Chem Lab #3 Buffers

NAME: QUIZ SECTION: AD LAB PARTNER: Chem 152 Lab #3: Buffers Note: All sections of this report must be typed Total Points = 60 pts (10 notebook, 50 template) By signing below, you certify that you have not falsified data, that you have not plagiarized any part of this lab report, and that all calculations and responses other than the reporting of raw data are your own independent work. Failure to sign this declaration will result in 5 points being deducted from your lab score. Signature: Deprotonation of Acetic Acid 3 pt rxn, 2 pt for Ka, 2 pt for Kb (correct & entered) Provide the Ka value Provide the Kb value for the Provide the acid dissociation reaction for proton removal in water: for the acid conjugate base CH3COOH(aq) (+) H20(l)->H3O^+ (+) CH3COO^1.8E-05 DATA AND CALCULATIONS Parts A - D: Preparation of Acetic Acid/Acetate Solutions 5 pts: ALL solution prep data and expected initial pH values entered Molarity of Stock Soln (M) 0.10 Volume of Water *Molarity (mL) (M) 100.0 0.10 *(from calculation on pg 2) CH3COOH CH3COONa·3H2O Solution ID #1 #2 #3 #4 #5 DI Water Molar Mass (g/mol) 136.079 mL of CH3COOH 20.00 15.00 10.00 5.00 0.00 -- Mass of Solid Used (g) 1.36 g Moles of Solid (mol) 0.010000 mL of CH3COONa 0.00 5.00 10.00 15.00 20.00 -- 5.6E-10 mL Water 30.00 30.00 30.00 30.00 30.00 50.00 Total Volume 50.00 50.00 50.00 50.00 50.00 50.00 *Summary of Expected Initial pH 3.07 *(based on example 4.27 calculations on pg 2) 4.74 5.22 8.68 5.30 Part E: Preparation of Aspirin and Buffered Aspirin Solutions Aspirin Buffered Aspirin Mass of Solid (for acid test) (g) 0.208 0.431 Mass of Solid (for base test) (g) 0.209 0.428 Molarity of HCl for acid test: Volume (mL) of HCl added: Millimoles of HCl added: 0.5 M 0.25 0.125 Parts C-E: pH Measurements and Determination of Buffer Capacity Solution ID #1 #2 #3 #4 #5 DI Water Aspirin (acid) Buffered Aspirin (acid) Initial Measured pH 3.60 4.50 4.90 5.40 7.50 6.80 3.30 5.40 pH Measured After H+ Addition 3.60 4.20 4.75 5.10 5.70 3.20 3.00 5.50 *Buffer Capacity (mmol H+ added /|DpH|) N/A 0.417 0.833 0.417 0.069 0.035 0.417 1.250 Molarity of NaOH for base test: Volume (mL) of NaOH added: Millimoles of NaOH added: 0.50 0.25 0.125 5 pts: ALL pH values and calculated buffer capacities entered Solution ID #1 #2 #3 #4 #5 DI Water Aspirin (base) Buffered Aspirin (base) *example calculation for Solution #2 buffer capacity is on pg 3 Page 1 of 8 Initial Measured pH 3.60 4.50 4.90 5.40 7.50 6.80 3.30 5.70 pH Measured After OHAddition 4.20 4.80 5.20 6.10 11.10 11.00 3.90 7.60 Buffer Capacity (mmol OHadded /| DpH|) 0.126 * .15 mL NaOH added 0.250 * .15 mL NaOH added 0.250 * .15 mL NaOH added 0.179 0.035 0.030 0.208 0.066 Provide the calculation for the concentration of the stock CH 3 COONa solution you prepared (actual, not theoretical). (3 pts) 1.360 (g) CH3COONa weighed -> 1.36g/136.0796 (g/mol) = .01 (n) CH3COONa / 100 (mL) = 0.1 M CH3COONa Give the predominant chemical reaction that occurs when CH 3COONa is dissolved in water (omit spectator ions). (3 pts) CH3COONa(aq) +H2O(l) -> CH3COOH(aq)+ OH-(aq) Provide the calculation for the expected initial pH of solution #1. (3 pts) pH= Ka * [HA]= x^2 -> [HA]= (.1M x .020L)/ .050L -> √(1.8E-5(.04))= x -> -log(x)=3.07 Provide the calculation for the expected initial pH of solution #2 (as an example of the calculation for solutions #2-4). (3 pts) pH= pKa + log( [Base]/ [Acid]) = 4.745 + log([.01]/[.03])= 4.745 - 0.477 = 4.27 Provide the calculation for the expected initial pH of solution #5. (3 pts) pH=14-pOH -> pOH=Kb([B])=5.6E-10(.04)^1/2= x =[OH-] -> pH= 14 - (-log[OH]) -> pH=14-5.325 =8.68 Page 2 of 8 Define the terms buffer and buffer capacity, identifying the differences between