CHEMISTRY PAPER 3 DOUBTSS!!

[Zaki ali asghar]

can you explain me a bit the method u used please

 

[anastasia grey113]

can you explain me a bit the method u used please

i have another method
since u added 10 cm3 in the flask of 2moldm-3 n then diluted it the moles in the whole flask will be as follows
10/1000 x 2

then u will find the number of moles per cm3 in the flask as follows
calculated moles/0.25dm3 (v r doing all the calculations in dm till here)

then multiply this answer with the volume u used

did this help?

Correction Zaki ali asghar
u have to divide calculated moles by 250cm3 n then multiply it with the volume used which should also b in cm3...

 

[Zaki ali asghar]

yes,thank you so much.Easier to understand it this way.

 

[anastasia grey113]

yes,thank you so much.Easier to understand it this way.

haha ur welcome

 

[Zaki ali asghar]

actually the mark scheme confused me so much,ughh i have been thinking of it since hours

 

[anastasia grey113]

actually the mark scheme confused me so much,ughh i have been thinking of it since hours

dont think
it wastes time just post on xpc

 

[Zaki ali asghar]

how are you preparing for practical?

 

[anastasia grey113]

how are you preparing for practical?

being quite honest i never prep for chem p3
i just go n perform
m doing mcqs right now

 

[Aymen J]

I wanted to ask in the chemistry practical exam in the titration quetion do we have to thoroughly clean all the equipment wiht distilled water before use,and if so how exactly are we supposed to do that?

 

[amina1300]

I wanted to ask in the chemistry practical exam in the titration quetion do we have to thoroughly clean all the equipment with distilled water before use,and if so how exactly are we supposed to do that?

If youre really good at completing the Practical well in time then you might as well clean the equipment before use but If your not so good then dont waste your time with it but, if some piece of equipment looks used or not washed then dont take a risk by not washing it. I'd recommend that if you have to wash it take no more than 3 seconds because in a Chem Practical timing is everything.

 

[Zaki ali asghar]

i have this bad habit of spilling chemical over my exam paper and breaking the equipment as the station is small and there are so many chemicals all together,any tips to avoid this :/

 

[amina1300]

i have this bad habit of spilling chemical over my exam paper and breaking the equipment as the station is small and there are so many chemicals all together,any tips to avoid this :/

lol I get the spilling of chemicals but how exactly do you BREAK equipment!

 

[amina1300]

Can anyone tell me the stuff I should learn for the Organic Questions because I haven't attempted any at all Plus some hard calculation questions( e.g with some approximate value for titre or temperature change from the Q's which they have attempted in a Lab.)

 

[techgeek]

--> Place a small spatula measure of FB 5 in a boiling tube, add a 1cm depth of dilute hydrochloric acid and warm the contents of the tube gently.
Observation : FB 5 + HCl: effervescence/ fizzing/ bubbling gas pops with lighted splint

-->Place small spatula measures of FB 5 and FB 6 in a single boiling tube. Use a test-tube holder to hold the tube. Add a 2cm depth of aqueous sodium hydroxide. CARE
Observation: FB 5 + FB 6 + NaOH: vigorous / violent/ exothermic / great/ extreme/ lots of and effervescence/ fizzing/ bubbling gas/NH3 turns (damp) red litmus (paper) blue.

How do we identify FB 5 is???

Fb5 is Al or Zn (s)
Is there a logical way to deduce this???

--> "Al and Zn reduce NO3- to ammonia in basic solutions"?

I searched, found out that this reaction is called "a nitrate test using Devarda's alloy", but there is no specific description of the process in our syllabus so how do the examiners expect us to know this!!

In the qualitative analysis, marks are awarded for observations and drawing conclusions only. We don't need to know the process of what exactly is happening during the reactions.
While testing for FB5 in 3 a)i), since we are told that FB5 is an element, it most likely could be a metal and remember to test for gases whenever you are told to add acid or alkali and heat in a boiling tube, since this was a metal, reacting with acid gives off H2 gas which "pops" with a lighted splint. Plus you record the observation about effervescence as the gas is being produced.
When you add FB5+FB6+NaOH, it gives off ammonia and when you match with the table of 'reactions of anions' you see it could be either nitrate or nitrite, this part may also give you hint that the metal was Al since it's being added in NaOH to test.
Our identification for nitrate is confirmed when dilute acid (HCl) does not liberate NO or NO2, which would have happened if it was nitrite (but even if you didn't happen to notice that, they'll still mark you if you say nitrite).
When there's no reaction with NaOH, looking it up in the table for cation reactions, you see there's only one cation (Ba) with which no reaction occurs when NaOH is added. So you could either say it's unknown or Ba.
As you said it's not in syllabus so we aren't supposed to know it, that's not the case, all we have to do is make observations and draw conclusions based on given information in the two tables at the end of paper.

 

[amina1300]

In the qualitative analysis, marks are awarded for observations and drawing conclusions only. We don't need to know the process of what exactly is happening during the reactions.
While testing for FB5 in 3 a)i), since we are told that FB5 is an element, it most likely could be a metal and remember to test for gases whenever you are told to add acid or alkali and heat in a boiling tube, since this was a metal, reacting with acid gives off H2 gas which "pops" with a lighted splint. Plus you record the observation about effervescence as the gas is being produced.
When you add FB5+FB6+NaOH, it gives off ammonia and when you match with the table of 'reactions of anions' you see it could be either nitrate or nitrite, this part may also give you hint that the metal was Al since it's being added in NaOH to test.
Our identification for nitrate is confirmed when dilute acid (HCl) does not liberate NO or NO2, which would have happened if it was nitrite (but even if you didn't happen to notice that, they'll still mark you if you say nitrite).
When there's no reaction with NaOH, looking it up in the table for cation reactions, you see there's only one cation (Ba) with which no reaction occurs when NaOH is added. So you could either say it's unknown or Ba.
As you said it's not in syllabus so we aren't supposed to know it, that's not the case, all we have to do is make observations and draw conclusions based on given information in the two tables at the end of paper.

Thankyou so much.

I have another question, some people say that doing the gas tests its that necessary as you can draw a conclusion from the given information but as you said that whenever we are told to add acid or alkali and heat in boiling tube we test the gases so, are there any other occasions when you should and shouldn't test for the gases and in what sequence should we test for the gases ?

 

[Roe Mir]

N

Thankyou so much.

I have another question, some people say that doing the gas tests its that necessary as you can draw a conclusion from the given information but as you said that whenever we are told to add acid or alkali and heat in boiling tube we test the gases so, are there any other occasions when you should and shouldn't test for the gases and in what sequence should we test for the gases ?

Normally people dont do the gas tests because they are time consuming. So if you're sure of what the anion is then just check for the gas in the tables given.

 

[Roe Mir]

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?

 

[anastasia grey113]

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?

dude the only way is to take time out
it just takes 2 mins

 

[amina1300]

dude the only way is to take time out
it just takes 2 mins

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

 

[amina1300]

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