IIT-JEE problems

[Jaskanwar Singh]

guys shouldnt there be a thread where we can discuss several questions sometimes with each other? we can try to solve each others problems.

when replying to questions put answers in spolier tags. so that others dont want to see it.

[Ishu Gupta]

Sure.
Include AIEEE too.

[Jaskanwar Singh]

yes yes sure. all problems of different levels.

some questions i dont know -
1. alkali metals are paramagnetic but their salts are diamagnetic. explain.
2. why ammonia has higher melting point than phosphine?

answer them fast please. my XI chemistry exam on monday.

[abhijangda]

alkali metals are paramagnetic
They are paramagnetic because they have one unpaired electron.
Take the case of Sodium, it's atomic number is 11. so its configuration is [Ne] 3s1. Hence one unpaired electron will make it paramagnetic.
And their compounds are diamagnetic because they have no unpaired electron in their compounds.
And for last question
Dipole moment of Ammonia is greater than that of Phosphine or you can say that as Nitrogen is more electronegative than Phosphorus so Nitrogen-Hydrogen bond is more polar than Phosphorus-Hydrogen bond which increases the forces between two atoms.

Best of Luck for exams, also feel free to ask!!

[Jaskanwar Singh]

Thanks buddy. Also for 2nd one hydrogen bonding can be considered then i suppose?

[pauldmps]

^^ Both NH3 & PH3 have polar molecules. But N is much more electronegative than P. Hence NH3 has intermolecular hydrogen bonding while PH3 has only dipole-dipole forces.

[abhijangda]

Thanks buddy. Also for 2nd one hydrogen bonding can be considered then i suppose?

Yes why not!!

[Jaskanwar Singh]

thanks everyone.

BTW what will be the structure of NNO. N'triplebond'N'coordinatebond'O?

[ssk_the_gr8]

god..... how i hated all of this....
best of luck ... all of you'll

[pauldmps]

^^ Actually, there are two resonating stuctures: One you mentioned while another is N=N=O

[Jaskanwar Singh]

ok thanks.

why is there + overlap hybrid orbitals? i mean why + lobes overlap?

[abhijangda]

ok thanks.

why is there + overlap hybrid orbitals? i mean why + lobes overlap?

sorry, but i cant understand your question. Did you mean why is there positive overlap hybrid orbitals or anything else??

[Jaskanwar Singh]

Like why +ve lobes of hybrid orbitals are large and overlap with other elements' p or s orbitals etc? Why -ve lobes are small?

[abhijangda]

Like why +ve lobes of hybrid orbitals are large and overlap with other elements' p or s orbitals etc? Why -ve lobes are small?

See, you know that lobes represents electron density or probability of finding an electron around nucleus. Lets take example of CH4 it has 4Hydrogen and one Carbon. This Carbon has 2p orbitals which has +ve and -ve lobes. When s-orbital is attached with +ve lobe then the final hybrid orbital lobe becomes large because probability of finding electron there increase and when s orbital combines with -ve lobe then it decreases because probability of finding electron there decreases.

[Jaskanwar Singh]

paper went good. thank you. now english

[Zangetsu]

all r students here?

study hard.

[Jaskanwar Singh]

Btw solubility of alkaline earth metal carbonates decreases down the group?

And why is KO2 paramagnetic?

[pauldmps]

^^ 1. Size of cations & hydration enthalpy increases down the group, hence solubility decreases.

2. It has one or more unpaired electrons (duh!)

[Jaskanwar Singh]

^^i know it will be beacuse of unpaired electrons!!. but i am asking for electronic config or some explanation to write in exam.

[abhijangda]

all r students here?

study hard.

i m not a 12th or 11th student.

@jaskanwar: KO2 contains super oxide ion. Write configuration of super oxide ion according to Molecular orbital Theory and u will get an unpaired electron

[pauldmps]

^^i know it will be beacuse of unpaired electrons!!. but i am asking for electronic config or some explanation to write in exam.

I thought it was a thread meant for IIT-JEE & other competition exams in which you'll have objective questions.

Anyways, here are some questions from me:

1. Chemistry - What is the minimum number of carbon atom an alkane molecule must have in order to show optical activity ?

A. 5 B. 6 C. 7 D. 8

2. Maths - Suppose f(x) = x^3 + ax^2 + bx + c, where a,b,c are chosen respectively by throwing a die three times. Then the probability that f(x) is is decreasing in [1,3] is:

A. 5/9 B. 4/9 C. 0 D. 2/5

3. Physics - A solid sphere rolls on a smooth horizontal surface at 10 m/s and then rolls up an inclined plane of inclination 30° with horizontal. The mass of the sphere is 2kg. Find the height attained by the sphere before it stops in metre. (g= 10 m/s^2)
[Answer will be a digit between 0 to 9]

Let's see who can crack these IIT-JEE level question on his own. No cheating please. I'll answer them after anyone replies their answers.

[abhijangda]

I thought it was a thread meant for IIT-JEE & other competition exams in which you'll have objective questions.

Anyways, here are some questions from me:



Let's see who can crack these IIT-JEE level question on his own. No cheating please. I'll answer them after anyone replies their answers.

this thread is for asking doubts and not for taking tests or testing abilities of individuals. Just ask ques. If u have any doubt.