them. For a buffer, what are the required solution components? (4 pts) buffer: CH3COONa -> CH3COO- resists the change in pH by combining with either OH- or H+ ions by changing the buffer components : HA or A-, to more of the same components( if H+ added, more HA is made, or if OH is added more A- is made when the proton dissociates to water) and Water. buffer capacity: The ability, or duration, for the Buffer components to resist the change in PH. This is dependant upon the concentration of the buffering ions in solution. (5 mol of HA and Ahave a higher buffering capacity than 1mol of HA and A-) Provide the calculation of the buffer capacity for solution #2 after the addition of HCl. (3 pts) Buffer capacity= H+ (mmol added) / (|pHf-pHi|) -> .125 mmol HCl added / (|4.2 - 4.5|) = .125 mmol HCl / (0.3) = 0.42 mmol/∆pH Based on your experimental results, which acetic acid/acetate (#1-#5) solution was the bestat resisting pH change upon addition of acid? of base? Which acetic acid/acetate solution(s) (#1-#5) had the leastability to resist pH changes? Explain these observations. Also comment on the buffer capacity of DI water relative to the other solutions. (3 pts) Solution 1 was the best at buffering acids since the buffering capacity was uncalculatable because the pH of the two were so similar. However it was poor in buffering bases. Solution 2 and Solution 3 was the best at buffering bases with a capacity of 0.250, despite the amount of NaOH added being less than other runs due to the dispenser being incorrect. Solution 5 and DI Water were very poor at buffering both acids and bases. DI water: It was an interesting discovery that the DI water was slightly acidic . This leaves evidence that the water truly isn't de-ionized and allows why the buffer capacity is slightly different for that of NaOH then that of HCl. Page 3 of 8 1) Look up the chemical name of aspirin and then its conjugate base (as the sodium salt) and provide those names here: (1pt) acetylsalicylic+acid+(HA)+++->+acetylsalicylate+(A-) 2) The manufacturer's label for the "aspirin" lists aspirin as the active ingredient and various fillers and binders as inactive ingredients. The manufacturer's label for the "buffered aspirin" lists aspirin as the active ingredient and calcium carbonate, along with fillers and binders, as inactive ingredients. Given these lists of ingredients, is "buffered aspirin" a true buffer? Explain why it is or is not. If it is a buffer, what are the relevant conjugate acid/base pairs. (2 pts) It is not a true buffer, it "buffers" by neutralizing the acidic nature of the acidic aspirin through calcium carbonate. How do your measured initial pH values compare to the expected initial pH values for the acetic acid/acetate solutions?. Calculate the % error for the initial pH values for solutions #1 - #5 and comment on the accuracy of your results. Identify possible sources of error. (4 pts) Solution %error: 1: 14.7% 2: 5.39% 3: 3.38% 4: 3.45% 5: 13.6 % Possible sources of error could originate from inadequate cleaning of the cylinders and beakers, adding too little or too much solution. Having an incorrect concentration of CH3COONa·3H2O solution0created. Laboratory Waste Evaluation (1 pt) Laboratory waste is considered anythinggenerated during an experiment that is disposed of down the sewer drain, thrown in the garbage, collected in a container for disposal by the UW Environmental Health & Safety department, or released into the environment. Based on the written lab procedure andyour actions during the lab, list the identity and approximate amount (mass or volume) of waste that you generated while performing this experiment. ~ 750 mL of waste inside the Waste Bottle ~ 2 L of water Page 4 of 8