NiCKEL Posted November 5, 2015 Report Share Posted November 5, 2015 Based on a table of Indicators, Acids, and their colours created, how do you determine what is the strongest acid?The row headers are a selection of general acids HA1, HA2, HA... and the column headers are a selection of general indicators in their acid form, Ind.1, Ind.2... I don't need the exact answer so I didn't provide the exact question but how do you compare the strengths when the acid solution and the indicator is in the same comparing "batch"? I would compare pKa of indicators if I was just looking at indicators, but there's no numbers at play here. How do I approach the question (hopefully this is enough info)? Just a hint to get me going should be good Reply Link to post Share on other sites More sharing options...
kw0573 Posted November 5, 2015 Report Share Posted November 5, 2015 (edited) 1) Each buffer/indicator only works within 1pH difference from its pKa2) pKa + pKb = 14, or greater the pKa, lower the pKb.If the table is talking about titration with strong base, then the pH at equivalence point can be calculated using the pKa.Greater pKa means weak acid dissociation or strong base behaviour for the conjugate, means there would be more OH- produced, and there would be a higher than 7 pH.So if one such titration's equivalence point falls into range for phenolphthalein (pKa = 9.3) vs another falls within bromothymol blue (pkA = 7.3) then the phenolphthalein one is suitable for a relatively weaker acid (stronger base behaviour for the conjugate) while the bromothymol blue is suitable for a strong or relatively strong acid. EDIT:You would have the databooklet for Paper 2 and 3 so you would have the pKa values available. I would assume IB will not ask these type of questions in Paper 1. Is the new syllabus allowing data booklet in Paper 1? I am not sure. Edited November 5, 2015 by kw0573 Reply Link to post Share on other sites More sharing options...
NiCKEL Posted November 5, 2015 Author Report Share Posted November 5, 2015 1) Each buffer/indicator only works within 1pH difference from its pKa2) pKa + pKb = 14, or greater the pKa, lower the pKb.If the table is talking about titration with strong base, then the pH at equivalence point can be calculated using the pKa.Greater pKa means weak acid dissociation or strong base behaviour for the conjugate, means there would be more OH- produced, and there would be a higher than 7 pH.So if one such titration's equivalence point falls into range for phenolphthalein (pKa = 9.3) vs another falls within bromothymol blue (pkA = 7.3) then the phenolphthalein one is suitable for a relatively weaker acid (stronger base behaviour for the conjugate) while the bromothymol blue is suitable for a strong or relatively strong acid. EDIT:You would have the data booklet for Paper 2 and 3 so you would have the pKa values available. I would assume IB will not ask these type of questions in Paper 1. Is the new syllabus allowing data booklet in Paper 1? I am not sure. Thanks for the reply, but I'll try to replicate the question because I don't think I successfully asked the question I meant to ask and it's my fault probably nobody will answer me. The question is 39 Analyze the table below and then answer the questions.c) Of HIn1, HIn2, HA1, HA2, which has the strongest conjugate base? d) Of HIn1, HIn2, HA1, HA2, which is the strongest acid? | NaOH | HA1 | HA2 | HCl |_______|__________|________|_________|________|HIn1 | purple | purple | purple |colourless|HIn2 | yellow | yellow | blue | blue | Reply Link to post Share on other sites More sharing options...
