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last SG · 2015-08-19T22:54:00+00:00

Topic : atomic radius Hi! This is from singapore cambridge nov a level exam year 2010,Paper 1 qn 1Qn: which element has the largest atomic radius?BrKKrScSo...

Flying grenade SG

OMG ULTIMA SIBEI SMART

THANKS BOSS

Flying grenade SG

zai kia

Flying grenade SG

err.. so do we take Volume of KMnO4 to turn soln into a permanent pink color, minus 1cm3 ? since that 1 extra drop is in excess

UltimaOnline SG

err.. so do we take Volume of KMnO4 to turn soln into a permanent pink color, minus 1cm3 ? since that 1 extra drop is in excess

No. Becoz in titration, no matter how many drops (and each drop isn't exactly 1cm3 in the first place) you use to reach endpoint, the moles of titrant used will always technically be either slightly before, or slightly after, the equivalence point anyway.

So if you minus 1 drop, your moles of titrant used will be slightly before equivalence point. If you don't minus 1 drop, your moles of titrant used will be slightly after equivalence point. Since either way it'll be slightly inaccurate (can't be helped), so it makes more sense to just use the titre volume to reach endpoint, without further complicating errors by minusing away 1 drop.

Now you understand why we call this 'endpoint' instead of 'equivalence point'? Endpoint merely approximates equivalence point.

Flying grenade SG

wait.

the answer provided by sean chua(SAP publisher) is wrong .

2013/p3/2cii

KMnO4 is added from burette.

solution is colorless. end point is detected when one extra drop of KMnO4 turns the solution a permanent pink color soln.

Flying grenade SG

it doesn't make sense if purple,(high conc mno4-) (implies kmno4 is inside the conical flask, and is being titrated onto) -> colorless(all mno4- reacted) -> one extra drop MnO4-(pale pink)

Flying grenade SG

Now you understand why we call this 'endpoint' instead of 'equivalence point'? Endpoint merely approximates equivalence point.

i see! thanks god !

Flying grenade SG

this is sibei confusing.

in the first place, small concentration of MnO4- is pale purple right? (pale pink not accepted?)

Mn2+ is pale pink in color.

if Mn2+ is in low concentrations, almost colorless, very faint pink

also, don't have titration whereby we add Mn2+ ions. only MnO4-.

Flying grenade SG

i cant think of any scenarios whereby add mno4- into a colorless soln, and the color change is purple->colorless-> pale pink/purple(with extra drop of MnO4-)

except u have dilute soln of KMnO4. add lots of water, turn colorless. add one drop, color of soln perhaps turn pale pink

Flying grenade SG

i got it alr Ultima. no need address this liao. thanks for your previous replies.

the only answer for 2013/3/2cii is just first permanent pale pink coloration when 1 excess drop of KMnO4 is added, thats all.

Decolorisation of purple to colorless is wrong.

once u add kmno4 into soln while stirring, purple immediately decolorise

Flying grenade SG

N11/p1/26

is it possible for 1,2-butandiol to exist(and with only one chiral centre )

oxidised to 1-hydroxybut-2-one or 2-hydroxybutan-1-al

oxidised to butan-1,2-dione ? or is it too unstable

sorry my blunder.

butan-1,2-dione doesn't exist.

only 2-oxobutan-1-al (aldehyde and ketone in the same molecule)

Flying grenade SG

can tertiary alcohol undergo elimination to give alkene?

both this sources say can , but nvr show the mechanism of the final alkene cpd , i dk and cannot visualise

http://chem.libretexts.org/Core/Organic_Chemistry/Hydrocarbons/Alkenes/Synthesis_of_Alkenes/Alkenes_by_Dehydration_of_Alcohols#Secondary_and_tertiary_alcohols_dehydrate_through_the_E1_mechanism

http://www.masterorganicchemistry.com/2015/04/16/elimination-reactions-of-alcohols/

Flying grenade SG

e.g. N07/1/27

was thinking can (CH3)3COH undergo elimination rxn to give alkene?

