AS Chemistry Doubts

[iFuz]

I'm starting this thread just to separate AS students... from that one thread were all are posting and things are pretty messed up!

So, ONLY AS STUDENTS post your doubts and challenging questions here...

I'm sure it will help me and YOU!

 

[smartangel]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s11_qp_22.pdf
In the first question part iv we calculate the no.of moles of NaOH that REACT with ethanoic acid right?? so shouldnt the equilibrium moles be 0.1 - 0.04 therefore giving us 0.06 as moles of acid n alcohol at equlilbrium..but this gives a wrong anwer..plz explain..im worried!!

 

[zainab1661]

Can you help me out with this question plllzzz!!!!
The compound known as ‘gamma-linolenic acid’ is found in significant amounts in the seeds of
the Evening Primrose plant. There is evidence that the compound may help patients with
diabetes.
CH3(CH2)4CH=CHCH2CH=CHCH2CH=CH(CH2)4CO2H
gamma-linolenic acid
How many cis-trans isomers does gamma-linolenic acid have?
A 3 B 6 C 8 D 1

 

[workaholic]

The answer is 8...........
In order to identify the no of cistrans isomers u need to first identify the stereocentres.If a compound has a C-C double bond and each of the C in the double bond is bonded to seperate groups then the compound has a stereocentre.In the above case there are 3 such centres.The total number of cistrans isomers is given by the formula 2^n so 2^3=8. There you go......

 

[smartangel]

please tell me why is the answer A and not B?

 

[smartangel]

.8 The equation below represents the combination of gaseous atoms of non-metal X and of hydrogen to form gaseous X2H6 molecules.
2X(g) + 6H(g) -----> X2H6(g) ^H = –2775 kJ mol

The bond energy of an X–H bond is 395 kJ mol
.
What is the bond energy of an X–X bond?
A – 405.0 kJ mol

B – 202.5 kJ mol

C +202.5 kJ mol

D +405.0 kJ mol

how do we get D as the answer..please explain!!!!

 

[workaholic]

please tell me why is the answer A and not B?

bcoz deltaH=Enthlpy of Products - Enthalpy of Reactants.................As the enthalpy of (Y+Z) is less than (W+X) so the answer is bound to come +ve.....

 

[workaholic]

X2H6 has a one X-X bond and 6 X-H bonds.The eqn given above is thatenthalpy change of formation meaning all these bonds were formed.......The energy released during bond formation is equal in magnitude but with a negative sign to the bond energy(coz its exothermic).........So we have 2775 = 6 * 395 +x .................Answer is 405

 

[smartangel]

X2H6 has a one X-X bond and 6 X-H bonds.The eqn given above is thatenthalpy change of formation meaning all these bonds were formed.......The energy released during bond formation is equal in magnitude but with a negative sign to the bond energy(coz its exothermic).........So we have 2775 = 6 * 395 +x .................Answer is 405

thanks a lot! please tell me this one also..

24 Compound X on reaction with hot concentrated sulphuric acid gave a mixture of three alkenes.
What could X be?
A butan-2-ol
B propan-2-ol
C 2-methylbutan-2-ol
D 2-methylpropan-2-ol

hows the answer A?

 

[smartangel]

this one also please..
the answer is B

 

[workaholic]

thanks a lot! please tell me this one also..

24 Compound X on reaction with hot concentrated sulphuric acid gave a mixture of three alkenes.
What could X be?
A butan-2-ol
B propan-2-ol
C 2-methylbutan-2-ol
D 2-methylpropan-2-ol

hows the answer A?

A substitution reaction occurs.......Butan-2-ol has the formula.CH3CH(OH)CH2CH3.....Now the rules for substituton says a water molecle is eliminated with OH from the carbon containing hydroxy group and the H from one of the neighbouring carbon atoms.......As you can see that there are two carbon atoms neighbouring the (OH) carbon......So two different products but-1-ene and but-2-ene are formed....but2-ene has two isomers (cis and trans but-2-ene) thus giving a total of 3 isomers i.e but-1-ene, cis but-2-ene and trans but-2-ene.......Thus ans is A.

 

[workaholic]

this one also please..
the answer is B

In A and D there is onlo one alkene possible as there is only one C atom neighbouring the (OH) wala carbon
In B there are 4 possible (cis and trans pent-3-ene and cis and trans pent-2-ene)
In C there are 3 possible (cis and trans 3-methyl but-2-ene and 2-ethyl but-1-ene)

 

[zainab1661]

The answer is 8...........
In order to identify the no of cistrans isomers u need to first identify the stereocentres.If a compound has a C-C double bond and each of the C in the double bond is bonded to seperate groups then the compound has a stereocentre.In the above case there are 3 such centres.The total number of cistrans isomers is given by the formula 2^n so 2^3=8. There you go......

Thank youu sooo much

 

[zainab1661]

25 Which reaction would not give propene as one product?
A adding an excess of concentrated sulfuric acid to propan-1-ol
B adding warm aqueous sodium hydroxide to 2-bromopropane
C adding warm ethanolic sodium hydroxide to 1-bromopropane
D passing propan-2-ol vapour over heated aluminium oxide
why is the answer B???

 

[zainab1661]

29 2-Methylbuta-1,3-diene, CH2=C(CH3)–CH=CH2, is used as a monomer in the manufacture of
synthetic rubbers.
Which compound would not produce this monomer on treatment with concentrated sulfuric acid
at 170°C?
A (CH3)2C(OH)CH(OH)CH3
B HOCH2CH(CH3)CH2CH2OH
C HOCH2CH(CH3)CH(OH)CH3
D HOCH2C(CH3)(OH)CH2CH
gosh i'm soo confused :s

 

[zainab1661]

26 Glycol, used in anti-freeze, has the formula HOCH2CH2OH. It can be oxidised to give a number of
products.
What is the molecular formula of an oxidation product of glycol that will not react with sodium?
A C2H2O2 B C2H2O3 C C2H2O4 D C2H4O

 

[kkhizar1]

The question is in May/June 2003 Q2. Please I asked many people but they didnt know the answer. Question is attached

 

[workaholic]

Hey if u are talking about this qst.....



B and D cant be cuz the oxidation state of S in both the first and second reaction is the same.
C cant be because SO3^-2 has gained an H+ thus meaning it has gained a proton to become HSO3^- showing that it is acting as a base cuz bases are proton acceptors.
A is the answer cuz HSO3 loses a H+ ion to form SO2 thus it is acting as a acid as they are proton donors.

 

[iFuz]

Can you help me out with this question plllzzz!!!!
The compound known as ‘gamma-linolenic acid’ is found in significant amounts in the seeds of
the Evening Primrose plant. There is evidence that the compound may help patients with
diabetes.
CH3(CH2)4CH=CHCH2CH=CHCH2CH=CH(CH2)4CO2H
gamma-linolenic acid
How many cis-trans isomers does gamma-linolenic acid have?
A 3 B 6 C 8 D 1

Sorry for late reply.. but answer is C
Number of Cis Trans isomers is calculated with formula 2 ^ (number of double bonds present in compound)
3 in this case .. so 2^3 = 8

 

[TSZ]

Hey, thnx for starting this separate thread for AS.....i have some questions
http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_s10_qp_11.pdf
Q.35, why can't the ans be carbon as well....if carbon monoxide is formed, then it will also undergo same reaction....right?
http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w09_qp_11.pdf
In this I want to know how to do Q. 21 and 31
http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w08_qp_1.pdf
Q.38
http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s08_qp_1.pdf
In Q.29, why is C not an acid?