Another morning, another booklet, another series of codes sent:
| Q1-10 | CDDAA CBDBA |
| Q11-20 | BABDA DAADB |
| Q21-30 | BADBD ADDDA |
| Q31-40 | CDBCB ABCCA |
Note: After staring long and hard at the ang moh anomaly that is Q22, I’ve decided to change its answer to A. Reason being that scrap steel, containing a higher percentage (98%) of iron, will lead to a better yield and reduce mining wastes when iron is produced from it via recycling. The highlighted portion would definitely qualify as a correct reason if only it had been included in the original statement – instead of sparking intense debates like this 🙁
Do they tally with yours?
Update: Once again, the list of workings and explanations for each of the answers (where applicable) have been compiled (along with the questions at the end)!
You may access here by clicking the 
button (if you haven’t yet done so).
As usual, hopefully this will help in some way to settle some of the debates regarding some of the answers (and hopefully not create more in the process!)
87 Comments
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Could you explain question 8, 22, 24 and 28? Lol
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@Lucius: In response to Lucious,
For qn 24, since pH is very low, soln is highly acidic, so high concentration of H+ ions. But there will be still some OH- ions because water has small amount of H+ and OH- ions. ( because its a aqueous solution)
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35/40... Is that bad?
Can you kindly explain 9,22 and 35? Thanks!
Cheers!
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@Gary Lee: qn 35 unsure....i gt A as ans
1st and 2nd option confirm correct. but 3 and 4 donno. wont the molecules formed from reaction with bromine, hydrogen have different structural formula, even though molecular formula would be same since its a addition reaction.
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@Dannie: Haiz D= D= D= D= D=(smiley spam)
Unluckily No matter where the double bond is, add hydrogen= become alkane.
So ya become butane, XD, no difference liao ba...
So, the answer is B
D= D= D= D= D=
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@Gary Lee: for qn 9,
No of mol of h2o= 27/18 mol
No of mol of NH4NO3=27/18 x 1/2 ( because molar ratio of NH4NO3:H20=1:2)
Mass of NH4NO3=no of moles x Mr=27/18 x 1/2 x Mr of NH4NO3= 27/18 x 1/2 x 80= 27/36 x 80 ----------> thus, (B)
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dont agree with qn8; heres my working:
% of C= 72/222 x 100=32.4%
% of H = 18/222 x 100=8.1%
% of Si= 84/222 x 100=37.8%
% of O=48/222 x 100=21.6%
thus, Si has highest % by mass.
so answer should be D rite?
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@Dannie: ps sry see wrong but thats the explanation
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I think the answer for question 3 is B...I mean, ammonium salts are heated with alkalis will give off amonium gas, isnt it right?
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@April Nguyen: April Nguyen
question 3 should be D. This is because the question said that ammonia is "only" given off after aluminium foil is added to the hot solution. If the aqueous salt was ammonium nitrate, ammonia gas would have been given off the moment sodium hydroxide was added to it. ( This is QA). Thus, it can be seen that the aqeous salt has nitrate as it gives off ammonia when aluminium foil is added but cannot be ammonium nitrate because ammonia would already have been given off when sodium hydroxide was added. Hence, the only other possible nitrate would be sodium nitrate.
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and isnt the answer for 17 is D? I mean, sure, metals from Group I have decreasing melting point down the group but not all alkali metals are from Group I?
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@April Nguyen: group I elements are referred to as alkali metals. check page 289 of chemistry matters textbook if you have it.
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@anonymous: oh, thanks T.T gosh stupid mistake lah...do you know how much i should get in order to gain A1 from the Chem paper? like, how much was the score for A1 last year?
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@April Nguyen: When Group I metals react with water, they form soluble hydroxides, not oxides.
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my mark is 38/40 this time round but i think i might be in danger because alot of people found the paper quite easy. could you explain question 22 on the iron and steel thing. why is the answer not A? option 2 seems quite relevant. scrap steel does have a higher percentage of iron than iron ore and hence it has to be recycled to so call "conserve the iron" or something? can explain? thanks!
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y qns 35 is b???
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@shivaanand: - Since both molecules consists of the same number of C atoms, they both will form the same number of carbon dioxide molecules, and hence the same moles
- Both will decolorise bromine water due to the double bond between C atoms
- Because the double bond is situated between the 1st and 2nd C CH2=CHC2H5 and the double bond is situated between the 2nd and 3rd C atom of CH3CH=CHCH3, they form different compounds as the Br atoms bind to different C atoms.
