POST-LAB QUESTIONS Type your answers on a separate page and attach it to the bac
ID: 899022 • Letter: P
Question
POST-LAB QUESTIONS Type your answers on a separate page and attach it to the back of your report. Hand written answers will not be accepted. 1. (7 points) In this experiment, phenolphthalein was used as an indicator Drawing from your knowledge of acid-base chemistry, predict whether the pKa of another indicator, chlorphenol red, would have a higher or lower pKa than phenolphthalein. Explain. a. CI HO OH CI Chlorphenol red b. Would the structure above be responsible for the yellow or red color of the indicator? Explain. (You may find it helpful to refer to your lecture notes from 10/09/15 on the chemistry of phenolphthalein and also the website on absorbance spectroscopy available on canvas from experiment 2.) 2. (6 points) The method of preparing a buffer in this experiment was by a titration. In another experiment, imagine you were asked to prepare a phosphate buffer with pH = 7.00 from solid NaH2PO4 (pKa 7.21) and Na2HPO4. a. Calculate the ratio of [CB/[acid] b. Calculate the mass of each component required to make 1 L of a 0.10 M buffer solution. c. If the pH of your prepared buffer solution is 7.10, what do you do to adjust the pH to 7.00?Explanation / Answer
1)
a)
it is easier for chlorphenol red to lose electrons (making it acidic) and therefore a low pKa value
For phenophtalein, the pka value should be high since it is easier for the phenolphtalein to donate a proton in BASIC conditions, this case bmut have a pka value hihger than 7.
b)
The molecule above (chorphenol red) will have a red color of "purple" to "pink" in acidic conditions. In basic conditions the color will shift to yellow
For pehnolphtalein, expect transparent in acidic conditions and "purple" or "pink" in basic conditions
NOTE that for this specific case of the Chlorphenol red, we will have it in acidic conditions, therefore yellowish color
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