sample acid base titration lab

sample acid base titration lab - Titration of a Diprotic...

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Titration of a Diprotic Acid: Identifying an Unknown Aj Klatch Kyle Tisi Mr. Edmondson Ap Chemistry 4/19/02 2/3
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Abstract The following titration lab emphasizes several concepts important to solution chemistry and acid-base equilibrium. Titrations are careful procedures used in chemistry to determine the concentration of a particular solution. A solution containing a known concentration of base, called the standard solution, is slowly added to the acid until a neutralization reaction occurs between the two. The point at which stoichiometrically equivalent quantities are brought together is called the equivalence point of the titration. This lab involved the titration of solid maleic acid, a diprotic acid, with 0.25M NaOH base. The molecular formula for maleic acid is H 2 C 4 H 2 O 4 . A diprotic acid differs from a conventional monoprotic acid because it has two ionizable H atoms instead of just one. Each of the H atoms in maleic acid ionize in successive steps. Specifically, maleic acid dissociates in water in two stages: H 2 C 4 H 2 O 4 i H C 4 H 2 O 4 + H 3 O H C 4 H 2 O 4 i C 4 H 2 O 4 + H 3 O The acid dissociation constants for these equilibrium stages are labeled Ka1 and Ka2, respectively. The numbers on the constants refer to the particular proton of the acid that is ionizing. It is important to know that Ka2 is much smaller than Ka1. On the basis of electrostatic attractions, we would expect a positively charged proton to be lost more readily from a neutral H 2 C 4 H 2 O 4 molecule instead of a H C 4 H 2 O 4 molecule. It is always easier to remove the first proton from a diprotic acid than the second proton. Because Ka1 is so much larger than Ka2, almost all the H protons come from the first ionization stage. In this lab, my partner and I performed steps comparable to any acid-base titration. Our first step was to set up the apparatus and make sure that everything was working properly. For example, we rinsed out all of the glassware we were going to be using, including the burette and beakers. We made sure that the burette was properly calibrated and also checked that the stopcock was working correctly. Because we were using a solid diprotic acid (maleic), our next step was to weigh out the designated .120 grams of the solid acid on an electronic balance. It is important to know that we didn’t use an acid-base indicator in this titration, but rather a pH probe with an amplifier and a hot plate equipped with a stirring feature. The use of the hot plate and magnetic stirrer eliminated the need to swirl the beaker as the base is added to it. It was important to calibrate the pH probe before beginning any data collection. To calibrate this device, we attached the sensor to a computer port and placed it in 3 different solutions, each of which had a different pH value (4,7, and 10). In the first step of the calibration, the probe was placed in a buffer solution of pH 4.00. From here, we recorded the data on a computer and rinsed off the probe in distilled water. The probe
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This note was uploaded on 05/25/2010 for the course CHEM 101 taught by Professor Fricke during the Spring '10 term at Canisius College.

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sample acid base titration lab - Titration of a Diprotic...

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