Chemistry: Post your doubts here!

[A n g e l a ♥]

can anyone please explain 1f http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_s10_qp_34.pdf

 

[iKhaled]

i did but wht i actually want to know is that why doesn't the colour change from green to red-brown??? y is it changing to colourless or pale yellow???

idk man thats how i see it when i did the titration and then when u add excess of KMno4 it goes completely dark purple

 

[Nab900]

idk man thats how i see it when i did the titration and then when u add excess of KMno4 it goes completely dark purple

aha

 

[Nab900]

and wht abt 4(c)?? how to do tht???

 

[knowitall10]

Allah Yawaffagkum!!!
Good Luck for your Chemistry P4 tomorrow!!!
May Allah Succeed us all!!!!!!!!!!!!!!!!!!!!

 

[raamish]

hey guys can anybody tell me the different purposes of glc, mass spectrometry, nmr, hplc and tlc

 

[kalpaveen]

can anyone give me a link for some good revision notes of section B applications.....

 

[geek101]

Allah Yawaffagkum!!!
Good Luck for your Chemistry P4 tomorrow!!!
May Allah Succeed us all!!!!!!!!!!!!!!!!!!!!

good luck to you tooo and in sha Allah

 

[biba]

hey guys can anybody tell me the different purposes of glc, mass spectrometry, nmr, hplc and tlc

NMR is used to obtain the information about the number of hydrogen atoms and their chemical environment (to predict the structure of organic molecules ).
MASS SPECTROMETER is used to predict the relative atomic mass of the element,the different isotopes of the element and the relative abundance of the isotopes..
All chromatography techniques are used separate the different components in a copmound by using a mobile phase and a stationary phase..
but in each technique better mobile and stationary phases are used to make the results more reproducible and the separation more efficient.
TLC is similar to paper chromatography (PC) it has the same mobile phase (a polar solvent) as PC but a different stationary phase (slurry of silica or Al2O3),but the separation in TLC is better than that of PC because of the small size of the particles of stationary phase.
GLC uses an inert gas (N2 or Ne) as the mobile phase whereas stationary phase is inert porous diatomaceous rock which is packed in a glass column of narrow diameter (2mm), again the separation in GLC is better than TLC as the narrow diameter ensures the components to b in close contact with both stationary and moblie phase that increases the efficiency of separation... GLC is used to detect drugs in blood.
HPLC uses a high purity solvent as mobile phase that is puped under high pressures in the HPLC instrument , the stationary phase are the porous silica beads packed in a glass column of a narrow diameter, the results of HPLC is better than GLC... HPLC is used in the detection of explosive residues on the skin, it is used in the detection of pesticides in vegetables and fruits!

 

[Nab900]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s10_qp_43.pdf

Can Someone please explain me Question 4b (i) and 4(c)

 

[gary221]

I requote:

the whole ques?

 

[gary221]

No, just part e iv.

gary221 thanks in advance

okay, first off, find no of moles of NaOH → conc = moles/vol
moles of NaOH = 0.100 * (10 * 10^-3) = 0.001 moles

Moles of CH3CO2H = 0.250 * (10 * 10^-3) = 0.0025 moles
So, clearly, moles of CH3CO2H are in excess.
So, 0.001 moles of NaOH will react with 0.001 moles of CH3CO2H, leaving (0.0025-0.001) = 0.0015 moles of acid in the solution
Now, total volume of solution = 0.02 dm^3

So, conc of acid in soln = moles/vol
conc = 0.0015/0.02 = 0.075 mol/dm^3

Now, substiute values in pH = pKa + log ([salt]/[acid])
conc of salt → moles of salt = moles of NaOH
vol of soln = 0.02 dm^3
conc = 0.001/0.02 = 0.05 mol/dm^3

Hope u gt it!

 

[biba]

in 4b (i) we have FeSO4 which has a pale green colour due to Fe^2+ ions... when we start adding KMnO4 (oxidising agent) to FeSO4 the pale green colur starts fading away,the solution turns very pale yellow (almost colourless) because the Fe2+ ions are getting oxidised into Fe3+.. and Fe3+ are yellow coloured when u add more KMnO4 the sloution turns pale pink as the MnO4^- ions are reduced to Mn2+ and Mn2+ ions are pink in colour... but when excess of KMnO4 is added the solution will turn purple because now there are many MnO4^- ions and MnO4- ions are purple in colour!
in 4(c) they said Fe2+ ions (acidified) are stable to oxidation in air.. u open the data booklet and pick ub the equation for Fe2+ in which it is being oxidised to Fe3+ this equation had E* value +0.77.. now u pick up the equation of O2 which is in acidic medium i.e it has H+ ions in it this equation indata booklet has E* value of +1.23.. the reaction occurs between these two equation and E* becums (+1.23)-(0.77)=+0.46V...
in its second part u pick up an equation in which we have Fe(OH)2 and Fe(OH)3.. this equation has E* of -0.56V , now pick up an equation under ALKALINE medium for O2 this one has E* of =0.40.. the reaction occurs between these so the overall E* becums (+0.40)-(-0.56)= +0.96V.. now u comment that the oxidation of Fe(OH)2 is rapid because it has a more positive E* value (remember the more positive the E8 value, the more feasible the reaction is )
​

 

[Oliveme]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w11_qp_43.pdf

Its a Ka question:
Q no 3 (e) part iv

Can you please help me out.

