For all students learning O Levels Chemistry (and other equivalents), it is important that you understand the difference between the 3 types of formulae commonly used – namely Molecular Formula, Empirical Formula and Structural Formula.

Previously, we have discussed on this topic before. You can find them HERE.

Let’s define them now, for better understanding and application:

Molecular Formula

- shows the exact number of atoms of each element in a molecule

Empirical Formula

- shows the types of elements (as well as the simplest ration of different types of atoms) in the compound

- commonly known as the Simplest Formula

Structural Formula

- shows the covalent bonds involved (single, double or triple covalent bonds) between each atom in the compound

- more for Covalent Molecules

Let’s look at some examples to look at the difference between Molecular Formula and Empirical Formula:

Example 1: Hydrogen Peroxide

Molecular Formula is H₂O₂

Empirical Formula is HO

Example 2: Ethane

Molecular Formula is C₂H₆

Empirical Formula is CH₃

Example 3: Ammonia

Molecular Formula is NH₃

Empirical Formula is NH₃

Enjoying your self? Now, try out a tricky question that was emailed to me by my Sec 4 O Level Chemistry student.

Question 1:

A student can only recall the formula of a mineral as K₂Al₆(SO₄)_x(OH)_y, but remembers clearly that its empirical formula is KAl₃S₂O₁₄H₆. What are the values of x and y in the formula?

Try it out and leave your suggested answer below. Feel free to challenge each other in your answer. That’s the way how learning should be.

PS: Feel free to send this blogpost link to your classmates, friends or students (if you are an educator) if you think it will help them.

Air or some like to call it Atmosphere is considered a ‘minor’ chapter in GCE O Levels Chemistry examination, so much so that many teachers do not emphasise on their existence, not to mention importance.

As described by my Sec 4 Chemistry students, many of their Chemistry School teachers:

1. Ask the students to read textbook on their own
2. Give them 1 page summary mindmap or worksheet and ask them to just memorise it
3. Never even mention that Air is part of examination

Now, i beg to differ on this. If you are an educator (teacher, HOD or tutor) or even a parent that follows closely with Ten Years Series, you would realised that questions on Air have been appearing in recent year’s GCE O Levels Chemistry Examination.

Many a times, they appear in Section A (Short Questions) of Paper 2, and commonly in the form of Tables, Charts (Pie or Bar) or even Graphs. They are Application-Based questions and require the student to apply their concepts, rather than simply regurgitate what they memorise. TAKE NOTE especially if you are aiming to ACE your Chemistry!

Students must know the essential concepts of this topic well in order to be able to understand and comprehend these application fast, and solve them in limited time in examination conditions.

Let’s check out some questions on Air (Atmosphere):

Question 1:

Unpolluted rain water has a pH of about 6. Which of the following gas causes this pH?

A. Carbon Dioxide

B. Oxygen

C. Nitrogen Dioxide

D. Sulfur Dioxide

&

Question 2:

Which gas is NOT produced when hydrocarbons are burnt in the internal combustion engines?

A. Carbon dioxide

B. Carbon monoxide

C. Hydrogen

D. Oxide of nitrogen

&

Question 3:

Powdered calcium carbonate is commonly placed near the chimneys in coal-burning power station, in order to reduce atmospheric pollution by removing the waste gases. Which waste gas will not be removed by the powdered calcium carbonate?

A. Sulfur trioxide, SO₃

B. Nitrogen dioxide, NO₂

C. Carbon monoxide, CO

D. Phosphorous (V) oxide, P₂O₅

Now is your time to do some work. Try out the questions and see how much you know about this topic.

PS: Leave your answers and any suggested solutions in the “Leave a Reply” section below.

The following Mole Calculation question is not difficult but many O Levels Chemistry students tends to make mistake in exams, where time is not a luxury. When the same question was posed to my regular classes of Sec 3 and 4 O Levels and IP students, i realised that many fall into the trap also.

Let’s take a look at the question.

