You will want to have it plot ph as a function of

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Unformatted text preview: much. How would this influence your calculation of Ka? Of ∆ Go ? Of ∆So ? Which of these values could you trust, and why? Dakota State University page 1 50 of 232 Experiment 13: Molecular Mass of an Acid General Chemistry I and II Lab Manual Experiment 13: molecular mass of acid Purpose: To use a titration method to determine the molar mass of an unknown acid Purplunk: The noise my heart makes when she says “what…with you?” Background: See “Using the Pasco System” and “Basic Laboratory Procedures”; pipettes, burets, balance Introduction: Titrations are the second most accurate form of quantitative analysis tool available to chemists, second only to gravimetric techniques. Between the two, however, titrations are far more common, because they are far easier than gravimetric techniques, and still afford (typically) four significant figures in accuracy. Today you will be using titrations to determine a very fundamental property of an unknown acid; its molecular mass. Remember, molecular mass is nothing more than the mass of the material present, divided by the moles of material in the sample. Mass is easy; put it on a balance, and you get mass. Moles we can get from the titration. If you are VERY careful, you can get VERY accurate results, and you will be graded on accuracy. Procedure: You will want to set up the Pasco system for manual data collection using the pH probe. You will want to have it plot pH as a function of volume of base added. Set up a buret (see general procedures) for a titration. Standardization of the Base: Follow this procedure IF the base solution provided has not already been standardized. If the base has been standardized for you, write down the exact concentration (four significant digits) of the base and proceed to titration of the acid. For the standardization of the base, we will be using potassium hydrogen phthalate (KHP) as our primary standard (molecular mass 204.22 g/mol). Use a CLEAN DRY spatula to measure out about 0.4 grams of KHP (you do not need exactly 0.4 g, but you do need to know exactly how much you do have to as many significant figures as possible). If your lab instructor requests you to do so, add a couple of drops of phenolphthaliene. Titrate the sample either to the endpoint, or using the Pasco system to collect data as your lab instructor requests. Perform at least three titrations; if any of the runs are off by more than 0.04 mL from any other run, perform a fourth titration and discard the titration that seems to most significantly disagree with the other runs. Determine the exact concentration of the base from this step. Dakota State University page 1 51 of 232 Experiment 13: Molecular Mass of an Acid General Chemistry I and II Lab Manual Titration of the Acid: Measure out about 0.1 gram of the unknown acid (you do not need exactly 0.1 g, but you do need to know exactly how much you have and you must have at least 0.1 g to have as many significant figures as possible). If your lab instructor requests you to do so, add a couple of drops of phenolphthaliene. Titrate the sample either to the endpoint, or using the Pasco system to collect data as your lab instructor requests. Perform at least three titrations; if any of the runs are off by more than 0.04 mL from any other run, perform a fourth titration and discard the titration that seems to most significantly disagree with the other runs. Deter...
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