (1.6) + log (10 -3 )} pH = -{ ) pH = 2.80

18 © 2006 Brooks/Cole - Thomson pH and [H + ] If the pH of Coke is 3.12, its [H + ] is ____________. Because pH = - log [H + ] then log [H + ] = - pH log [H + ] = - pH Take antilog and get [H + ] = 10 -pH [H + ] = = 7.6 x10 -4 M

19 © 2006 Brooks/Cole - Thomson Zinc reacts with acids to produce H 2 gas.Zinc reacts with acids to produce H 2 gas. Have 10.0 g of ZnHave 10.0 g of Zn What volume of 2.50 M HCl is needed to convert the Zn completely? What volume of 2.50 M HCl is needed to convert the Zn completely? SOLUTION STOICHIOMETRY

20 © 2006 Brooks/Cole - Thomson GENERAL PLAN FOR STOICHIOMETRY CALCULATIONS Convert something to Moles Coefficient Ratio Convert to what you’re looking for

21 © 2006 Brooks/Cole - Thomson Write the balanced equation Write the balanced equation Zn(s) + 2 HCl(aq) --> ZnCl 2 (aq) + H 2 (g) Step 1: Calculate amount of Zn Zinc reacts with acids to produce H 2 gas. If you have 10.0 g of Zn, what volume of 2.50 M HCl is needed to convert the Zn completely? Step 2: Use the stoichiometric factor

22 © 2006 Brooks/Cole - Thomson Step 2: Use the stoichiometric factor Zinc reacts with acids to produce H 2 gas. If you have 10.0 g of Zn, what volume of 2.50 M HCl is needed to convert the Zn completely? Step 3: Calculate volume of HCl req’d

23 © 2006 Brooks/Cole - Thomson ACID-BASE REACTIONS Titrations

24 © 2006 Brooks/Cole - Thomson Setup for titrating an acid with a base Active Figure 5.23

25 © 2006 Brooks/Cole - Thomson Titration 1. Rinse buret with titrant. Remove bubbles. Add solution from the buret to flask. 2. Flask contains analyte and indicator. Base reacts with acid in solution in the flask. 3. Indicator shows when exact stoichiometric reaction has occurred. 4. Net ionic equation H + + OH - --> H 2 O H + + OH - --> H 2 O 5. At equivalence point (end point) moles H + = moles OH - moles H + = moles OH -

26 © 2006 Brooks/Cole - Thomson mL of 1.2 M H 2 C 2 O 4 (oxalic acid) requires mL of NaOH for titration to an equivalence point. What is the concentration of the NaOH? mL of 1.2 M H 2 C 2 O 4 (oxalic acid) requires mL of NaOH for titration to an equivalence point. What is the concentration of the NaOH? LAB PROBLEM #1: Standardize a solution of NaOH — i.e., accurately determine its concentration.

27 © 2006 Brooks/Cole - Thomson mL of 1.2M H 2 C 2 O 4 (oxalic acid) requires mL of NaOH for titration to an equivalence point. What is the concentration of the NaOH? Step 1: Calculate amount of H 2 C 2 O 4 Step 2: Calculate amount of NaOH req’d

28 © 2006 Brooks/Cole - Thomson Step 1: Calculate amount of H 2 C 2 O 4 = mol acid = mol acid Step 2: Calculate amount of NaOH req’d = mol NaOH = mol NaOH Step 3: Calculate concentration of NaOH [NaOH] = M mL of 1.2M H 2 C 2 O 4 (oxalic acid) requires mL of NaOH for titration to an equivalence point. What is the concentration of the NaOH?