Chemistry: Post your doubts here!

[Metallic9896]

Metallic9896 tips!!! Especially for timing! i seem to take extra 10-15 minutes for chem!

Haha, scary thing is I suck at Chem timing too. The best tip I can give you is what I follow myself. Don't think about revising the paper. Do it with such precision, care, and attentiveness that no need arises for a recheck. I put all my effort in the question I'm doing and once I'm done I forget about it, cause Chem questions are time taking on their own anyway, so one might as well put all of their energy and do it right the first time. If any question seems to be utterly difficult and takes more time than the average question, mark it with your pencil and come back to it and only it later. Forget the rest. Also, leave the isomer questions for the end. They take so much time and at least I tend to mess them up anyway so huge time waste.

 

[anastasia grey113]

Anyone?

answer is A
cuz since O has an oxidation of -2 and H of +1
Cl should have an oxidation of +1 in order to make a total of zero.
so as there are two elements i.e. H and Cl with same oxidation states i.e. +1 the answer should be A.

i hope this helps
if m still not clear u can let me know

 

[darks]

Haha, scary thing is I suck at Chem timing too. The best tip I can give you is what I follow myself. Don't think about revising the paper. Do it with such precision, care, and attentiveness that no need arises for a recheck. I put all my effort in the question I'm doing and once I'm done I forget about it, cause Chem questions are time taking on their own anyway, so one might as well put all of their energy and do it right the first time. If any question seems to be utterly difficult and takes more time than the average question, mark it with your pencil and come back to it and only it later. Forget the rest. Also, leave the isomer questions for the end. They take so much time and at least I tend to mess them up anyway so huge time waste.

I also seem to mess up isomers a lot! thanks for the advice : p we now have a few hours to discuss q's. I have decided to also revise the entire syllabus. So that will take around 3-4 hours too.

 

[Metallic9896]

I also seem to mess up isomers a lot! thanks for the advice : p we now have a few hours to discuss q's. I have decided to also revise the entire syllabus. So that will take around 3-4 hours too.

Yeah sure I'll be on and off here too. Good luck man! I recommend especially periodicity equations of group 17 and the general oxides etc. as well as the AS Organic equations conditions etc. it's always good to revise them.

 

[SohaibButt]

I

answer is A
cuz since O has an oxidation of -2 and H of +1
Cl should have an oxidation of +1 in order to make a total of zero.
so as there are two elements i.e. H and Cl with same oxidation states i.e. +1 the answer should be A.

i hope this helps
if m still not clear u can let me know

Tysm i think we can take hydrogen oxidation state -1 right?

 

[anastasia grey113]

I

Tysm i think we can take hydrogen oxidation state -1 right?

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

 

[Metallic9896]

I

Tysm i think we can take hydrogen oxidation state -1 right?

H only exists as -1 in very rare circumstances. Not in this one. Here's a tip. If anything is joined to a more electronegative atom, the more electronegative one will take the cake and get the negative oxidation state. For example, N is more electronegative than H so in NH3, N is -3 and H is +3. O is more electronegative than N, so in NO, O is -2 and N is +2. F is the most electronegative, more than O. So in F2O, O is +2 and both F are -1 each. H is only -1 when it is more electronegative than the atom it is attached to or is bonded ionically too, and that is in the case of metal hydrides. THERE, in NaH for example, Na is +1 while H is -1. But normally H is +1. So for metal hydrides it has -1 but otherwise it's +1.

 

[amina1300]

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?

 

[anastasia grey113]

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?

in q4 they r just metals neptunium n plutonium
this questions like radioactive decay in physics

n in q33 paper 13
it means that no particle is stationary
all particles have kinetic energy to some extent n they r moving n colliding
so its correct

 

[Zaki ali asghar]

Did someone else too found March 16 and 17 chem p1s really really easy?

 

[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?

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.

 

[anastasia grey113]

Did someone else too found March 16 and 17 chem p1s really really easy?

i scored a 38 in m17
havent solved 16 yet

 

[Zaki ali asghar]

i scored a 38 in m17
havent solved 16 yet

Got 37 in both,i hope some questions from past papers are repeated tomorrow. :/

 

[Zaki ali asghar]

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?

 

[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