Plan for Tues/Wed, 21/22 Oct 08

1. Turn in Exp 4 Pre-lab Today: Exp 4, Volumetric Analysis Purpose: To standardize an ~0.1 M aqueous solution of NaOH using a primary standard (KHP – potassium hydrogen phthalate). To use the standardized NaOH solution as a secondary standard to determine the unknown molarity of an acid solution. THIS EXPERIMENT WILL BE WRITTEN UP AS A FORMAL LAB REPORT. The formal report format will be discussed in lecture. THIS EXPERIMENT IS DUE AT THE BEGINNING OF NEXT WEEK’S LAB SESSION. Plan for Tues/Wed, 21/22 Oct 08

2. Tips for success TECHNIQUE • DO NOT let the level of solution in your buret fall below the markings!! • DO NOT force the small volume of solution out of the bottom of your pipet tip!! • MAKE SUREto have me check your NaOH dilution calculation and your percent-difference calculations (these must be within 1%!!). • MAKE SUREto mix your diluted NaOH solution really really well!! • YOU NEED YOUR DILUTE NaOH SOLUTION IN PART B!!Don’t throw it away!! PROCEDURE CHANGES: • PART A: Weigh out only 0.4-0.5 g of KHP (not 0.9-1.0 g). • PART B: Obtain only 50 mL of unknown acid from stockroom (not 100 mL). • Do not check out a magnetic stir bar…you will swirl your titration flasks with your hand. WASTE • Everything goes in the sink. If you have excess 6 M NaOH, run the tap for 10-20 sec when you dump it out. • Return unused KHP solid to the stockroom. • NEVERput a spatula, etc into a reagent bottle. NEVER return unused reagent to the reagent bottle. Any unused reagent must be placed in the waste container. • CLEAN UP THE BALANCE AREA!! THROW AWAY THE WEIGHING PAPERS!! SAFETY • KHP(s) and NaOH(aq) are toxic and corrosive!! BE CAREFUL!! Do not touch your face!! Gloves are available at the back of the lab. • Be sure to wash and dry your lab bench to remove all traces of any spilled chemicals. • Wash your hands with soap and water before you leave lab.

3. NaOH after eq. pt: OH-,Na+, X-, H2O @ eq. pt: X-,Na+, H2O before eq pt: H+, X-, Na+, H2O HX Acid-Base Neutralization Rxns • Neutralization: when an acid and a base react to form a molecule (water or another weak acid) and an aqueous salt. • If the acid and base we use is strong, and we add enough base to just consume all the available acid, we say the acid has been “neutralized.” HX(aq) + KOH(aq) KX(aq) + H2O(l) [H+] = [OH-]

4. What does an endpoint look like? TOO DARK!!! That’s more like it…

5. M2V2 M1 Making your ~0.1 M NaOH Solution • You have 6 M NaOH, you want to make a solution that is approximately 0.1 M. • Using the KHP primary standard, you will determine the actual concentration of this solution to 4 sig figs. • Recall the dilution formula: M1V1 = M2V2 where… M1, V1 = molarity and volume of first solution M2, V2 = molarity and volume of second solution V1 = ? V2 = 400 mL M1 = 6 M M2 = 0.1 M This volume of NaOH goes in your 500 mL Erlenmeyer flask, and you dilute it up to ~400 mL. M1V1 = M2V2 V1 =

6. 1 mol NaOH mol KHC8H4O4 1 mol KHC8H4O4 g KHC8H4O4 mol NaOH x L NaOH soln Standardizing the KOH Solution • You will dissolve a known mass of KHP in water, and then titrate it with your ~0.1 M NaOH. • The known mass of KHP is converted to moles of NaOH. • Since you know the volume of NaOH(aq) you delivered to reach the end point, you can determine the molarity of your NaOH solution. = x g KHC8H4O4 ? mol NaOH g of KHP you weigh out MM of KHP vol of NaOH solution you deliver to the flask from the buret ? [NaOH] =

7. 1 mol HX mol NaOH 1 mol NaOH L NaOH mol HX x L HX soln Determining Unknown Molarity of an Acid • You will titrate a sample of a monoprotic acid of unknown concentration using your standardized NaOH solution. • Since you know the molarity of your NaOH and the volume needed to reach the end point, you can determine the moles of acid present in your sample. • Since you know the volume of you acid sample, you can determine the molarity. = x L NaOH ? mol HX vol of NaOH you delivered [NaOH] you determined previously vol of HX solution you measured out ? [HX] =