Chemistry: Post your doubts here!

[qwertypoiu]

Which compound might X be? In its reaction with sodium, 1mol of a compound X gives 1mol of H2(g).

A CH3CH2CH2CH2OH B (CH3)3COH C CH3CH2CH2CO2H D CH3CH(OH)CO2H

Answer is D, but how?

One MOLE of H2 means there must be TWO Hydrogen atoms released per molecule being released.

Compound A and B both only have one alcohol functional group. Both will release ONE Hydrogen atom per molecule, but that will only be HALF a mole of H2.
Compound C only has one carboxylic acid group and so releases only HALF a mole of H2.
Compound D has one carboxylic acid group and one alcohol group. In total, it will release TWO hydrogen atoms per molecule which is same as saying ONE mole of H2 per mole of compound.

 

[qwertypoiu]

What is the enthalpy change under standard conditions for the following reaction? The standard enthalpy changes of formation of iron(II) oxide, FeO(s), and aluminium oxide, Al2O3(s), are and –266kJmol–1 and -1676kJmol-1 respectively.

What is the enthalpy change under standard conditions for the following reaction?


2O3(s)(s) 3Fe(s) + Al 3FeO(s) + 2Al

Shouldn't it be Reactants - Products?
So, 3x(-266) - (-1676)= +878kJmol-1?

What you have written doesn't look like a reaction to me.

 

[Sunit Saha]

Hey guys! I wanted to ask that do you guys have any idea where I can get the sample paper of May/June 2016 for As level chem?

 

[Syed Umar]

What you have written doesn't look like a reaction to me.

oh sorry.




3FeO(s) + 2Al(s) = 3Fe(s) + Al2O3(s)

 

[Syed Umar]

One MOLE of H2 means there must be TWO Hydrogen atoms released per molecule being released.

Compound A and B both only have one alcohol functional group. Both will release ONE Hydrogen atom per molecule, but that will only be HALF a mole of H2.
Compound C only has one carboxylic acid group and so releases only HALF a mole of H2.
Compound D has one carboxylic acid group and one alcohol group. In total, it will release TWO hydrogen atoms per molecule which is same as saying ONE mole of H2 per mole of compound.

THANKK YOU!!

 

[Syed Umar]

Which reagent would react with prop-2-en-1-ol to form a product that could exist as optical isomers?
Why is it bromine and not PCl5?

 

[Syed Umar]

oh sorry.




3FeO(s) + 2Al(s) = 3Fe(s) + Al2O3(s)

Nvm, I got the answer, thanks anyways!

 

[qwertypoiu]

Which reagent would react with prop-2-en-1-ol to form a product that could exist as optical isomers?
Why is it bromine and not PCl5?

Prop-2-en-1-ol:

CH2=CHCH2OH

If you react this with PCl5, the alcohol group will be replaced by Cl:

CH2=CHCH2Cl

There is no chiral carbon in this compound.

If you react prop-2-en-1-ol with Br2 instead, the alkene group reacts:

CH2BrCHBrCH2OH

The pink carbon atom above is chiral.

 

[Syed Umar]

Prop-2-en-1-ol:

CH2=CHCH2OH

If you react this with PCl5, the alcohol group will be replaced by Cl:

CH2=CHCH2Cl

There is no chiral carbon in this compound.

If you react prop-2-en-1-ol with Br2 instead, the alkene group reacts:

CH2BrCHBrCH2OH

The pink carbon atom above is chiral.

Right, right, ofc. I was thinking more on the lines of could form after another reaction. Now I see what was wrong, thanks!

 

[Ibtehaj Munir]

AOA
Please people, I need serious help with this boring subject. I've tried reading the course book twice, tried solving some topicals but still I cant seem to get more than 30/40 in MCQs ...... HELP ME PLZ. Any good way to retain all the syllabus would do the work. i'm not looking for notes. Just tell me an effective way to study this subject

 

[bakhita]

AOA
Please people, I need serious help with this boring subject. I've tried reading the course book twice, tried solving some topicals but still I cant seem to get more than 30/40 in MCQs ...... HELP ME PLZ. Any good way to retain all the syllabus would do the work. i'm not looking for notes. Just tell me an effective way to study this subject

This same with me too!!
Just keep doing the papers, that's what I'm going to do and I'm improving day by day. There still are two months and the more we practice past papers, the better!

 

[The Sarcastic Retard]

AOA
Please people, I need serious help with this boring subject. I've tried reading the course book twice, tried solving some topicals but still I cant seem to get more than 30/40 in MCQs ...... HELP ME PLZ. Any good way to retain all the syllabus would do the work. i'm not looking for notes. Just tell me an effective way to study this subject

30 is not bad.

 

[Ibtehaj Munir]

30 IS bad if people expect more from you :'(

 

[drowning-in-studies]

Which pollutant, present in the exhaust fumes of an internal combustion engine, has an element in the +2 oxidation state and an odd number of electrons in one molecule of the pollutant? A) CO B ) H2S C) NO D )NO2
Ans C Please explain!

 

[The Sarcastic Retard]

30 IS bad if people expect more from you :'(

Dont give a damn about others. You know what you are and u do ur level best that's it. Marks dont judge ur knowledge..
30 is not that bad that u are thinking. I use to get happy if I score around 30.. *Cheers*

 

[drowning-in-studies]

Hey guys! I wanted to ask that do you guys have any idea where I can get the sample paper of May/June 2016 for As level chem?

Do you mean specimen paper?
http://www.cie.org.uk/programmes-an...al-as-and-a-level-chemistry-9701/past-papers/

 

[The Sarcastic Retard]

Which pollutant, present in the exhaust fumes of an internal combustion engine, has an element in the +2 oxidation state and an odd number of electrons in one molecule of the pollutant? A) CO B ) H2S C) NO D )NO2
Ans C Please explain!

Its either A or C.
A has 14 electrons
C has 15 electrons

 

[Konstantino Nikolas]

30 IS bad if people expect more from you :'(

I understand.
First, to be able to do well in any subject, you will have to clear your negative mindset about it. Same goes with chemistry. Think of it as a challenge instead of a burden.
Having said that, for chemistry you will first have to understand each concept very well to go any further. It's best if you can have a good teacher who can help you out with that. So if you do, you need to pay attention in class. Taking notes during the lecture helps you stay awake and grasp things. Then, study each chapter from the textbook (highlight important points as you study if that helps you concentrate). Once you're done with a single chapter, do questions from around 5 - 10 past papers. Repeat that with every chapter.
Then in the end you will have to revise and do paper-wise. More the practice, better will be your marks.
For paper 1 specifically, doing the questions from last to first helps you finish sooner. (Don't ask how, but it helped me when I had a problem with time-management. ) But considering the fact that you are already scoring a 30, the only thing that will help you score more is practice. All the best! Above 30 is perfectly achievable.

 

[Copy Cat]

 

[qwertypoiu]

View attachment 59567

Moles of thiosulphate ions = 19.5 x 0.02 / 1000 = 0.00039mol
This is equal to moles of copper ions:

Moles of copper ions = 0.00039mol
Concentration of copper ions = 0.00039mol / 50 * 1000 = 0.0078mol/dm^3
This is concentration in moles per volume. Let's convert it to mass per volume:

0.0078mol/dm3 of copper ions = 0.0078*63.5 = 0.4953g/dm3

So there is 0.4953g of copper dissolved in every 1000cm3 of water.

We need to find percentage of copper by mass. Note that 1000cm3 of water has 1000g of mass.

Percentage by mass = 0.4953g / (1000 + 0.4953)g x 100% = 0.05%