Chemistry: Post your doubts here!

[Metallic9896]

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_11.pdf
Q4 I dont understand what Np Pu means??
Q6 ??
Q17 ??
Q30??

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_13.pdf
Q9 ???
Q21 ??
Q33 Statement 2 ; No particles have zero energy at either temperature. ?? What does this mean in context?

Q6: First get the moles of Al2O3 present in the mixture by using the amount of NaOH that was required. Moles of NaOH that were needed = 2 x 100/1000 = 0.2 moles of NaOH. As per the equation they gave us, 2 moles of OH- react with 1 mole of Al2O3. 2 moles of OH- of course means 2 moles of NaOH since one NaOH contains one OH-. Using the ratio method:

If 1 mole of Al2O3 requires 2 moles of NaOH
Then Y moles of Al2O3 require 0.2 moles of NaOH
0.2/2 = 0.1 moles of Al2O3 react with 0.2 moles of NaOH.

This tells us that the mixture contained 0.1 moles of Al2O3.

Now use the amount of HCl needed to get the total moles of HCl needed to react with both the moles of Al2O3 and MgO. 800/1000 x 2 = 1.6 moles of HCl were involved in total. From the equation of HCl's reaction with Al2O3 and the moles of Al2O3 that we calculated above, we can calculate the moles of HCl that reacted only with Al2O3. Once we get that, we subtract it from the total moles of HCl that reacted and get the moles of HCl that reacted with MgO. We will then use the equation of the reaction of HCl with MgO and the moles of HCl that reacted with MgO that we would have just calculated to get the moles of MgO. Let's do it this now:

From the equation of Al2O3 and H+, each single HCl gives a single H+ ion, so in these equations we can treat H+ the same as HCl.

6 moles of HCl are needed to react with 1 mole of Al2O3. Then how many moles of HCl would be needed to react with 0.1 moles of Al2O3 (as calculated above)? 0.1 x 6 = 0.6 moles of HCl reacted with Al2O3. Thus, 1.6 - 0.6 = 1.0 moles of HCl reacted with MgO.

From the equation of MgO and H+, 2 moles of HCl need 1 mole of MgO. So 1 mole of HCl would need 1/2 = 0.5 moles of MgO. Hence, our mixture contains 0.5 moles of MgO and 0.1 moles of Al2O3, option D.

 

[amina1300]

Q4: The point here is to be concerned with the nucleon number and proton number. They already told you above that in a nuclear reaction, both the nucleon numbers and proton numbers remain constant, i.e. if you add up the nucleon number of the reactants, the sum would be equal to the nucleon number of the product, and the same would be in the case of proton numbers. So in the second equation, the reactant nucleon number is 238 + 2 = 240 while proton number is 92 + 1 = 93. Hence, the product nucleon and proton numbers should be equal to 240 and 93 respectively too. They gave you only one option, J, and have 2 neutrons added to it. The 2 neutrons contribute 2 nucleon numbers and no proton numbers. So J must be an element that has 240-2 = 238 nucleon number and 93 proton number. Both A and B qualify for the nucleon number. But how do you know which one has a 93 proton number? Go to the Periodic Table and if you search hard enough, you'll find that in the f-block, (the bottom two rows) we have Np and Pu. Np has a 93 proton number so A must be the answer. I'll do the rest and post them as I type them out. Hope I helped.

wow !!!!! THANKYOU SO MUCH!!

 

[anastasia grey113]

http://justpastpapers.com/cie-9701-...question-paper-and-mark-scheme/9701_m17_qp_12 in Q39 why would propan-2-0l give yellow ppt with iodine?

it gets oxidised to propanone by NaOH first
this propanone formed gives a positive test

 

[Metallic9896]

wow !!!!! THANKYOU SO MUCH!!

Haha no problem at all, happy to be of help.

 

[darks]

http://justpastpapers.com/cie-9701-...question-paper-and-mark-scheme/9701_m17_qp_12 in Q39 why would propan-2-0l give yellow ppt with iodine?

CH3CHOH/CH3CO branch (Or should i say group?) will always give a yellow ppt.

