Q7 MC from 2010 HSC paper.... whats the correct answer!! (1 Viewer)

[y510920]

Equal volumes of four 0.1 mol L−1 acids were titrated with the same sodium hydroxide solution.
Which one requires the greatest volume of base to change the colour of the indicator?
(A) Citric acid
(B) Acetic acid
(C) Sulfuric acid
(D) Hydrochloric acid

Well, i would say C, becoz A and B are weak acid. C and D are strong acid. sulfuric acid is diprotic so it should be a stronger acid which means it will require more (strong)base to change the colour of the indicator. but then the suggested answer in BOS is A, which is citric acid. but...WHY???HOW COME???? anyone? thanks

 

[concon]

firstly the equivalent point is reached when equal moles of H+ and OH- are been added together. citric acid is a triprotic acid(c6h8o7) which means it is able to produce 3 moles of H+ ions.. therefore it requires the most volume of base...

 

[concon]

equivalence*

 

[Bobbo1]

true, in the past board of studies has said that hcl is more acidic than citric acid, and therefore sulfuric acid must be more acidic than citric...
For e.g. see q 24 from the 2004 hsc: http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2004exams/pdf_doc/chemistry_04.pdf

can someone clear this confusion up or is it just board of studies up to its normal confusion again??

 

[roryclifford]

Hahaha I no joke came on Bored Of Studies to post this question! I jumped straight away to HCl, and was shocked by the Citric answer.

Hope someone can explain it!

 

[kr73114]

write out the balanced equations for each and calculate the volume required from these mole ratios. U'll find that the strength of the acid has nothing to do with how much volume is required and it is purely the mole ratio. Since citric acid has a mole ratio of 1:3 it requires the most volume of base.

 

[roryclifford]

write out the balanced equations for each and calculate the volume required from these mole ratios. U'll find that the strength of the acid has nothing to do with how much volume is required and it is purely the mole ratio. Since citric acid has a mole ratio of 1:3 it requires the most volume of base.

Ohhh, I see. Cheers, I understand

 

[Bobbo1]

write out the balanced equations for each and calculate the volume required from these mole ratios. U'll find that the strength of the acid has nothing to do with how much volume is required and it is purely the mole ratio. Since citric acid has a mole ratio of 1:3 it requires the most volume of base.

aha, learn something today

 

[hscishard]

write out the balanced equations for each and calculate the volume required from these mole ratios. U'll find that the strength of the acid has nothing to do with how much volume is required and it is purely the mole ratio. Since citric acid has a mole ratio of 1:3 it requires the most volume of base.

That's using the fact that every mole of citirc will produce 3 moles of hydronium ions, which doesn't happen

 

[kr73114]

my answer is still right

 

[zhiying]

Strength of the acid doesn't effect the volume needed, I think concentration does, which is why they said they were of the same concentration. So yes citric acid would be the answer since its triprotic

 

[b3kh1t]

yes although citric acid does not ionise completely; ie 100%, whilst sulfuric acid completely ionises (just for your information, in the core sulfuric acid is known to ionise 100%, however only the first ionisation goes to completion and the second ionisation []), this is a neutralisation reaction meaning the will form with the to form water. This means the will be removed, forcing more and more of the acid will ionise and form more , until all the acids have become ionised. Therefore the one that will be able to donate the most protons will be the one that will need the most volume, thus citric acid will be the answer as it is triprotic (gives 3 protons per molecule), while sulfuric acid is diprotic (gives 2 protons per molecule), and hydrochloric acid and acetic acid is monoprotic (gives 1 proton per molecule)