Chem 171 Quiz 8

Chem 171 Quiz 8 Version A 1. A solution of malonic acid (a diprotic acid H 2 A), H 2 C 3 H 2 O 4 , was standardized by titration with 0.1000 N NaOH solution. If 23.95 mL of the NaOH solution were required to neutralize completely 10.00 mL of the malonic acid solution, what is the molarity of the malonic acid solution? Write the relevant molecular equation (you can write H 2 A in the eq.). H 2 C 3 H 2 O 4 (aq) + 2NaOH(aq) ? Na 2 C 3 H 2 O 4 (aq) + 2H 2 O(l) Or: H 2 A (aq) + 2 NaOH (aq) ? Na 2 A(aq) + 2H 2 O(l) N NaOH = M NaOH 23.95 mL x 0.1000 mmol / mL = 2.395 mmol of base 2.395 mmol base x 1 mmol base / 2 mmol acid = 1.198 mmol acid 1.198 mmol acid / 10.00 mL = 0.1198 M acid 2. The following question pertains to the lab entitled ?Evaluating Commercial Antacids?. Why is it that when you are titrating, you do not let a buret dry? If you let all the liquid run out through the tip, you run the risk of getting an air bubble trapped in the stopcock. Bubbles can impede the proper functioning of the buret or come out in the middle of a titration by making the volume inaccurate. Chem 171 Quiz 8 Version B 1. Sodium carbonate is a reagent that may be used to standardize acids. In such a standardization, it was found that a 0.212 g sample of sodium carbonate (molar mass = 106 g/mol) required 24.5 mL of a sulfuric acid solution to reach the endpoint for the reaction. What is the molarity and the normality of the sulfuric acid solution? Write the relevant molecular equation. Na 2 CO 3 (aq) + H 2 SO 4 (aq) ? Na 2 SO 4 (aq) + CO 2 (g) + H 2 O (l) 0.212 g Na 2 CO 3 / 106 g / mol = 0.00200 mol Na 2 CO 3 Molarity of acid = 0.0020 mol H 2 SO 4 / 0.0245 L = 0.0816 M 0.0816 M x 2 equiv / mol acid = 0.163 N 2. The following question pertains to the lab entitled ?Evaluating Commercial Antacids?. Do you blow out the last drop in the 25 mL pipet? Explain. After we dispense a solution from a pipet, a small amount of liquid will remain in the pipet tip. The 25 mL pipet is calibrated to deliver (TD) 25.00 mL of solution excluding the small amount remaining in the pipet tip. Chem 171 Quiz 8 Version C 1. A solution contains 0.252 g of oxalic acid (a diprotic acid H 2 A), H 2 C 2 O 4 .2H 2 O (molar mass = 126 g/mol), in 500.00 mL. What is the molarity of the oxalic acid solution? What is the normality of the oxalic acid solution if it is titrated with sodium hydroxide. Write the relevant molecular equation (you can write H 2 A in the eq.). H 2 C 2 O 4 (aq) + 2 NaOH (aq) ? Na 2 C 2 O 4 (aq) + 2 H 2 O (l) Or: H 2 A (aq) + 2 NaOH (aq) ? Na 2 A(aq)+ 2H 2 O (l) 0.252 g / 126 g/mol = 0.00200 mol oxalic acid 0.00200 mol oxalic acid / 0.500 L = 0.00400 M 0.00400 M oxalic acid x 2 equiv / mol acid = 0.00800 N 2. The following question pertains to the lab entitled ?Evaluating Commercial Antacids?. Why do you boil the antacid-HCl solution? The reason you boil the antacid - HCl solution is to expel the dissolved CO 2. Any CO 2 present during the subsequent titration will interfere with the determination of the endpoint. Chem 171 Quiz 8 Version D 1. Define the following two items: 1) a primary standard and 2) a titration A primary standard is any highly stable substance whose purity is extremely high from which we can determine the concentration of another solution. A titration is the use of a buret to quantitatively deliver a known amount of a solution of known concentration as it reacts with a known amount of solution of unknown concentration. 2. The following question pertains to the lab entitled ?Evaluating Commercial Antacids?. When you are titrating the antacid solution, how do you know that you are approaching the titration endpoint? At this point, how should you modify your titration? As the reaction approaches the endpoint, you will notice momentary blue-green flashes in the area where the NaOH solution enters the solution in the flask. When the reaction is very close to the endpoint, these blue-green areas will grow larger, taking longer to revert to yellow. At this point, you should add the NaOH solution dropwise. Chem 171 Quiz 8 Version E 1. Define the following two terms: 1) an indicator and 2) a titration error An indicator is usually an organic dye used in a titration that changes color when the reaction is complete. This color change indicates to the observer that the reaction is complete. In a titration, the indicator may not change exactly at the equivalence point. The difference between the endpoint and the equivalence point is called the titration error. 2. The following question pertains to the lab entitled ?Evaluating Commercial Antacids?. Explain the analytical procedure called ?back-titration?. In a back-titration, we add more standardized HCl solution to the antacid tablet that the tablet can neutralize. We then titrate the un-neutralized or excess HCl with standardized sodium hydroxide solution. We then subtract the moles of HCl neutralized from the moles of HCl added. siegel Microsoft Word - Chem 171 F07 Q8 Tit Antacid Key.doc