Dispose all your solutions in the acid base waste container in the fume hood, DO
ID: 498992 • Letter: D
Question
Dispose all your solutions in the acid base waste container in the fume hood, DO NOT discard your solutions down the drain unless you are cortain that they are of a pH between 6 and 8 Since many natural and synthetic compounds have acid-base properties which influence their color, such compounds can be used as visual indicators of pH. Besides allowing us to estimate the pH of a solution by color, the use of indicators is a way of visually detecting the end point (close to the equivalence point) of an acid-base titration. The pH at which an indicator changes color is dependent on its pKa As you have seen. an abrupt change in pH coincides with the equivalence point of a titration. An indicator must be chosen such that its color changes concurrently with the pH change at the equivalence point. That is, the useful will have close the point pH. characteristics of some common acid-base indicators are tabulated below. Table 7.1: Properties of some common acid-base indicators. Color transition Color transition range pKa Indicator name (pH units) acidic- basic 4.2 3.1 -4.4 red- orange Methyl orange 4.7 3.8 5.4, Yellow blue Bromocresol green 5.0 4.2-6.2 Methyl red red yellow 6.0 4.8 -6.4 Yellow red Chlorophenol red 7.1 Yellow blue Bromothymol blue 7.4 Yellow red Phenol red 9.7 8.0-9.8 colorless pink Phenolphthalein 9.9 9.3 10.5 Thymolphthalein colorless blue 11.0 10.1 12.0 Alizarin yellow R Yellow red 11.4 13.0 12.2 ndigo carmin Blue yellow As a general rule, the color change of an acid-base indicator takes place over a pH range of pKa-1 to pKa 1: however, note from the table above that the range over which color change occurs is actually somewhat different than pKat 1 pH unit. Thus, the general rule is typically applied only when the experimentally determined transition range is not available It is important to distinguish a titration's equivalence point from its endpoint. Equivalence occurs when the number of moles of acid equals the number of moles of base (in an acid-base whereas the endpoint is the point at which we observe the indicators color change. An indicator will always change color just before or just after equivalence. This error, called "indicator error. s minimized by careful choice of theExplanation / Answer
Q!.
determine molar absorptivity of Bromothymol blue...
if
l = 14.45 mm = 1.445 cm
e = ?
M = x-axis
A = y-axis
From your graph, we got a striagh line, which is a perfect fit ( R^2) = 0.999
so
A = 48048*M - 0.0105
ignore y intercept:
A = 48048*M
From beer's law:
A = e*l*M
since l = 1.445 cm then
m = slope = e*l = 48048
e * 1.445 = 48048
e = 48048 /1.445 = 33251.2110
molar absorptivity = 33251.2110 1/(M-cm)