 

[Zaki ali asghar]

Sorry to post it here but can someone explain why is option A right? for Q7 papers.gceguide.com/A%20Levels/Physics%20(9702)/9702_m16_qp_12.pdf

 

[Metallic9896]

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_11.pdf
Q4 I dont understand what Np Pu means??
Q6 ??
Q17 ??
Q30??

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_13.pdf
Q9 ???
Q21 ??
Q33 Statement 2 ; No particles have zero energy at either temperature. ?? What does this mean in context?

Q17: You have to mentally or physically make a note of the equation of hydrochloric acid and calcium carbonate.

2HCl + CaCO3 = CaCl2 + CO2 + H2O

In this question, 36/1000 x 0.5 = 0.018 moles of HCl reacted with CaCO3.

As per equation, 2 moles of HCl reacted with 1 mole of CaCO3. How many moles of CaCO3 react with 0.018 moles of HCl then? That turns out to be, 0.018/2 = 0.009 moles of CaCO3. How much is the mass of 0.009 moles of CaCO3? Moles = mass/Mr. So, 0.009 = mass/Mr of CaCo3 = mass/100. Mass = 100 x 0.009 = 0.9 grams. This rock contains 0.9 g of calcium carbonate, and itself the entire thing weights 2 grams. So % of CaCO3 = 0.9/2 x 100 = 45%. Answer should be B.

 

[Metallic9896]

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_11.pdf
Q4 I dont understand what Np Pu means??
Q6 ??
Q17 ??
Q30??

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_13.pdf
Q9 ???
Q21 ??
Q33 Statement 2 ; No particles have zero energy at either temperature. ?? What does this mean in context?

Q30: At first it seems like, "Okay all C=C double bonds means cis trans isomer!" But no. Start by the extreme left C=C. The left C in that C=C has two CH3 groups attached to it, so it doesn't qualify for cis-trans isomerism. The only other C=C is in the middle so that gives us 2 cis-trans isomers. Options C and D are ruled out. Only question now is how many chiral centres. As far as X is concerned, when Br2 is added the C=C will turn into Br-C-C-Br. Draw it out on the diagram yourself. You will find that, from the extreme left, on the main chain, the third C will become a chiral centre, the sixth C will become a chiral centre, and the seventh C will become a chiral centre. Basically, out of the four C involved in the first C=C and the second C=C, the left most WON'T be a chiral centre, while all the other 3 C will be. Answer will be A.

 

[Metallic9896]

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_11.pdf
Q4 I dont understand what Np Pu means??
Q6 ??
Q17 ??
Q30??

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_13.pdf
Q9 ???
Q21 ??
Q33 Statement 2 ; No particles have zero energy at either temperature. ?? What does this mean in context?

Now to variant 13.

Q9:

Kc = [products]/[reactants] (each raised to appropriate power from equation)

For this equation:

Kc = [NH3]^2 / [N2] x [H2]^3

They gave you equation, Kc, and conc. of reactants, so:

2 = [NH3]^2 / [2] x [2]^3

2 = [NH3]^2 / 2 x 8

2 x 2 x 8 = [NH3]^2

32 = [NH3]^2

sqrt 32 = [NH3]

Answer should be C.

 

[Metallic9896]

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_11.pdf
Q4 I dont understand what Np Pu means??
Q6 ??
Q17 ??
Q30??

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_13.pdf
Q9 ???
Q21 ??
Q33 Statement 2 ; No particles have zero energy at either temperature. ?? What does this mean in context?

Q21:

These are the 3 isomers I drew up. You should be able to do this. Draw it out on the question paper if you have to. The original molecule was CH2=CHCH=CH2, and you had 2 HBr molecules you could add. So they could be added in this fashion (same order as drawn above) (From left most C to right most C):

H then Br, then H then Br
OR H then Br, then Br then H
OR Br then H, then H then Br

(there is no Br then H, then Br then H cause that's the same as the first one, just a mirror image, hence why total 3 isomers).

In these 3 the first 2 contain chiral centres as I've underlined.

 

[Metallic9896]

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_11.pdf
Q4 I dont understand what Np Pu means??
Q6 ??
Q17 ??
Q30??

http://papers.gceguide.com/A Levels/Chemistry (9701)/9701_s16_qp_13.pdf
Q9 ???
Q21 ??
Q33 Statement 2 ; No particles have zero energy at either temperature. ?? What does this mean in context?