Flying grenade SG

ok realized it can. form (CH3)2C=CH2

UltimaOnline SG

can tertiary alcohol undergo elimination to give alkene?

both this sources say can , but nvr show the mechanism of the final alkene cpd , i dk and cannot visualise

http://chem.libretexts.org/Core/Organic_Chemistry/Hydrocarbons/Alkenes/Synthesis_of_Alkenes/Alkenes_by_Dehydration_of_Alcohols#Secondary_and_tertiary_alcohols_dehydrate_through_the_E1_mechanism

http://www.masterorganicchemistry.com/2015/04/16/elimination-reactions-of-alcohols/

U coconaden. Some tertiary alcohols can kena dehydrated into alkenes, some cannot. It's *not* a matter of whether it's a pri or sec or tert alcohol, rather it's a matter of availability of beta protons to be abstracted by the Bronsted-Lowry base in the E1 or E2 mechanism.

Figure out the mechanism for yourself, or go ask your school teacher or private tutor.

Flying grenade SG

2014 p1 qn 22

important theory qn

for alkaline hydrolysis of amide, let's say for example, there are 4 amide linkages in a cpd. is it 4 moles of NaOH will react with the cpd?

or is it 8 mol of NaOH? Angeline Tan and Jasmine Wong writes NaOH will react with the COOH formed from the alkaline hydrolysis of amides.

is it straight away will from COO- liao instead of COOH then COO- , as suggested by Angeline Tan and Jasmine Wong?

i havent encountered this in all books and notes so far, except in the solution written by A&J. seems like they are the first few who thought of this

UltimaOnline SG

2014 p1 qn 22

important theory qn

for alkaline hydrolysis of amide, let's say for example, there are 4 amide linkages in a cpd. is it 4 moles of NaOH will react with the cpd?

or is it 8 mol of NaOH? Angeline and jasmine writes NaOH will react with the COOH formed from the alkaline hydrolysis of amides.

is it straight away will from COO- liao instead of COOH then COO- , as suggested by Angeline and Jasmine?

i havent encountered this in all books and notes so far, except in the solution written by A&J. seems like they are the first few who thought of this

"Angeline and jasmine writes..."

Wah seh, intimate first name basis leh, machiam they ur ex-girlfriends liddat...

Instead of spoon-feeding you the answer (and I don't like it when you openly criticize/point out the author's typos on the forum, even though in your mind you think you're doing them a favor by pointing out their typos ; or worse, when you ask *me* to publicly confirm who's right who's wrong, eg. Chan Kim Seng vs George Chong vs CS Toh etc), so I'll leave it to yourself to solve this problem, quite easily done, by drawing out the hydrolysis mechanism (try drawing both acidic vs alkaline mechanisms), and you'll get your answer.

And don't post the answer here on the forum (as you are wont to do). Anyone else interested can go draw out the mechanisms for themselves to arrive at the answer.

Flying grenade SG

idk if its typo

i am asking becos i dunno the chemistry concept, not becos i am questioning their typo

im not and never my intention to criticise authors. my words never show that. i am asking qn. but if other people interpret in that way, let them be. i never write anything offending, and my words not against law

UltimaOnline SG

idk if its typo

i am asking becos i dunno the chemistry concept, not becos i am questioning their typo

Fair enough, I meant you "criticize/point out the author's typos", rather than criticize the authors themselves (I'll go edit my previous posts to be fair to you).

Regardless, like everyone else who asks me Chem qns here on the forum, I'd rather you focus on asking about TYS or Prelim Papers qns, instead of comparing notes and books and narrowly zooming in on nitty-gritty details and OCDing over who's right who's wrong (which you can actually settle yourself by cross referencing and applying common, ok, chemistry sense).

Flying grenade SG

OCD ppl like me like to learn true knowledge, after learning many wrong concepts/knowledge/things all my life(i dont blame anyone) , I am Tired of learning false and wrong concepts, i only desire to learn the Truth.

i am curious, i desire to understand the Sciences, i work extremely hard cross reference excessively, think, analyse, and then if truly in need, seek help and the Truth from one of the rare and true teachers, Ultima.

self discovery and learning and independent learning is supremely important (must put in effort think critically yourself first before asking) , with the guidance from extremely desired,sought after and Trustworthy, knowledgeable, credible educators like BFJC

yes i truely thank all the authors who write these books to help me gain knowledge ,and i love all these books they wrote, they are great books i treasure them

Love the Sciences , work hard to learn the Truth(extremely important)

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