Chemistry: Post your doubts here!

[L-Girl]

guys, does anyone know how the marks are calculated for AS levels to get a mark out of 130?

 

[unseen95]

guys, does anyone know how the marks are calculated for AS levels to get a mark out of 130?

i guess its
obtained marks/130 × 100

 

[mamoon123]

guys, does anyone know how the marks are calculated for AS levels to get a mark out of 130?

Every paper (i.e: MCQs, Structured and Practical) holds a specific weightage.

 

[Jspake]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_s12_qp_21.pdf
I do not understand how to solve question no. 2(b) iv
Please explain how the answer is derived.
Thanks

 

[asd]

Moles of NaOH that reacted with HCl will be the same as HCl's moles (ratio is 1:1) = 0.03.
Moles that you'll calculate in (iii) will be the total moles of NaOH in the initial reaction that took place with (NH4)2SO4 --> 0.1.
0.03 mol of NaOH are the moles that didnt react with (NH4)2SO4 and were titrated with HCl, as the question reads "The REMAINING NaOH was then titrated with 1.00moldm–3 HCl and 31.2cm3 were required for neutralisation."
So, moles that did react, 0.1 - o.03 = 0.7.

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s12_qp_21.pdf
I do not understand how to solve question no. 2(b) iv
Please explain how the answer is derived.
Thanks

 

[Jspake]

Moles of NaOH that reacted with HCl will be the same as HCl's moles (ratio is 1:1) = 0.03.
Moles that you'll calculate in (iii) will be the total moles of NaOH in the initial reaction that took place with (NH4)2SO4 --> 0.1.
0.03 mol of NaOH are the moles that didnt react with (NH4)2SO4 and were titrated with HCl, as the question reads "The REMAINING NaOH was then titrated with 1.00moldm–3 HCl and 31.2cm3 were required for neutralisation."
So, moles that did react, 0.1 - o.03 = 0.7.

oh yeah.. thanks a lot mate

 

[snowbrood]

i have two questions from chemical energetics as level
molar enthalpy change of the reaction of pure sodium carbonate with hydrochloric acid is 49.5KJmol^-1
2.6g of washing soda was added to 50cm^3 of dilute hydrochloric acid in a plastic cup the temperature rose by 5C(specific heat capacity of the resulting solution=4.3) calculate the molar enthalpy change for the reaction of washing soda with dilute hcl
answer -43.8kjmol^-1

 

[Alice123]

http://papers.xtremepapers.com/CIE/...d AS Level/Chemistry (9701)/9701_w02_qp_1.pdf
no 8, 9, 11, 13
please help, thanks in advance

 

[scouserlfc]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w02_qp_1.pdf
no 8, 9, 11, 13
please help, thanks in advance

13= Only Aluminum can hydrolyse and form the complex acid ions (Al(H2o)6)+ + Cl- and also react with an acid to form a salt that is it has the amphoteric property !
8=C use PV=nRT and get the answer ! P and T have to remain the same,you can negate R here as its value is always same so it has no effect on the equation,remember Deuterium exists just like hydrogen,that is,D2 so its Mr is 4 now see which equation balances both side in such a way that the pressure p is exerted ! (ask if u still dont get )
9=A as C and D can easily be eliminated at first,the former being eliminated because the elements have no energy so they are at 0 and the latter because CO2 is formed in the reaction and its forming releases energy (exothermic) so it is the most stable compound having the least energy ! Now between A and B you have to see which if the options has actually used up most energy from outside in B the carbon has been changed to gaseous C but in A not only this happens but O2 has been broken into its radicals so it has to have the highest energy !
11= Answer is D,check if you are taking the mole ratio in account when doing the calculation of H2 and I2 moles at equi. At equi. HI has b-x and H2 and I2 have x/2 each ! now use partial pressure equations for all and u will use b as the total moles in it and when u get on to calculating Kp u will see that b andp (both squared ) get cancelled !

 

[snowbrood]

13= Only Aluminum can hydrolyse and form the complex acid ions (Al(H2o)6)+ + Cl- and also react with an acid to form a salt that is it has the amphoteric property !
8=C use PV=nRT and get the answer ! P and T have to remain the same,you can negate R here as its value is always same so it has no effect on the equation,remember Deuterium exists just like hydrogen,that is,D2 so its Mr is 4 now see which equation balances both side in such a way that the pressure p is exerted ! (ask if u still dont get )
9=A as C and D can easily be eliminated at first,the former being eliminated because the elements have no energy so they are at 0 and the latter because CO2 is formed in the reaction and its forming releases energy (exothermic) so it is the most stable compound having the least energy ! Now between A and B you have to see which if the options has actually used up most energy from outside in B the carbon has been changed to gaseous C but in A not only this happens but O2 has been broken into its radicals so it has to have the highest energy !
11= Answer is D,check if you are taking the mole ratio in account when doing the calculation of H2 and I2 moles at equi. At equi. HI has b-x and H2 and I2 have x/2 each ! now use partial pressure equations for all and u will use b as the total moles in it and when u get on to calculating Kp u will see that b andp (both squared ) get cancelled !

help me bud

 

[Alice123]

13= Only Aluminum can hydrolyse and form the complex acid ions (Al(H2o)6)+ + Cl- and also react with an acid to form a salt that is it has the amphoteric property !
8=C use PV=nRT and get the answer ! P and T have to remain the same,you can negate R here as its value is always same so it has no effect on the equation,remember Deuterium exists just like hydrogen,that is,D2 so its Mr is 4 now see which equation balances both side in such a way that the pressure p is exerted ! (ask if u still dont get )
9=A as C and D can easily be eliminated at first,the former being eliminated because the elements have no energy so they are at 0 and the latter because CO2 is formed in the reaction and its forming releases energy (exothermic) so it is the most stable compound having the least energy ! Now between A and B you have to see which if the options has actually used up most energy from outside in B the carbon has been changed to gaseous C but in A not only this happens but O2 has been broken into its radicals so it has to have the highest energy !
11= Answer is D,check if you are taking the mole ratio in account when doing the calculation of H2 and I2 moles at equi. At equi. HI has b-x and H2 and I2 have x/2 each ! now use partial pressure equations for all and u will use b as the total moles in it and when u get on to calculating Kp u will see that b andp (both squared ) get cancelled !