Question:

The relative atomic mass of chlorine is 35.5. What is the mass of 2 mol of chlorine gas?

A. 142g

B. 71g

C. 35.5g

D. 2g

Before we continue, let’s do a quick recap on the formula we need to use to solve this question, which is:

Mass = Mole x Mr of Molecule

Try it first on your own! Once you have an answer, find out the Correct Answer, Common Mistakes made by Students as well as my Suggested Solution below.

Enjoy!

Suggested Answer:

A

Common Mistakes made by students:

Group of students will make the mistake by choosing answer as B. This is because they didn’t realised that Chlorine Gas is a Diatomic Gas and has the chemical formula as Cl₂.

Suggested Solution:

If you are aware that Chlorine Gas is Cl₂, then the working would look like this:

Mr of Cl₂ = 2 x 35.5 = 71

Mass = Mole x Mr of Cl₂ = 2 x 71 = 142g

Hope you learn something here today.

Stay tune to the next post. =)

PS: Leave me a comment. I would love to hear from you.

In the previous post, we have discussed on a MCQ question on Electrolysis that requires students to:

1. Write out discharge half equations

2. Do Molar Ratio Comparisons

Question:

Which of the following requires the least  number of eletrons for discharge?

A. 2 mol of Al³⁺ (aluminium) ions

B. 5 mole of OH^- (hydroxides) ions

C. 3 mol of O^2- (oxides) ions

D. 6 mol of H⁺ (hydrogen) ions

Write your Answer and Suggested Working in the “Leave A Reply” section below.

PS: If you are not sure how to proceed, you can refer to the previous blogpost by clicking HERE

In the previous post, we have discussed on 1 MCQ question on Electrolysis that requires basic Mole Calculations strategy to solve.

Do note that Electrolysis topic is only applicable to students taking:

• GCE ”O” Level Pure Chemistry (Code: 5072)
• Integrated Programme (IP) – it will be classified under Electrochemistry chapter
• IB programme
• IGCSE and other equivalents

It is NOT applicable to GCE ”O” Level students taking Combine Science syllabus.

Today, we shall continue to look into another challenging Electrolysis question.

Question:

Which of the following gives the largest number of eletrons for discharge?

A. 4 mol of Al³⁺ (aluminium) ions

B. 5 mole of OH^- (hydroxides) ions

C. 4 mol of O^2- (oxides) ions

D. 10 mol of Na⁺ (sodium) ions

Try it out! Once you have an answer, proceed to click the link below to see if you have gotten the correct answer. Suggested answer and thought process is given also.

Read the rest of this entry »

A few students from my weekly chemistry sec 4 classes have been asking me about the following Multiple Choice Questions (MCQ) on Electrolysis, more specifically on Electrolytic Cell.

It seemed like many secondary schools in Singapore had set this question in the recent school’s O Levels Preliminary Exams.

Question:

In an  electrolysis experiment, the same quantity of electricity deposited 16g of copper and 6g of titanium.

What was the charge on the titanium ion?

A. 1+

B. 2+

C. 3+

D. 4+

What do you think?

Clue: Although this is question is on Electrolysis, you need to realise that it is calculation-based and you should be thinking of Mole Calculations at the same time.

Work it out on your own, before you click the link below to check the answer and my suggested solution (with thought process).

Read the rest of this entry »

I would like to share with readers of SimpleChemConcepts.com an Organic Chemistry mindmap I’ve drawn for my coaching classes when we were doing a summary of the topics.

Go to the bottom of this blogpost. You may download and print it out, put it in your notebook and use it for revision. Click on image to have a bigger view. =)

Quick Announcement:

I’m conducting my annual event : Organic Chemistry Mastery Workshop on 31 July, 7 & 14 Aug 2010 (830 am – 1230 pm) to help students (Pure Chemistry or Combine Science) to

• Understand this easy-to-score major Chemistry topic step-by-step (which many schools rush through)
• Link up the various chapters & Identify the inter-relationship for effective understanding of the concepts
• