Acid-Base #3 Indicators - KEY

Acid Base #3Indicators1) What is an acid/base indicator? Define and give an example.A weak acid whose protonated colour differs from its unprotonated colour *OR*A weak acid whose protonated colour differs from the colour of its conjugate base.2) Why are the terms “acidic solution” and “acidic colour” not necessarily the same thing?Give an example of a solution and indicator that is basic but shows the indicator’s“acidic colour”.“Acidic solution” implies the concentration of hydronium is greater than theconcentration of hydroxide. (or pH < pOH) “Acidic colour” is what happens when an indicatoracts like a Bronsted-Lowery base and accepts a proton from the solution. Depending on theindicator, this can occur in a solution that is acidic, basic or neutral.eg Alizarin yellow will have a yellow colour (the acidic colour) at any pH value less than10.1. So from pH = 7.0 up to pH = 10.1, you have a basic solution exhibiting the acidic colour.below pH = 7.0 the solution is acidic and displays the acidic colour of the indicator.3) Calculate the Ka’s of each indicator;a) Alizarin yellow(10.1 + 12.0)/2 = 11.05 Ka = [H 3 O + ]b) Neutral redKa = antilog (-11.05)Ka = 8.9 x 10 -12(6.8 + 8.0)/2 = 7.4 Ka = [H 3 O + ]c) Methyl orangeKa = antilog (-7.4)Ka = 4.0 x 10 -8(3.2 + 4.4)/2 = 3.8 Ka = [H 3 O + ]

Ka = antilog (-3.8)Ka = 1.6 x 10 -44) The following data is collected, describe the original solutions in as much detail as thedata permits;a) Phenol red is red, thymol blue is green, and thymolphthalein remains colourless.Phenol Red pH > 8.0Thymol Blue 8.0 < pH < 9.6Thymolphthalein pH < 9.4Therefore; 8.0 < pH < 9.4b) Indigo carmine is blue, thymol blue is yellow and phenolphthalein appears pink.Indigo carmine pH < 11.4Thymol blue 2.8 < pH < 8.0Phenolphthalein pH > 8.2Therefore; pH ~ 8.0c) Orange IV is yellow, methyl violet is blue and bromcresol green appears yellow.Orange IV pH > 2.8Methyl Violet pH > 1.6bromcresol green pH < 3.8Therefore; 2.8 < pH < 3.85) Determine the colour each indicator would appear in each solution;a) Methyl orange b) Phenol red c) Thymolphthaleini) 0.500 M HClSince HCl is a strong acid, [H3O+] = 0.500 M so the pH = 0.30a) Methyl orange = redb) Phenol Red = yellowc) Thymolphthalein = colourless

ii) 2.00 M KOHSince KOH is a strong base, [OH-] = 2.00 M so the pOH = -0.30 and pH = 14.30a) Methyl orange = yellowb) Phenol Red = redc) Thymolphthalein = blueiii) 0.080 M HOAcSince HOAc is a weak acid, [H 3 O + ] is determined by an ICE table.a) HOAc (aq) + H 2 O (l) H 3 O + (aq) + OAc- (aq)I 0.080 M - - -C -x +x +xE 0.080 - x assume x

Kb = Kw = 1.0 x 10 -14 = [HF - ][OH - ] = 2.86 x 10 -11 = x 2 1.43 x 10 -13 = x 2Ka 3.5 x 10 -4 [F - ] 0.0050 [OH - ] = 3.78 x 10 -7 M = xa) Methyl orange = yellowb) Phenol Red = orangec) Thymolphthalein = colourlesspOH = -log[OH - ] = 6.42pH = 7.586) Determine the [H 3 O + ] in each solution;a) Alizarin yellow is orangeAY is orange means the pH is between 10.1 and 12.0b) Methyl violet is greenMV is green implies the pH is between 0.0 and 1.67) If neutral red was to be placed on your data booklet’s K a table (p #6), between whichtwo acids would it be located?Neutral Red has a range from 6.8-8.0. Assuming the midpoint between those twonumbers describes a situation with [HA] = [A - ], the pH of the midrange = 7.4 = pKaSince pH = pKa = 7.4[H 3 O + ] = antilog (-7.4) = 3.98 x 10 -8 M = KaA Ka of 3.98 x 10 -8 places this weak acid between Boric and thedihydrogen phosphate ion.8) Give two reasons why phenolphthalein is the indicator of choice for most acid basetitrations.Phenolphthalein changes colour at roughly pH = 8.2 which is the pH range for theendpoint (equivalence point) of a titration. Assuming strong acid/strong base, the pHwould be roughly 7.0, assuming strongbase/weak acid, the pH will be above 7.0 which iseven closer to the initial colour change of phenolphthalein.

When phenolphthalein changes colour it goes from colourless to pink. Changingfrom no colour to a colour is far easier to see than colour to colour (such as neutral red“amber-red”)Все еще не могу поверить что мне за это платят...