- Both compounds become butane after reaction with H
- Both compunds form different polymers when polymerrised since the double bond is situated between the 1st and 2nd C CH2=CHC2H5 and the double bond is situated between the 2nd and 3rd C atom of CH3CH=CHCH3
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@Little Miss Loi: i think i get it. thanks
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is there a possibility Q31 is A?
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@Goh Rui Yi: For the reaction of ethanoic acid with sodium carbonate, the end salt product is CH3COONa. So you can see from the product that the O-H in COOH was broken and Na took the position of H. It can't be A because ethanoic acid CH3COOH and the product is CH3COONa so all the C-H bonds actually still remain.
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@anonymous: i replaced magnesium with sodium just for an example. However, if you go to page 424 of your textbook, there is an example of a reaction between ethanoic acid and magnesium. From there you can see that the O-H part disappeared.
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@anonymous: thanks alot , damn i only got 31/40! is it stupid to think about an A1 now? My written paper is around 75% too
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@Goh Rui Yi: don't be too worried. you should enjoy your break and wait till the results are out. who knows? you might just about get an A1. what is done is done and there is no turning back so look AHEAD and stay positive!
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Can someone please explain qn 22 35 and 37? Shouldn't 37 be D?
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Hi can you explain q3,5,12,22? For q3 why is it not ammonium nitrate since ammonia is given off? For q5 shouldn't the electrons become lesser cuz electrons are used to carry charges? For q12 why is it a and not c? They intersect at a point for volume of gas doesn't mean they are same rate isnt it? Doesnt rate refer to the gradient of volume time graph? For q22 I chose d as I thought recycling would not produce less co2 as the co2 produced is due to the reaction of coke with oxygen and not the purity of iron. Thank you!
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I got 36 only. Is that bad?
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can u estimate the belll curve tis time for a1?
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@LM: if you got 35 and above for your MCQ and possibly about 60 something for the paper 2 you should be quite safe already. don't have to worry so much... just wait for next year when the results are out. enjoy the break first! After all, we can only guess but cannot tell for sure what we will get. 😀
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@anonymous: Hmmm...true I think the bell curve would be slightly higher than last year
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yea can someone pls explain Qn 22,
I put my ans as 1,2,3 (option A) I know 1 and 3 is correct cos it's in the TB , but im not sure about 2!
um can someone explain qn 35 too? i put option a too :O
i know for qn 37, the ans is B cos since the number of carbon atoms increases, its harder to burn THUS less flamable. it becomes more viscous as number of carbon atoms increase! so D is wrong.
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why qn4 ans is not B? and qn19, why is th ans not B?
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@unknown: i thought valency electrons are electrons used in bonding. across a period, it will increase up to +4 and then decrease. so i think the answer should be A, increase in electrons across a period.
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@rainie ong: i know le.. another careless mistake of mine.. Argh! 🙁
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@unknown: D= superscript/subscript not working
Q4
Formula of the ionic compound X2Y, this means 2 X ions and 1 Y ion.
By the converse of the cross-multiplication method.
The formula of X ion is X+, Y ion is Y2-
Then work it out.
Q19
hello it's across a period, how does the number of electron shells increase across a period!!!
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i hope i can get a1, had 38/40 for this mcq but im not rly confident for paper 2. found this years papers harder than last years ones 🙁 quite a few weird qns i never see before! the qn 22 esp.
depending on chem + bio . my physics cmi one hahahah
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Hi can you explain q5,12,22? For q5 shouldn't the electrons become lesser cuz electrons are used to carry charges? For q12 why is it a and not c? They intersect at a point for volume of gas doesn't mean they are same rate isnt it? Doesnt rate refer to the gradient of volume time graph? For q22 I chose d as I thought recycling would not produce less co2 as the co2 produced is due to the reaction of coke with oxygen and not the purity of iron. Thank you ^^
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@2: according to my friend,
for qn 5, ans is A cos number of electrons must match the protons! and it doesnt become lesser over time. im not sure if this is the right explanation but i also put my ans as A.
for qn 12 i know its A definitely becos i did that qn so many times in exam papers.... they are at different rates cos exp 1 has a steeper gradient than exp 2! different rates of reaction but same vol of gas produced. so exp 1 takes less time for the reaction to end.
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@2: Q5
The incoming electrons from one end of the metal displaces electrons at the other end of the metal, period.
