Hi everyone,
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Today i am going to discuss on some “tricky” exam-based questions that were sent in by my own student under our Chemistry Ultimate Leap Programme, for discussion below:
Hi Sean, I have a few chemistry questions which I am unsure about.
Question 1: on Properties of Gases
A sample of polluted air is bubbled through water. The pH of the solution is less than 7. What gas causes this?
A. Carbon dioxide
B. Carbon monoxide
C. Sulfur dioxide
D. AmmoniaSuggested Answer by Student X:
My answer is carbon dioxide because it is an acidic gas and would thus form an acidic solution. But the correct answer is C. Sulfur dioxide. Why?
&
Question 2: on Qualitative Analysis (QA)
Aqueous sodium hydroxide and aluminium foil are warmed with a substance Y. An alkaline gas is liberated. Deduce which anion Y contains.
A. Carbonate
B. Chloride
C. Nitrate
D. SulfateSuggested answer by Student X:
My answer is C, nitrate, which is correct. But I don’t understand why. My reason for putting C is because carbonate produces carbon dioxide which is acidic, which chloride produces chlorine which I know is not alkaline. Then I chose nitrate over sulfate through guess work, and hence I want to know why nitrate is correct.
Do you agree with the above answers by my Student X?
The best way to learn is to work the answers out on your own and prove to yourself that it is correct. I always tell my students: “Challenge me (nicely) if you think something i said is incorrect!” – cos i believe the best way to learn is to experience it yourself..do not believe everything others say.
Confucius famous quote: “I hear and I forget. I see and I remember. I do and I understand.”
PS: Work out the answers, and leave your answers (and reasonings) in the “LEAVE A REPLY” section below. I will come in to give my suggested answers soon.
For question 1, my answer is also C. Is it because Carbon Dioxide is only weakly soluble in water, unlike sulphur dioxide, which is vry soluble in water? I think question 1 is about choosing the best correct answer and not just the correct answer.
Question 2 definitely C. Ammonia gas would be given off. Ammonia is an alkaline gas.
Reply
sean Reply:
April 24th, 2009 at 2:21 pm
Hi “Dcma 1993″,
Question 1: Answer is (C) indeed. But your explanations is not correct. CO2 is soluble in water just like SO2. There is a “clue” in the questions. Read it again. And it is linked to the chapter of “ATMOSPHERE / AIR”.
Question 2: Answer is (C) indeed. That is the standard test for Nitrates Anion (NO3-). NH3(g) is produced which is an alkaline gas.
PS: Think about it, based on the comments i have written. Hear from you soon!
To Your Academic Success in Chemistry.
Sean Chua
Master Trainer & Author
WINNERS Education Group
“Experience Learning with A Difference”
Reply
[...] Again, these 2 questions were sent in via our own coaching students via email. I applaud him for taking actions in sending me emails to clear his doubts. This platform has been well received by our students over the years. (You can check the previous 2 questions by him over HERE) [...]
i read that air pollution question whose answer is “c” “sulphurdioxide” i want
to know why??
Reply
sean Reply:
April 24th, 2009 at 2:23 pm
Hi “hina”,
Good that you want to know why!
Now, i want you to think why the answer is (C) sulphur dioxide and give me your suggested answers.
The best way to learn is to participate…to seek the truth with your own thoughts.
PS: Look forward to hearing from you.
To Your Academic Success in Chemistry.
Sean Chua
Master Trainer & Author
WINNERS Education Group
“Experience Learning with A Difference”
Reply
Question 1 is simple.
Basically, there are 2 answers, that will indeed form acidic solution when bubbled in water. They are carbon diovide and sulphur dioxide. Both, will definitely form a solution with pH of less than 7. Then we look back at the question. it states “sample of polluted air “. We have to recognise the fact that carbon dioxide is only a greenhouse gas, and is NOT an air pollutant. However, sulphur dioxide, instead, is one of the air pollutant, that is poisonous, and can cause acid rain. there, since they wanted “polluted air”, it is therefore sulphur dioxide.
Question 2 the answer is C.
Radically, we have to know that this question is a test of what anion is present.
In here, we see that substance Y is our main test for this whole reaction, and sodium hydroxide is added. And Ammonia will be given out after the reaction is completed.
thereotically speaking, from all i know for OLVL syllabus, we just have to recognise this trend in our QA, and acknowledge, that this is a test of NITRATE ions.