[Jaskanwar Singh]

Well is the ans to physics question - 7m?

[nims11]

guyz i need Tips for studying organic Chemistry. i am good at Physics and Mathematics but my Chemistry is poor due to organic Chem and my overall performance gets affected due to it. teachers say that Chemistry is skoring as well as less tym taking. My friends too score good in Chem. i am really comfortable with basic reaction mechanisms, basics of org chem like isomerism, optical activity,etc. i am also comfortable in hydrocarbons, alkyl halides and alcohols. but as the number of reagents increase, i lose track and am unable to grasp stuffs.

i need to get Computer Science as branch in a very good college so i think its very imperative for me to increase my overall performance in any possible ways.

@paul
answer to the chemistry one -> 7
answer to the maths one -> 0
answer to the physics one ->its coming 7 after assuming that it "pure rolls" on the smooth horizontal surface.
i hope these are right

[pauldmps]

^^ Absolutely Right all three of them.
Where are you taking your coaching BTW ?

@Jaskanwar Singh
Yes the physics answer is right. The chem & maths were on the tougher side, right ?

@abhijangda
I think such surprise question really help in preparation. Isn't it ?

[Jaskanwar Singh]

Actually buddy these chem and maths if from 12th i dont know as i in 11th. But if 11th i need to study more

but that was phy rotation level of iit?

[nims11]

^^ physics rotation question was of IIT's easy level questions as it could have been made quite complex by introducing few conditions.

@paul i take my coaching at FIITJEE. the classes are now over and i am preparing for 12th boards along with my revision for IITJEE.
i think that these surprise questions comes as a challenge and helps us to revisit the past topics which is necessary because the syllabus is vast and its difficult to keep track of the past topics while absorbing the new topics. for eg, i solved a question of rotation after many days.

[pauldmps]

@Jaskanwar Singh
IIT does not ask you very difficult questions. From last 3-4 years, the questions have been comparatively easier. You solved the question because you knew the concept. Give the same question to some of your friends & then see what they do. Some may not be able to identify that this question requires energy conservation & some may forget to add the rotational KE term.

More questions from class XI to come.......


@nims11
My classes at Prerna Classes are also over. Why don't you post some questions from your side here ?

[abhijangda]

^^ Absolutely Right all three of them.
Where are you taking your coaching BTW ?

@Jaskanwar Singh
Yes the physics answer is right. The chem & maths were on the tougher side, right ?

@abhijangda
I think such surprise question really help in preparation. Isn't it ?

i didnt said that these are not helpfull. Presently I m in NIT and also I have qualified JEE. I know the importance of surprise questions. But you should not write that 'lets see who can solve these problems' etc. Because if some one is not able to solve these problems his confidence will go down. Also the questions you have asked are of JEE levels for sure.

guyz i need Tips for studying organic Chemistry. i am good at Physics and Mathematics but my Chemistry is poor due to organic Chem and my overall performance gets affected due to it. teachers say that Chemistry is skoring as well as less tym taking. My friends too score good in Chem. i am really comfortable with basic reaction mechanisms, basics of org chem like isomerism, optical activity,etc. i am also comfortable in hydrocarbons, alkyl halides and alcohols. but as the number of reagents increase, i lose track and am unable to grasp stuffs.

i need to get Computer Science as branch in a very good college so i think its very imperative for me to increase my overall performance in any possible ways.

@paul
answer to the chemistry one -> 7
answer to the maths one -> 0
answer to the physics one ->its coming 7 after assuming that it "pure rolls" on the smooth horizontal surface.
i hope these are right

see if you are good at Maths and Physics then do not study for CBSE exams now. Study for exam only in March. I was also good at PCM and mostly at P and M. I prepared for exam in March and got 97 in P and 95 in M. For chem you have to study hard. Because ques. In chem are more theory oriented. So, prepare for chem only. Also at present do Organic only. Those reaction based questions are frequent in JEE. Good luck!!

guyz i need Tips for studying organic Chemistry. i am good at Physics and Mathematics but my Chemistry is poor due to organic Chem and my overall performance gets affected due to it. teachers say that Chemistry is skoring as well as less tym taking. My friends too score good in Chem. i am really comfortable with basic reaction mechanisms, basics of org chem like isomerism, optical activity,etc. i am also comfortable in hydrocarbons, alkyl halides and alcohols. but as the number of reagents increase, i lose track and am unable to grasp stuffs.

i need to get Computer Science as branch in a very good college so i think its very imperative for me to increase my overall performance in any possible ways.

@paul
answer to the chemistry one -> 7
answer to the maths one -> 0
answer to the physics one ->its coming 7 after assuming that it "pure rolls" on the smooth horizontal surface.
i hope these are right

see if you are good at Maths and Physics then do not study for CBSE exams now. Study for exam only in March. I was also good at PCM and mostly at P and M. I prepared for exam in March and got 97 in P and 95 in M. For chem you have to study hard. Because ques. In chem are more theory oriented. So, prepare for chem only. Also at present do Organic only. Those reaction based questions are frequent in JEE. Good luck!!

[asingh]

Really nice thread. Feels good to see a platform like TDF being used so constructively. J.Singh, great start up, and hope you all maintain the rigor. Heck brings back old memories -- am B.Sc. in Physics (Electronics) -- but cannot even attempt these questions now.

Kudos to you all.