I know it has used the formula, pH=pKa + log (salt/acid)
but how the values?

I'll be honest here - this took me a long while to work out, too.

Let's first biuld an equation = CH3COOH + NaOH ------> CH3C00Na + H2O
Moles of acid = 10/1000 x 0.25 = 0.0025 moles
Moles of NaOH = 10/1000 x 0.1 = 0.001 moles
From the equation, the molar ratio should be 1:1 so the acid is clearly in excess.
the total volume of solution is 20 cm^3, right? Because we add 10 and 10.
so moles of salt CH3COOH are 0.001 and so concentration of salt is -----> 0.001/0.02 = 0.05 mol dm^3
The moles of unreacted acid are 0.0025 - 0.001 = 0.0015 moles
so conc. of unreacted acid = 0.0015/0.02 = 0.075 mol dm^3
Plug the values into pH = pKa + log ([acid]/[salt])
this gives ----> pH = 4.76 + log (0.05/0.075) = 4.58

Hope you get it.

 

[messi10]

Wow, that was really tricky! :/ Would have never thought of that.

Bro, in the same paper, how you found the rate constant in Q2 (b) part iii..


Thank You so much guys for helping.. You are saviour!

 

[D0cEngi]

Plz tell me how to balance an ionic equation.

 

[Nab900]

in 4b (i) we have FeSO4 which has a pale green colour due to Fe^2+ ions... when we start adding KMnO4 (oxidising agent) to FeSO4 the pale green colur starts fading away,the solution turns very pale yellow (almost colourless) because the Fe2+ ions are getting oxidised into Fe3+.. and Fe3+ are yellow coloured when u add more KMnO4 the sloution turns pale pink as the MnO4^- ions are reduced to Mn2+ and Mn2+ ions are pink in colour... but when excess of KMnO4 is added the solution will turn purple because now there are many MnO4^- ions and MnO4- ions are purple in colour!​

in 4(c) they said Fe2+ ions (acidified) are stable to oxidation in air.. u open the data booklet and pick ub the equation for Fe2+ in which it is being oxidised to Fe3+ this equation had E* value +0.77.. now u pick up the equation of O2 which is in acidic medium i.e it has H+ ions in it this equation indata booklet has E* value of +1.23.. the reaction occurs between these two equation and E* becums (+1.23)-(0.77)=+0.46V...​

in its second part u pick up an equation in which we have Fe(OH)2 and Fe(OH)3.. this equation has E* of -0.56V , now pick up an equation under ALKALINE medium for O2 this one has E* of =0.40.. the reaction occurs between these so the overall E* becums (+0.40)-(-0.56)= +0.96V.. now u comment that the oxidation of Fe(OH)2 is rapid because it has a more positive E* value (remember the more positive the E8 value, the more feasible the reaction is )​

​

So Fe3+ ions are basically yellow in colour??? aren't fe3+ red-brown in colour

and for part(c), can you please explain why are we taking acidic and alkaline medium into consideration :/

Anyway, THNKS for ur time

 

[biba]

So Fe3+ ions are basically yellow in colour??? aren't fe3+ red-brown in colour

and for part(c), can you please explain why are we taking acidic and alkaline medium into consideration :/

Anyway, THNKS for ur time

wel its wriiten in chemistry book that in aqueous solutions Fe3+ is yellow..
we have to take the acidic and alakine conditions into consideration because they have specified it in the question, if u donot take the conditions into consideration u wil get stuck in the question because there are 2 different equations for O2 in th data booklet

 

[nidanas]

umm hello ppl!
I wanna ask is it true dat the duration of d chem paper for tomorrow will be 2 hours?!
I mean I jst noticed dat 2011 and 2012 ques paper wz 2 hrs long... so any idea abt 2mrw' paper?!

 

[biba]

umm hello ppl!
I wanna ask is it true dat the duration of d chem paper for tomorrow will be 2 hours?!
I mean I jst noticed dat 2011 and 2012 ques paper wz 2 hrs long... so any idea abt 2mrw' paper?!

yup it is 2 hours!