However other than that, i would think that this is a redox reaction, as Nitrate ions will be reduced to nitrogen by aluminium, which will react with sodium hydroxide, to form NH3. This is only a suggested reason.
Reply
sean Reply:
April 24th, 2009 at 2:25 pm
Hi “huaan”,
This is a good set of explanations above.
Keep it up!
And i look forward to more participation from you.
Cheers,
To Your Academic Success in Chemistry.
Sean Chua
Master Trainer & Author
WINNERS Education Group
“Experience Learning with A Difference”
Reply
Great work! I understood it very well… Thanks
Reply
I read “Huan’s” comment after deciding my own answers and am glad to see that they did not differ from mine.
Sir, do u mind me asking a couple of questions?
Reply
sean Reply:
October 15th, 2009 at 12:50 pm
Hi Shahbuz,
Please go ahead and send me an email if you have chemistry questions that you would like to discuss with readers in the humble chemistry blogsite.
Thanks,
Sean Chua
Reply
I hope you say yes
Reply
Hi friends.Um a new student but facing same problems like u.
My question in which um struck is :
which of the following reacts with aqueous sodium hydroxide to produce salt?
1 calcium oxide
2 iron 2 oxide
3 copper 2 oxide
4 zinc oxide.
Plz solve it for me.
Reply
sean Reply:
May 6th, 2010 at 12:39 am
One of them is an AMPHOTERIC Oxide – it will react with both acids and alkalis.
Think about it.
Reply
Dears,here 1 more.
which equation represents chemical change caused by exposing a photographic film
to sun light?
1 Ag=Ag+ + e-
2 Ag+ + e- = Ag
3 AgBr=Ag+ +Br-
4 2Ag + Br2=2AgBr
Reply
sean Reply:
October 15th, 2009 at 1:34 pm
Hi Al Nasr Ahmed,
See my reply to Marcus & Jie below.
All the best to your Chemistry,
Sean Chua =)
Reply
Hey sean, I wanna try one reply no. 8 – Al Nasr Ahmed’s qn.
Is the ans zinc oxide?
Because it is am amphoteric oxide, thus reacting with both acid and alkali to produce salt (and water)
The other three choices are basic oxides(aka metal oxides), which will only react with acids to form salt (and water)
Is this explanation correct?
p/s just trying out
Reply
sean Reply:
October 15th, 2009 at 1:28 pm
Well Done!
I like your explanations on this question posted by Al Nasr Ahmed.
Keep up the good work!
Sean Chua =)
Reply
very well Jie
try out no 9
Reply
Marcus Reply:
September 30th, 2009 at 10:45 pm
To Jie:
Yes, zinc oxide is an amphoteric oxide, and reacts with the sodium hydroxide to produce salt and water.
The other options are not acidic oxides or amphoteric oxides, so they cannot react with the sodium hydroxide.
And for Q9, this is just a guess: photographic film contains silver, and since the film and sunlight have no bromine whatsoever, options 3 and 4 are out. It would be options 1 and 2 left.
Reply
sean Reply:
October 15th, 2009 at 1:33 pm
Hi Marcus,
Your reasoning for the question on Zinc Oxide is correct.
Keep it up!
For the question on photography chemistry, your steps in analysis is also spot on. Answer would be options 2 in which Ag+ ions has been reduced to Ag after taking in electrons.
Hope this information helps.
To Your Academic Success in Chemistry,
Sean Chua =)
Reply
I hope my answers are not too late. For question 1, CO2 will not be accepted because in our air, this is already a small percentage of CO2, so we do not consider it that acidic. SO2 is chosen because SO2 forms H2SO3 in water which is much more acidic than HCO3-, thus it is much safer to choose SO2.
As for question 2, I learnt it in class 2/3 years ago. It’s a reaction that is not required of O level candidates. (:
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Question 1)Ans is C;Though CO2 and SO2 both result for the acidity of acid rain,it is mainly because of SO2.When CO2 dissolve in water it will form carbonic acid and when SO2 gets dissolved in water it will finally form sulfuric acid.As sulfuris acid is much more acidic than carbonic acid,SO2 IS THE COORECT ANS I FILL…
Question 2:Ans C;Aluminium is an reducing agent.When Nitrates in NaOH solution are treated with Al Nitrates are reduced to Ammonia gas.This is a special test used to detect the nitrates called Devardas test.
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