thanks a lot. May the Almighty bless u!
i'm still a bit confused about number 8. first i found the p of the given sample i e p=nT/V where n=2/2 which equals 1. In C, Mr is 4, so the no of moles is 3(total mass)/4, so the equation is 3T/4V which is not equal to p help me out. Why isnt the ans A?or B?

 

[hassankhan]

can anyone help me with good notes on organic as i missed these classes and am having lots of problem

 

[scouserlfc]

can anyone help me with good notes on organic as i missed these classes and am having lots of problem

go through whichever topic you are looking for,organic is in the topics at the bottom !
http://www.chemguide.co.uk/CIE/index.html

 

[scouserlfc]

i have two questions from chemical energetics as level
molar enthalpy change of the reaction of pure sodium carbonate with hydrochloric acid is 49.5KJmol^-1
2.6g of washing soda was added to 50cm^3 of dilute hydrochloric acid in a plastic cup the temperature rose by 5C(specific heat capacity of the resulting solution=4.3) calculate the molar enthalpy change for the reaction of washing soda with dilute hcl
answer -43.8kjmol^-1

You have two questions can u tell me the two seperately by numbering em because i cant get which one is which ???? Ill solve it as soon as u do it !

 

[scouserlfc]

thanks a lot. May the Almighty bless u!
i'm still a bit confused about number 8. first i found the p of the given sample i e p=nT/V where n=2/2 which equals 1. In C, Mr is 4, so the no of moles is 3(total mass)/4, so the equation is 3T/4V which is not equal to p help me out. Why isnt the ans A?or B?

Heres why !
First the answer we are looking for !
PV=(2/2)RT

A- total moles = 2/4 =0.5
and so the overall equation is !
PV=0.5nRT which is impractical !

B- total moles = 4/4 =1
P(V/2)=nRT the volume if it was in A,then it would have been right but thats not the case !

C- remember moles for different compounds or elements cant be found by one equation u have to find them individually thats one basic mistake !
total moles = 1/2(H2) + 2/4 (D2) =1
PV=nRT (the correct answer )

D- total moles = 2/2 + 1/4 = 1.25
2PV=1.25nRT (not possible)

 

[sweetjinnah]

Please any1 solve this: oct/nov 2011/22 Q4 (a)http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w11_qp_22.pdf

 

[aalmuhannadi]

Anyone know how to do this please?

I know the answer has to be either A or C since it's trans, but I have no clue how to calculate the number of double bonds... I thought it would be two as there's a C=C double bond in a cyclohexene ring and the other is given in the question between Carbons 11 and 12 (aldehyde doesn't count as there's no C=C bond). So how would you arrive at a total of 5?

 

[Warrior66]

Anyone know how to do this please?
View attachment 21434
I know the answer has to be either A or C since it's trans, but I have no clue how to calculate the number of double bonds... I thought it would be two as there's a C=C double bond in a cyclohexene ring and the other is given in the question between Carbons 11 and 12 (aldehyde doesn't count as there's no C=C bond). So how would you arrive at a total of 5?

I know the answer has to be either A or C since it's cis,

Asalam-o-Alaikum!
For this question, you have to draw the whole structure out. They tell you the molecular formula: C20H28O and they also tell you that it has: a cyclohexene , the aldehyde group (at the end of the aliphatic chain) and the long aliphatic chain.
So you start out with drawing the cyclohexene whose molecular formula would be C6H9 . This means that now you cancel out this from the C20H28O, so that you're now left with: C14H19O. Now as you have drawn the cyclohexene, you join the aliphatic chain containing 14 carbons. and then add in the aldehyde group in the end. this now means that you're left with 18 hydrogens. moving on, you have to count the eleventh carbon in this aliphatic chain and draw in the double bond and the cis bit. uptil now we have how many double bonds? only 2. yes? so now keeping in mind that we have 13 carbons that we need to bond the hydrogens to (14th carbon is not counted since it has the aldehyde group) and since we only have 18 hydrogens, we know that there MUST be some double bonds in this chain apart from the 11th & 12th double bond. how many? that is what we have to find out...either 4 or 3 MORE double bonds...after you've drawn out everything you'll find that we needed 3 MORE double bonds in the aliphatic chain...so now we add up all the double bonds in the structure! 1 in the cyclohexene, 1 between the 11th & 12th carbon, and 3 more! that makes 5 double bonds. your final answer is A.

 

[Alice123]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_s11_qp_11.pdf
please help me with no 4,510,11,14
and in 7, isnt the backward rate supposed to increase?
thanks in advance

 

[Alice123]

Heres why !
First the answer we are looking for !
PV=(2/2)RT

A- total moles = 2/4 =0.5
and so the overall equation is !
PV=0.5nRT which is impractical !

B- total moles = 4/4 =1
P(V/2)=nRT the volume if it was in A,then it would have been right but thats not the case !

C- remember moles for different compounds or elements cant be found by one equation u have to find them individually thats one basic mistake !
total moles = 1/2(H2) + 2/4 (D2) =1
PV=nRT (the correct answer )

D- total moles = 2/2 + 1/4 = 1.25
2PV=1.25nRT (not possible)

thanks