Q33: As anastasiagrey113 said, although going by the graph it looks like there is a o point at the origin. Still, that's what "appears" to be the case like when you see a curve that almost becomes and appears to become horizontal but never actually is horizontal if you zoom it up. Similarly, I believe, you can zoom in as much as you want to the least energy possessing particle but still discover that it has SOME energy in it. The only point when a molecule has no energy whatsoever is absolute O K, for which as Wikipedia says gorgeously, "Absolute zero cannot be achieved". So here you even have some temperature probably much higher than absolute 0, so of course there'll be molecules that have a very low energy, but energy nonetheless. Nothing will have 0 energy.

 

[amina1300]

Q30: At first it seems like, "Okay all C=C double bonds means cis trans isomer!" But no. Start by the extreme left C=C. The left C in that C=C has two CH3 groups attached to it, so it doesn't qualify for cis-trans isomerism. The only other C=C is in the middle so that gives us 2 cis-trans isomers. Options C and D are ruled out. Only question now is how many chiral centres. As far as X is concerned, when Br2 is added the C=C will turn into Br-C-C-Br. Draw it out on the diagram yourself. You will find that, from the extreme left, on the main chain, the third C will become a chiral centre, the sixth C will become a chiral centre, and the seventh C will become a chiral centre. Basically, out of the four C involved in the first C=C and the second C=C, the left most WON'T be a chiral centre, while all the other 3 C will be. Answer will be A.

Thankyou so muchh again!! I have a Q though can the Br be attached to the one end of broken C=C with 2 methyl groups attached (leftmost side) and the other Br at other end of the broken C=C at the same time? [ One as markovnikov's rule product and one not?]

 

[Metallic9896]

Thankyou so muchh again!! I have a Q though can the Br be attached to the one end of broken C=C with 2 methyl groups attached (leftmost side) and the other Br at other end of the broken C=C at the same time? [ One as markovnikov's rule product and one not?]

Wow, nice question. Well that is precisely what happens when you just add Br2 and not HBr. In Br2 since the only atom we have is Br, both Br are attached to the same C=C carbon atoms. However, when we have an HBr, the default programming is that the HBr will split as H+ and Br-, and because C=C is electron-rich and negative, Br- will automatically be repulsed and H+ will attach to one C from C=C, leaving the other and ONLY C open for Br. Even if you have two HBr, the MOMENT the HBr splits, the Br- from both HBr is going to be repelled by the C=C, and when it is able to come back, it's ONLY when one C is occupied by an H, leaving no possibility for 2 Br to attach to the same C=C pair. I hope this made sense. Basically, in HBr addition, Br will only exist as Br-, and Br- ain't got no chance with C=C unless HC already forms. And you're welcome!

 

[Metallic9896]

Thankyou so muchh again!! I have a Q though can the Br be attached to the one end of broken C=C with 2 methyl groups attached (leftmost side) and the other Br at other end of the broken C=C at the same time? [ One as markovnikov's rule product and one not?]

With the Br2 alone, there isn't much of a Markavnikov's rule going. They are all just going to saturate the C=C. And you need not worry about the mechanism if it isn't clear cause it's p1 tomorrow so probably nothing like that will show up.

 

[DeadbeatCIE]

Anyone?

 

[Thelastmoment]

Anyone?
View attachment 62475

KNO3 = 101.1g contains one N
CO(NH2)2 = 60g and 2N

So for it to have same mass of N as in KNO3 the mass of CO(NH2)2 must halve so you have 30g

Hope this helped

 

[amina1300]

Please post the hardest MCQs and Answer (A,B,C,D )from years 2014 and 2015 pleaseeeeeeeee!

 

[SohaibButt]

H always has a +1 state
so here it will be +1 too

n also recall the Cl reaction with NaOH
it makes NaClO right?
Cl has +1 state n Na has +1 too

HClO is a compound just like it
Cl reacts with water to make HClO in which Cl has +1 oxidation state n H also has +1
this can be found in the bleaching properties of Cl section
bleaching is in our syllabus

Exactly!

 

[SohaibButt]

Anyone?

 

[amina1300]

It was tough paper *sigh*

Can you tell the answers too plz.

25 B?
The one with Si02 and CO2 --> A?
Q32 D?