A very interesting questions sent in by a local students.

Try this out and leave your suggested answers in the “Comments” section below this post.

Questions:
A sample of sea-water contains 0.1 g/dm^3 of bromide ions.

(i) What volume of this sea-water produces one mole of bromine molecules?

(ii) What volume of chlorine gas is required to produce 8g of bromine from this sea water?

Suggested Answers:

Below is a question contributed by a local chemistry student (Student X) and an avid reader of this Chemistry Blogsite.

This will be a question that combines Mole Calculations & Qualitative Analysis topics. Give it a try!

Question:
In an experiment, 4.0cm^3 of 1.0mol/dm^3 aqueous copper(II) sulphate was mixed with 8cm^3 of 1.0mol/dm^3 aqueous sodium carbonate:

CuSO4 + Na2CO3 –> Na2SO4 + CuCO3

What did the reaction vessel contain when the reaction was completed?
A. A colourless solution only
B. A green precipitate and a blue solution
C. A green precipitate and a colourless solution
D. A blue precipitate and a blue solution

Student X’s suggested answer:
What I think: copper sulphate is the limiting reactant, hence sodium carbonate is in excess. Hence, when the reaction is completed, only excess sodium carbonate remains. So, the solution remains colourless and my answer is C.

My Suggested Answer:
I agree that the Answer is C. At the end of the reaction, we will see a Green Precipitate of Copper Carbonate(s) which is insoluble in water. There will also be a colourless solution that is contributed by both the product Sodium Sulphate(aq) and the excess reactant Sodium Carbonate (aq) which are soluble in water. (Click the image below to have a clearer understanding)

PS: Check out “Comments” below for Suggested Answers.

Below is a question that was sent to my email by a local student that is preparing for Chemistry examination.

Question:
Gallium has a relative atomic mass of 69.8. If only two isotopes of gallium were found to have nucleon numbers 69 and 70 respectively, what is the percentage abundance of gallium-69 that occurs naturally?
A. 20%
B. 40%
C. 60%
D. 80%

This will involve some simple mathematical manipulation after you understand the concepts of Isotope and Isotopic Abundance.

PS: Check out “Comments” below for suggested answers

This one may require abit more thinking to solve the question. Past years preliminary examination questions from local school.

Question:

Which of the following has the same number of atoms as 6 dm^3 of chlorine gas?

a) 20g of argon

b) 0.25g of hydrogen

c) 23g of sodium

d) 8.5g of ammonia

Show your working and reasoning on how you derive to the answer.

40 more days to Singapore Mid-Year Examinations…Stay focused!

“We all fail.
But it is not failing that hurts.
What hurts is knowing that you didn’t give your best.”
~ By Andrew Matthews “Happiness in a Nutshell”

Sean

PS: Check “Comments” below for Suggested Answers.

Here is another question on Mole Concepts / Chemical Calculations for you to try. Leave your answer and working in the “Comment Section” directly below this post so that we can come together to discuss on it.

Example:
A metal compound has the formulae XCl4 and a relative formula mass of 261. What is:
a) The relative atomic mass of X? (ans: 119)
b) Metal X? (ans: Tin)

PS: “Either You Practice & Use It, or You Loose It!”

PS: Check “Comments” below for Suggested Answers.

This is a question that i picked up and shared with a student that i am coaching at the moment. This will be a question that tests on the different sub-sections that is covered in Mole Concepts and Chemical Calculations.

Example:
Hydrogen Fluoride attacks glass and is used to draw on glass. It is prepared by adding concentrated sulphuric acid to calcium fluoride. The reaction takes place at r.t.p: CaF2 (s) + H2SO4 (l) –> 2HF (g) + CaSO4 (s)

1) When 15.6g of calcium fluoride was reacted with 12.5cm^3 of 20.0 mol/dm^3 sulphuric acid, 7.68dm^3 of hydrogen fluoride was formed.
i) What is the limiting reactant in this reaction?
ii) Calculate the theoretical volume of hydrogen fluoride that should be formed.

2) Calculate the mass of the calcium fluoride in the impure calcium fluoride.
3) What is the percentage purity of the calcium fluoride?

Please work out your answer on your own first on a piece of paper, before you click on “Read More” or the Headline itself for the suggested answer. (more…)

I had recevied a couple of emails from Secondary 5 Normal (Academic Students) asking for help to cope for their ‘O’ Level this year. Transiting from completing the N Levels to preparing for the O Levels in Sec 5 is not an easy journey indeed. This is also mentioned by Beetsma, the Singapore Top O-Level Performer among the Sec 5 cohort for 2007 GCE ‘O’ Level examinations.

As mentioned by Beetsma, “the gap is very wide because they teach us more in a shorter amount of time. The classes were getting harder and harder and I had difficulties.” In recent weeks, they has been controversies of cases that Sec 5 students attempting O levels are being “discouraged” by their school principal to transfer to the Institute of Technical Education (ITE) as they are unlikely to do well at ‘O’ Levels.

Even though the road to ‘O’ Level from ‘N’ Level is not easy, i personally felt the principal should have been more tactful in his speech. Perhaps, the principal (and thus the school) should have a meeting/breifing with parents and students together and let them know the options that are available. I remembered in my early school days, such “talks” on Options that are available, have always been very smooth and positive.

Having said that, i strongly urge Normal (Academic) Students to keep their focus and study hard. Starting from now, go learn all the fundamental concepts in Chemistry that you are not strong with. It is important to have a good understanding of the basic Chemistry Concepts in order to appreciate the Sec 5 chapters.

Following are some fundamental chapters In Chemistry that you need to master:

• Elements, Compounds & Mixtures
• Atomic Structure
• Bonding (Ionic, Covalent & Metallic)
• Writing Chemical Equations (including Ionic Equations) –> Most Students have problems
• Mole Concepts & Chemical Calculations –> Most Students have problems
• Acids, Bases & Salts
• The Periodic Table & Its Elements

You can do it! Start your revision early, so that you can find help. Do not revise on the 11th hour.

PS: Go download your Free Video on Tips for Balancing Chemical Equations on the right side of this webpage.

PSS: I will be sharing with you on questions that emphasize on Essential Chemistry Concepts in this post. Stay Tuned!