Q12 excellent explanation by X. above
Q 22 my opinion is A(deviation from miss loi)
1. The blast furnace process produces a lot of CO2 as CO is used to reduce hematite, and a lot of heat is needed, fuel burnt = more CO2. Iron products already have a high percentage of iron, such as steel, see no. 2
2. Scrap steel does contain a higher percentage of iron than iron ore, iron ore is mostly earth!
3 is always right.
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@SQhi: option 2, though economically benficial, would not be a appropriate option here as the question seems to ask for more environment-related issues for recycling.
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@Dannie: In life we should know how to zoom in and zoom out.
I think we should approach this question from multiple angles...
The question asks,
Which statements are correct reasons for recycling iron?
Aren't both environmental and economic reasons viable reasons?
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why is qn15 A? mine is D..
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@unknown: for qn 15,
lead II ions react with sulfate ions to form insoluble lead II sulphate..
Zinc sulfate and potassium sulphate(SPA salt) are soluble.
so should be A
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Hi! May i ask whether for question 39, can the answer be A?
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@린젠레: Ammonia is an alkali so it reacts with an acid. So answer is C
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Hey can i ask out of curiosity why is qs 22 the part about scrap steel having a higher percentage of iron is not a reason?
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Can you explain why the ans. for 22 and 35 isn't A?
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@Daniel & W: For Qn 22, if you go and calculate the mass percentage of iron in Fe2O3, you'll realise that the % is higher than the 40% that is contained in scrap metal.
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@린젠레: The percentage of iron in scrap steel is really 40%?
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@린젠레: I asked my Chem tutor.
Then my chem tutor says Scrap metal contains more Iron than Iron Ore!! :O
So isn't the ans suppose to be A?
*crosses fingers.*
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@린젠레: I thought iron present in steel is 98% and the remaining 2% are carbon and other elements.
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@린젠레: @the person who replied my comment
How'd you know that the percentage of iron in scrap steel is only 40%?
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@린젠레: Although I think although the fact that 2 may be a correct reason i think it isn't the reason for recycling
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@Daniel: Let us assume that we have some muddy soil and a tub of fresh water obtained from filtration of the muddy soil. Since the fresh water has a higher concentration of water than the muddy soil, you rather "recycle" it than take more muddy soil and try to filter it to obtain water. Right?
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can u explain qn 19?thanks.
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Hi may i ask why question 28 is D? can it be A since they asked for fossil fuels? CO(carbon monoxide) is from incomplete combustion of fuels that contain carbon i thought? and the SO2 was from incomplete combustion of fossil fuels/ volcanic eruptions?
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@Alissya: Hello! I actually wrote the same answer as you. But i realised that sulfur dioxide is not necessarily formed from incomplete combustion of fossil fuels but rather, it is formed from the combustion of fossil fuels. ( This is found in textbook under atmosphere and the environment). I guess carbon monoxide is formed from incomplete combustion of carbon-containing fuels such as petrol and petrol is a constituent of petroleum which is a fossil fuel. I am not very sure but the question is weird. Why couldn't they have asked us "Which of the following products are formed from incomplete combustion of petrol in cars?"
That would have been much easier and understandable.
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@anonymous: can someone explain question 5?
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@Dannie: this is what i think and i am not sure. when a current flows, the electrons displaced at one end of the metal should result in an equal displacement of electrons from the other. when the current is switched off, the electrons are no longer displaced so they probably remain at their normal positions as they should be. As for the positive ions, they are not mobile and so do not move.
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@anonymous: think i get it. thanks
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@Alissya: The question is very clear =(
c8h18, unburnt hydrocarbon.
CO, produced when oxygen supply is insufficient, leading to incomplete combustion.
NO2, produced due to high temp. in motorcar engine.
SO2, by product of combustion of fossil fuel.(nt related to incomplete combustion)
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@SQhi: but C8H18 is not a gas. Shouldn't it be in liquid??
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@Bell: But then C8H18 exists as a liquid only in room temperature. In the ignition chamber it's nt room temperature. Under high temp it could vaporise to become gaseous state
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@Bell: You are right that octane is a liquid. However, given the high temperatures in the car engine, the octane is most likely to be released as gas. The boiling point of octane is 125.52 degrees celsius and the temperature in the car engine is much higher than that.
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@SQhi: Although sulfur dioxide is formed from complete combustion, it is still formed during incomplete combustion of fossil fuels. Secondly, carbon monoxide is formed from incomplete combustion of carbon containing fuels such as petrol right? But petrol is not a fossil fuel! Petrol is formed from fossil fuel.
