Chemistry: Post your doubts here!

[sameen saeed]

can anyone tell me that in AS practical what does permanent colour change refer to ?
does it refer to first appearance of colour change or we have to continue to add titre to see any change.

 

[Namehere]

can anyone tell me that in AS practical what does permanent colour change refer to ?
does it refer to first appearance of colour change or we have to continue to add titre to see any change.

Permanent colour, just like the name suggests. So it would be the colour of the solution for which it does not change anymore.

 

[A star]

can anyone tell me that in AS practical what does permanent colour change refer to ?
does it refer to first appearance of colour change or we have to continue to add titre to see any change.

there is a seperate threads that creep up for the practical see them and one piece of advice I can give what ever you do keep calm cause in 1 hour 30 min there is no recovery time

 

[sameen saeed]

Permanent colour, just like the name suggests. So it would be the colour of the solution for which it does not change anymore.

sometimes it happens that the changed colour which seems to permanent disappears after some time then what to do?????consider that permanent or keep adding titre

 

[periyasamy]

Nah don´t get it I seem to get part iii, but now I dont understand what u mean with part ii.

K.Let me think agn.Sorry.

 

[goodluckayesha]

how much marks do we need to score to get an A in Chem P1 and P2?

abt 30 in ppr 1 and abt 48-50 in ppr 2 this way ull get an A easily and in A2 it will be easier to get an A*

 

[goodluckayesha]

How did you come to that conclusion , what i know about pka that its given by -log ka .. why pka is ph of buffer?

actually my teacher said so to assume so by tht i told so becoz in pst pprs 2-3 similar questions use the same assumption

 

[NinjaInPyjamas]

These are factual answers they dont really have any explanation

yeah, I wanted the factual answers. what reaction is it, and how to complete the reactions.

 

[chocolate_girl]

there is a seperate threads that creep up for the practical see them and one piece of advice I can give what ever you do keep calm cause in 1 hour 30 min there is no recovery time

Its 2 hours in practical

 

[chocolate_girl]

a free radical has one unpaired valence electron which gives it high reactivity.
to make CaSO4 from Ca(NO3)2, you will need a soluble (otherwise no reaction will occur as the salt will not dissolve) salt that has a metal(X) cation that can displace Ca to form XNO3 and a sulfate anion to replace NO3
Sodium Sulphate would do the job.
again the same kind of question. any soluble sulfate salt would do the job. sodium sulfate is, i am sure, soluble.
we have studied in hydroxy compounds chapter that -OH can be replaced by a halogen with help of PCl3 or PCL5 or red phosphorus+halogen(I2, Br2 etc) + acid + heat
that will replace OH group.
CH3(CH2)4CH(Cl)CH=CHCHO

Isnt the salt in second one CaCl as they have referred to reaction 1 and formation of CaCl is reaction 1
Also in the last one why dont we add Cl in double bonds?

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w10_qp_23.pdf in Q3 a-ii) how do we know what is the shape?

 

[chocolate_girl]

yeah, I wanted the factual answers. what reaction is it, and how to complete the reactions.

The mark scheme would say it all

 

[AbbbbY]

These are factual answers they dont really have any explanation

Everything, my dear Watson, has a logical explanation.

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w13_qp_21.pdf

q4 (c) (d) (e) can someone explain in detail please?

c)
S into T is hydration so I'd use steam + phosphoric acid catalyst @ 300C
S into U is di-ol so Cold Dilute Acidified KMnO4 in a heartbeat.
T into S is dehydration so I'd use conc H2SO4 @ 180C [Can also use H3PO4 @ 300C but that's not recommended]

T with Na (excess).
Replace all the OH's with O-+Na
NaOOCCH(ONA)CH2COONa

U with Na2CO3 (excess)
Replace the carboxylic OH's with O-+Na but not the alcohol OH's.

NaOOCCH(OH)CH(OH)COONa

Add the +'s and -'s on top. Was making it look all weird in typed format since I can't superscript it here.

e)
It's merely asking for cis trans isomers.

cis will be both COOH's on the same side of the two carbons and trans will be diagnol/opposite sides of the two carbons. If this doesn't make sense let me know and I'll draw it out for you.

why this sequence

Don't you know how to make protein chains from tri-peptides? You gotta join the chain in such a way that it forms the shortest chain with each tripeptide being used. You've to basically connect the dots. See where they overlap.

gly-ala-gly
ala-gly-ser
ser-val-lys

I'm taking these three as a random self created example. I state that the N terminal is glycine.

So, gly-ala-gly to begin with.
Now I'll see if I have tri-peptides starting with ala-gly or just gly and it's basically hit-and-trial and some logical thinking after this. Ofcourse, I don't have to repeat the common aminoacids. Like gly-ala-gly, second tripeptide is ala-gly-ser but I already have ala-gly ending the first tripeptide so my polypeptide with these 2 would be gly-ala-gly-ser

so, gly-ala-gly-ser-val-lys would be my polypeptide.

Soo sorry for the crappy explanation. I know I can do better because I've taught a lot of people how to do this but that involved a paper + vocal communication.

 

[AbbbbY]

okay.I see some things in chemistry are not very logic based

Wrong! Everything in every science is fully logic based. We, students, tend to ignore the logic in finding shortcuts/ways things are easy for US to remember/explain.

 

[AbbbbY]

I know its because of the size.But why? :/

Two words: Steric Hindrance!

 

[kitkat <3 :P]

Two words: Steric Hindrance!

Whats that ??

 

[Suchal Riaz]

Isnt the salt in second one CaCl as they have referred to reaction 1 and formation of CaCl is reaction 1
Also in the last one why dont we add Cl in double bonds?

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_23.pdf in Q3 a-ii) how do we know what is the shape?

why would we add cl in double bonds?
Cl will add in double bonds as result of electrophilic addition. whereas the mechanism of substitution is very different.
there are two carbons and one oxygen. the carbons has four electrons available for bonding. oxygen has 2 electrons available for bonding. the sequence is Carbon-Carbon-Oxygen
so carbon C=C=O will be most feasible structure.

as the chain is straight the shape is linear.

 

[Suchal Riaz]

Whats that ??

in simple words atoms take some space. so for them to bond there must be enough space. fluorine is very small so not more than one atom can bond with. as the two atoms will have to be very close to each other to bond with fluorine at same time.
other explanation might be that because the Fluorine atom is very small because the atom outer shell is very small. so not it cannot allow two more electrons in it. the repulsion of electrons will be very large.

 

[kitkat <3 :P]

in simple words atoms take some space. so for them to bond there must be enough space. fluorine is very small so not more than one atom can bond with. as the two atoms will have to be very close to each other to bond with fluorine at same time.
other explanation might be that because the Fluorine atom is very small because the atom outer shell is very small. so not it cannot allow two more electrons in it. the repulsion of electrons will be very large.

Thunkooo ^_^

 

[Marwaa]

can anyone tell me what the limits of the threshold are? (minimum and maximum mark required to get an A) in as level

 

[daredevil]

can anyone tell me what the limits of the threshold are? (minimum and maximum mark required to get an A) in as level

A levels has a percentile system. the threshold is decided ever year separately accordingly to the results