CHEMISTRY PAPER 3 DOUBTSS!!

[techgeek]

Guys, how do we distinguish between hydrogen and oxygen with our observations? Like i doubt i'll have time to carry out the actual tests so like how will i know which gas it is? If like OH then what would it be?

The sequence of gas tests? We can distinguish ammonia due to its smell but, how about the others?

guys it depends on the reagents you are using... Since the question paper guides us thoroughly, for instance, if we are asked to use Al foil and OH- then we can expect NH3 only, no other gas gets liberated with Al and OH-. If we are using dilute acids, then it could either be CO2 or SO2, if you are using dilute acids, keep a strip of filter paper moist with potassium manganate (VII), place the bung on test tube and wait for gas to collect, then test for SO2, if it's negative, you can immediately put the delivery tube of limewater, I think we get that ready-made. NO is also liberated by dilute acids, but since it immediately oxidizes to NO2, the brown color is evident.
And O2 is given off when we use H2O2 and H2 if we are using metal+ acid.

 

[techgeek]

There's this test which includes adding acidified KMnO4(aq) followed by aqueous starch. Does anyone know the purpose for this?

Can you quote the particular question? Because starch may have some purpose in that particular instance but if manganate ions are used, and the solution turns from purple to colorless, then it's because of sulfite ions. Basically the sulfite ions get oxidized to sulfate ions because potassium manganate (VII) is an oxidizing agent, it will oxidize the sulfite and itself get reduced, when it gets reduced, it decolorizes, so it indicates that sulfite is the anion present.

 

[amina1300]

guys it depends on the reagents you are using... Since the question paper guides us thoroughly, for instance, if we are asked to use Al foil and OH- then we can expect NH3 only, no other gas gets liberated with Al and OH-. If we are using dilute acids, then it could either be CO2 or SO2, if you are using dilute acids, keep a strip of filter paper moist with potassium manganate (VII), place the bung on test tube and wait for gas to collect, then test for SO2, if it's negative, you can immediately put the delivery tube of limewater, I think we get that ready-made. NO is also liberated by dilute acids, but since it immediately oxidizes to NO2, the brown color is evident.
And O2 is given off when we use H2O2 and H2 if we are using metal+ acid.

Wow thats really helpful thankyouuuuuuuuuuu!!!!

 

[amina1300]

I

Can you quote the particular question? Because starch may have some purpose in that particular instance but if manganate ions are used, and the solution turns from purple to colorless, then it's because of sulfite ions. Basically the sulfite ions get oxidized to sulfate ions because potassium manganate (VII) is an oxidizing agent, it will oxidize the sulfite and itself get reduced, when it gets reduced, it decolorizes, so it indicates that sulfite is the anion present.

Realised that it was being used as a test for the presence of Iodine. ( color change to blue-black on addition of starch)

 

[amina1300]

Do we add the rough titre in the table ?

 

[Zaki ali asghar]

Do we add the rough titre in the table ?

No.

 

[Zaki ali asghar]

Who is gonna come up with predictions

 

[amina1300]

No.

why not

Is this fine?

 

[amina1300]

Who is gonna come up with predictions

Some say titration isnt coming doubt that but, who knows and something with organic chemistry DNPH or fehlings solutions but Im not sure :/

 

[amina1300]

Can someone tell me the amount of time I should spend on each question.

 

[Zaki ali asghar]

I think no more than 25 minutes on titration,45 minutes on salt analysis.45 mins on Q2.

 

[Zaki ali asghar]

Some say titration isnt coming doubt that but, who knows and something with organic chemistry DNPH or fehlings solutions but Im not sure :/

Organic came the last whole year my guess is that it wont be coming this session probably

 

[Cherry lava]

Organic came the last whole year my guess is that it wont be coming this session probably

Exactly

 

[Zaki ali asghar]

i am not sure but in Q3 the cations are probably going to be Mg2+,Mn2+,Al3+
Anions=Sulphate and Chloride.

 

[Roe Mir]

i am not sure but in Q3 the cations are probably going to be Mg2+,Mn2+,Al3+
Anions=Sulphate and Chloride.

How do you know? but THANKS FRIEND!

 

[Zaki ali asghar]

How do you know? but THANKS FRIEND!

It has been circulating around so who knows it might even turn out to be true.

 

[sarmadimran]

i am not sure but in Q3 the cations are probably going to be Mg2+,Mn2+,Al3+
Anions=Sulphate and Chloride.

yes i have also heard the same

 

[Zaki ali asghar]

anyone knows how to calculate %uncertainty in burette reading

 

[techgeek]

this is the end of cambridge's standard...
Students know the answers before reading the question paper
We really proved ourselves an 'honest' nation

 

[Zaki ali asghar]

this is the end of cambridge's standard...
Students know the answers before reading the question paper
We really proved ourselves an 'honest' nation

We dont really know it,its not that we know the paper,Its just a guess and guesses are never accurate,its not that we are just going to write on paper without performing.