kw0573 Posted November 5, 2015 Report Share Posted November 5, 2015 1) Each buffer/indicator only works within 1pH difference from its pKa2) pKa + pKb = 14, or greater the pKa, lower the pKb.If the table is talking about titration with strong base, then the pH at equivalence point can be calculated using the pKa.Greater pKa means weak acid dissociation or strong base behaviour for the conjugate, means there would be more OH- produced, and there would be a higher than 7 pH.So if one such titration's equivalence point falls into range for phenolphthalein (pKa = 9.3) vs another falls within bromothymol blue (pkA = 7.3) then the phenolphthalein one is suitable for a relatively weaker acid (stronger base behaviour for the conjugate) while the bromothymol blue is suitable for a strong or relatively strong acid. EDIT:You would have the data booklet for Paper 2 and 3 so you would have the pKa values available. I would assume IB will not ask these type of questions in Paper 1. Is the new syllabus allowing data booklet in Paper 1? I am not sure. Thanks for the reply, but I'll try to replicate the question because I don't think I successfully asked the question I meant to ask and it's my fault probably nobody will answer me. The question is 39 Analyze the table below and then answer the questions.c) Of HIn1, HIn2, HA1, HA2, which has the strongest conjugate base? d) Of HIn1, HIn2, HA1, HA2, which is the strongest acid? | NaOH | HA1 | HA2 | HCl |_______|__________|________|_________|________|HIn1 | purple | purple | purple |colourless|HIn2 | yellow | yellow | blue | blue | Are you given any information on molarity or temperature or identity of the acids including indicators?Because this table does not make any sense. Reply Link to post Share on other sites More sharing options...
NiCKEL Posted November 5, 2015 Author Report Share Posted November 5, 2015 1) Each buffer/indicator only works within 1pH difference from its pKa2) pKa + pKb = 14, or greater the pKa, lower the pKb.If the table is talking about titration with strong base, then the pH at equivalence point can be calculated using the pKa.Greater pKa means weak acid dissociation or strong base behaviour for the conjugate, means there would be more OH- produced, and there would be a higher than 7 pH.So if one such titration's equivalence point falls into range for phenolphthalein (pKa = 9.3) vs another falls within bromothymol blue (pkA = 7.3) then the phenolphthalein one is suitable for a relatively weaker acid (stronger base behaviour for the conjugate) while the bromothymol blue is suitable for a strong or relatively strong acid. EDIT:You would have the data booklet for Paper 2 and 3 so you would have the pKa values available. I would assume IB will not ask these type of questions in Paper 1. Is the new syllabus allowing data booklet in Paper 1? I am not sure. Thanks for the reply, but I'll try to replicate the question because I don't think I successfully asked the question I meant to ask and it's my fault probably nobody will answer me. The question is 39 Analyze the table below and then answer the questions.c) Of HIn1, HIn2, HA1, HA2, which has the strongest conjugate base? d) Of HIn1, HIn2, HA1, HA2, which is the strongest acid? | NaOH | HA1 | HA2 | HCl |_______|__________|________|_________|________|HIn1 | purple | purple | purple |colourless|HIn2 | yellow | yellow | blue | blue | Are you given any information on molarity or temperature or identity of the acids including indicators?Because this table does not make any sense. Nope. We are supposed to deduce which acid HA1/HA2 is stronger based on the fact that it didn't change colour of HIn1 yet. Our answer only needs to be "X is stronger", not "pKa=" or any sort of number answer. I guess I'll delete this though, because my test is today ^.^ Reply Link to post Share on other sites More sharing options...
kw0573 Posted November 5, 2015 Report Share Posted November 5, 2015 (edited) HiI assume NaOH, Ha1, Ha2 and HCl all at same concentration.Recall that indicators and buffers only work with ± 1 pH grading.from row 1HInA has pKa range above 7 (possibly 13, 14 ish). because NaOH makes solution in the ±1 range of the pKa.2 conclusions made herea) HIn1 is a very very weak acid with pKa 13, 14 ish.b) HA1 and HA2 has produce strong conjugate bases to produce pH around that produced by NaOH. from row 2HIn 2 caused a split in the colours. It has (at least) two pKas. the lower one turned blue in strong acid HCl. It must have a low pKa1.Similarily HA2 also have a low pKa1. Both Ha2 and HIn 2 are diprotic (or more).There for HA2 is a stronger acid than HA1, looking at first protonations. HA2 gives an strong acid solution. HA1 gives strong basic solution. The second protonation of HA2 gives strong basic solution, but that is quite insignificant to its pKa1Best luck on your test! Edited November 6, 2015 by kw0573 Reply Link to post Share on other sites More sharing options...
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