Sometimes i really?wonder?on the?methods used by students to revise their academic subjects.
Over the years, i have received hundreds of emails enquiries from students asking for help in Chemistry questions.?I am definitely glad to help them sought out their correct concepts on Chemistry Theories, but certainly not by answering any questions posed to me, without even given me their answers, workings, reasoning or thoughts.
Learning is about acquiring the knowledge and applying them , be it in tests, exams, projects or as a working adult.? Just by trying to get?all answers to all the questions they have come across?(without going through their own thoughts)?does not mean they are going to Score the definite As in Exams!
Recently, i received severals emails from students asking me to supply them with answers asap to any questions that i have posted in this website. I will definitely share my suggested answers and reasonings/workings, but i believe the best way YOU CAN LEARN is to participate (by having your own answers and reasonings) before asking?for the correct answers.
Just by reading does?NOT allow?you to master something, especially in academic Chemistry! So if you a student preparing for your O Level Chemistry Paper 1 in less than 10 days time, i would strongly urge you to try out the questions on your own first.
Here is a set of MCQ questions on Qualitative Analysis to prepare you for Chemistry Paper 1:
Question 1:
Which ion, in aqueous solution, forms a white PPT when dilute sulphuric acid is added?
A. Ag+(aq)
B. Ba2+(aq)
C. Cu2+(aq)
D. Fe2+(aq)
Question 2:
Which gas is evolved when an alkaline solution of sodium nitrate is warmed with aluminium?
A. Ammonia
B. Nitrogen
C. Nitrogen Dioxide
D. Oxygen
Question 3:
A colourless gas is denser than air. It neither burns nor supports the burning of a splint. What could be the gas?
A. carbon dioxide
B. chlorine
C. helium
D. hydrogen
E. oxygen
PS: Rmbr to try out the questions using the knowledge that you know
im from Pakistan….and im doin GCE o-levels….yeah thanx alot its all clear now….Infact i gave dat explanation dat u have given here to my skool teacher n she agreed to it….Thanx to u dat now all my classmates know da right answer…
Hi zainub,
Where are you from? Singapore Secondary School?
In GCE O Levels Syllabus, we will calculate the MR of the gases and compare to check their density (lightness).
You are right that main gases in AIR is nitrogen (N2) and oxygen (O2), with a percentage composition of about 78% and 21% respectively.
MR of N2 = 28
MR of O2 = 32
Average MR of N2 + O2 in AIR = (78/100 * 28) + (21/100 * 32)
= 21.84 + 6.72
= 28.56
If you compare it to the MR of Carbon Dioxide (CO2) of 44, you realised that AIR is “lighter” than carbon dioxide.
Hope the above explanation is clear to you.
Let me know how you think.
To Your Academic Success in Chemistry.
Sean Chua
Master Trainer & Author
WINNERS Education Group
?Experience Learning with A Difference?
I am a bit confused. My chemistry teacher has taught us that Carbon dioxide is lighter
than air. Air has Mr greater than 60. The main gases in air are Nitrogen and Oxygen
and if we calculate their respective Mr they sum up to (28 + 32 = 60) 60 where as
the Mr of Carbon dioxide is 44. 44 < 60 therefore, carbon dioxide is lighter than air…
Yes, I am taking my O Level Chemistry Exams this year, however, I’m taking the IGCSE syllabus, not the GCE Chemistry syllabus. It’s roughly the same with just a few minor differences.
Thanks for the comment and I would try my best to improve myself at Chemistry.
Hi everyone,
Perhaps it will be good for me to sum all these up in one comment.
For MCQ question, we should use the Elimination Process to find the correct answer, based on information (clues) given in question.
Clue 1: Neither burns nor supports the burning of a splint
– Oxygen is eliminated since it supports burning of splint
– Hydrogen is eliminated since it burns in oxygen to form water vapour
Clue 2: Colourless Gas
– Chlorine gas is eliminated since it is a yellow greenish gas
Clue 3: Denser than Air
– Do note that Air consists 78% of Nitrogen and 21% of Oxygen
– As such we can take the relative molecular mass of Air as the Mr of Nitrogen (which is 28) since the majority of Air is made up of Nitrogen
– For Helium, it has a relative atomic mass of 4, and thus has to be eliminated.