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@anonymous: *takes a deep deep breath*
Alright. I think at this point of the discussion it is more useful to quote evidence. You may use Ctrl+F to validate the existence of the quote.
Point: Fossil fuels have sulfur content.
"When these fossil fuels are burned, the sulfur contained in them reacts with oxygen from the air to form sulfur dioxide (SO2)."
Source: http://www.chemistry.wustl.edu/~edudev/LabTutorials/Water/FreshWater/acidrain.html
Point: Petrol is a fossil fuel, it consists of hydrocarbons extracted from CRUDE OIL, hence incomplete combustion may lead to production of CO or C
"The incomplete combustion of fossil fuels results in the formation of carbon monoxide and soot (carbon)."
Source:http://www.chemicalformula.org/reactions/combustion
Anyway guys, time to focus on bio now, please!!!
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Hi, isnt question 22 answer be A instead of C? Because Steel contains 98% iron and 2% carbon?
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@Joseph: yeah! That's what my friends and I wrote too! In iron ore, iron should be about 40% but in scrap steel iron is about 90-99% i think. However, I think the trick in the question is probably about whether point 2 is relevant to the recycling process. The fact that it has more iron may not be relevant to recycling. Once again, I must say that I am not sure if this is the reason.
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@anonymous: i think option 2: scrap steel contains higher % of iron than iron ore
is more of a economical advantage of recycling, rather than environmental. It seems like the question wants more environment-related reasons
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@Dannie: But scrap steel with a higher percentage of iron can help reduce the amount of iron needed to be extracted thereby reducing the risk of depleting our resources. Furthermore, as stated on pg 241 of the textbk, it states that recycling steel is beneficial to the environment.
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hello:)
can someone pls explain Q28
cause i thought that C8H18 (octane) is a liquid and not a GAS resulting in incomplete combustion?
so why is the answer D?
and can it be B? lols
pls and thankyou:)
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@XOXO: B nope.
NO2 is due to high temp. in ignition chamber.
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@XOXO: Haha i put the same answer as you cause I thought the same way as you too. C8H18 is a liquid and they asked for "gases". I asked my Chem teacher and she said its possible to be CO and NO2 so i have my fingers crossed
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@Bell: haha thanks thanks 🙂
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@XOXO: for question 18,
incomplete combustion produces carbon monoxide and unburnt hydrocarbons, thus D
cannot be B becos NO2 is formed due to nitrogen and oxygen combining due to high temperatures created by complete combustion of fuel.
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Can someone upload the question paper? Cos our school haven return our question paper ...
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@Elaine: No!!!
1.The copyright belongs to Cambridge.
2.It is illegal, see no. 1
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shouldn't question 9 be option D?
Shouldn't question 19 be option A? (how could the last element in the period -noble gas have an increase in valency electron?)
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@crapper: for question 19,
just remember that valence electrons determine chemical properties of a substance, thus it tells whether it is mettalic or not.
thus,D
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i think i ald lost 1-2 marks: qn 5,35
paper 2 was ok but im afraid i might lose marks esp becos i did the organic chem qn, the calculations part
hopefully can get A1/A2:)
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can miss loi post the full soln ans, + qn pp just like physics, thanks very much.
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@marc: Look up look up ^^
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erm anyone has combine biology answer for the mcq link?
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any answers for 2011 bio o level paper?
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Qns 5 , very disputed. Some say A some say D. Can someone explain to me with a solid answer ? thanks in advance.
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@skylar: I believe those who chose D are looking only at the positive ions and electrons on the original diagram that have subsequently flowed to one side of the metal.
While the number of electrons in the metal remains constant due to the conservation of mass, the metal being part of a closed circuit will have more electrons coming from the circuit to take the place of those who have flowed on from one side to another, so the electrons cannot be permanently held at one end only.
Furthermore with the current switched off, the arrangement of the electronics should return to what it was like before the current was switched on i.e. the original diagram!
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Folks, after staring long and hard at the ang moh anomaly that is Q22, I’ve decided to change its answer to A. Reason being that scrap steel, containing a higher percentage (98%) of iron, will lead to a better yield and reduce mining wastes when iron is produced from it via recycling.
The highlighted portion would definitely confirm chop stamp qualify as a correct reason if only it had been included in the original statement – instead of sparking intense debates all over SIngapore 🙁
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@Little Miss Loi: YAHOO!!!