Question

The following graph shows the pH curve for the titration of 25 mL of a 0.1 M monoprotic acid solution with a 0.1 M solution of a monoprotic base.

#1's drop down menus both read weak/strong.
#2's drop down menu features a list of all the colors listed on the graph.

0 5 10 15 20 25 30 35 40 45 50 mL of 0.1 M base added (1) The pH curve represents the titration of a __ acid with a __ base. (2) Choose a suitable indicator for the endpoint of the titration from the following pulldown list. Malachite green Thymol blue 1.2 2.8 8.0 9.6 Methyl orange 3.2 4.4 Bromocresol green 3.8 5.4 Methyl red 4.8 6.0 Bromothymol blue 6.0 7.6 Cresol red 7.0 8.8 Phenolphthalein 8.2 10.0 Thymolphthalein 9.4 10.6 Alizarin yellow 10.1 10 12.01 12 13 pH 0 1 2 3 4 5 6 7 8 9 11 14

Answer 1

It is clear that pH started from ~ 3, and raises to a point where equivalence point is ~ 7.2 then steep at > pH = 9 represents weak acid vs strong base titration.

equivalence point is above 8, so best indicator always have pKa ± 1, so 7 - 9 pH range is observed, so best one goes to Phenolphthalein.

the indicator is colorless when the pH is less than pK_a – 1, and it is red for pHs greater than pK_a + 1.

0 5 10 15 20 25 30 35 40 45 50 mL of 0.1 M base added weak strong (1) The pH curve represents the titration of a __ acid with a base. phenolphthalein (2) Choose a suitable indicator for the endpoint of the titration from the following pulldown list. Malachite green Thymol blue 1.2 2.8 8.0 9.6 Methyl orange 3.2 4.4 Bromocresol green 3.8 5.4 Methyl red 4.8 6.0 Bromothymol blue 6.0 7.6 Cresol red 7.0 8.8 Phenolphthalein 8.2 10.0 Thymolphthalein 9.4 10.6 Alizarin yellow 10.1 10 12.01 12 13 pH 0 1 2 3 4 5 6 7 8 9 11 14

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