Answer: We are left with Carbon Dioxide which should be the answer.
Double-Check: Carbon Dioxide has Mr of 44, which is higher than Air (thus denser than Air).
This is the sequence that i always ask my students to used.
Hope it works for you too.
All the Best To Your Academic Success in Chemistry,
Sean
Master Trainer & Author
http://www.SimpleChemConcepts.com
i am so happy that all of you are learning something here.
Keep up the momentum. Just a few days more for your Chemistry Paper 1.
All the Best To Your Academic Success in Chemistry,
Sean
Master Trainer
http://www.SimpleChemConcepts.com
Hi Seng Chiy,
Are you taking your O Level Chemistry Exams this year?
Good concepts shown and i like the way you present your arguments and ideas.
Many students got confused when they see Air and wonder what is the relative molecular mass, Mr of Air.
Can’t agree more that when we asked for suggestions and answers from others, the best thing is still to determine the answer yourself. Make sure you know what is happening.
All The Best to Your Academic Success in Chemistry,
Sean
Master Trainer
http://www.SimpleChemConcepts.com
Hi “Hey”,
Good observation from what you have learned.
Contrary to many students’ belief that chlorine gas is colourless, it is indeed a yellow greenish gas.
PS: Beware when you check out older text-books and online source, where many still show chlorine as a colourless gas.
Cheers
Sean
Hi Sadiqin,
Glad you are benefiting from those questions.
Answers for Question 1 and 2 are correct, but for Question 3, you were tricked into making the mistakes.
Good to see your follow Chemistry students sharing their answers and thoughts on the process of selection.
Cheers
Sean
I’m sorry to have made such an error… Thanks for pointing it out! Luckily this isn’t the Os. I’ll make sure to remember this.
I forgot to elaborate on the point of the density of air.
The Mr of typical air at r.t.p. is 29.
Air is a mixture of mainly nitrogen and oxygen gas.
The Mr of nitrogen is 28 and the Mr of oxygen is 32.
78% of the air is nitrogen, 20% of the air is oxygen and the remaining gases are so minute that they are largely insignificant.
Therefore, you can take (78/100 * 28) + (20/100 * 32).
The answer should give you a value of 29, hence the Mr of air.
When you count the Mr of carbon dioxide, it is 12+16+16, giving an answer of 48. Since 48 > 29, carbon dioxide is denser than air.
While it is good that you actively ask others on things you don’t know, you should not believe what anyone says without checking it up yourself. Even if the source is supposed to be very reliable, like your teacher or in this case, the source being your brother, you should still double check to verify so that you will not make such mistakes in the exam.
Yeah, hey is right. Chlorine is yellowish green. I believe the right answer would be carbon as it is a colourless gas that does not aid the burning of a flame. In fact, it actually hinders the process of combustion. Furthermore, the point stated that it is denser than air. The only 3 gases that are denser than air would be chlorine, oxygen and carbon dioxide. Chlorine is out of the question as it is not colourless. Oxygen should also be cancelled off as it supports combustion. Therefore, by process of elimination, the only viable answer in there would be A. Carbon Dioxide.
chlorine is not a colourless gas
My answers are B, A and B respectively.
Barium Sulphate is the only product when sulphuric acid is added to all four answers which is insoluble and hence Ba2+ is the only one which forms a white ppt when sulphuric acid is added.
The second question I gather is quite standard, unless I’m wrong hahas.
The answer’s in the QT table yeah?
My answer is chlorine because 1) It’s the only gas which is heavier than air (my bro said since CO2 is in air, it technically isn’t heavier than air), and 2) it neither burns nor supports the burning of a splint.
if any of my answers are wrong for any question, please correct them ASAP =D and thanks for the questions. It’s good to flex my chem brain once in a while to make sure it’s all in there, though SS O levels is tomorrow hahas.