The aim of this series “Secrets to Score in Acids, Bases, Salts & Qualitative Analysis” is to help students to the understand the basic concepts surrounding these 2 topics: Acids, Bases & Salts and Qualitative Analysis, and to take a look at the type of questions that are usually associated with it in exams.
This blog-post will be the last post of this typical series and how about checking another typical type of exam-based questions in exams.
Question:
The following excerpts were taken from a student’s chemistry journal.A colourless gas P produced a white precipitate when bubbled into aqueous calcium hydroxide. Gas P was dissolved in water to form a colourless solution Q, which registers a pH of range of 5-6.
Solution Q was then added to a colourless barium solution, known as R. A white precipitate, S, in a colourless solution, T, was observed. Upon filtering, the colourless filtrate, T, was added into a test tube containing aqueous sodium hydroxide and Devarda’s Alloy. The gas evolved turns moist red litmus paper blue.
Identify the following unknowns P, Q, R, S and T. (5 marks)
PS: Leave your suggested answers in the “Comments Section” and we will come in to discuss on it real soon…
Hi, great website, do you mind adding more questions like these for practice?
Thanks,
Taya
Hi Simon,
Please check my earlier comments to other students.
I will suggest that you draw out a flow chart to see how the information given and linked from one to another.
You would need to have good knowledge of Tests for Cations and Anions though, in order to be able to comprehend that.
Cheers!
i am very very very tensed for mu o levels chemistry examination!i hardly know what to do,they are in 2011
gas p will be cabon dioxide
the solution Q will me carbonic acid
and i can’t do that further(look i just lost it in between)
that’s what happends with me! 🙁
Hi Raymond,
Well done. I live the way you present your workings towards getting the answers also.
Keep it up.
Cheers
Sean
Master Trainer
Author
P is carbon dioxide as it forms a white precipitate with limewater, which is calcium hydroxide.
Q is carbonic acid as that is the product formed when carbon dioxide is mixed with water.
R has to be Barium Nitrate as it is the only soluble barium out there in the syllabus. It could potentially be Barium Hydroxide, but the presence of T takes that possibility away.
S is barium carbonate, as that is the salt formed when carbonic acid reacts with barium nitrate, or any barium solution for that matter.
T is Nitric Acid, as well, I did not even bother thinking about this as the question stated that they had used Devarda’s alloy to test this and the result was ammonia was produced, so nitrate ions HAVE to be present, and therefore, it has to be nitric acid.
Well Done Gary.
You are spot on for this question.
For other readers that need more explanations on how gary came out with the answers, do drop us a comment so that we can revert back.
PS: Gary-Do share with us your working/thought process if there is someone that asks about it.
Cheers
Sean
I will try this.
P -Carbon Dioxide
Q -Carbonic acid
R-Barium nitrate
S-Barium carbonate
T-Nitric acid
I’m not sure if i am right.I know a NO3 ion is present but don’t know where to place it.
It also seems weird because if Barium carbonate is formed,it will react with nitric acid right?
=(
Gary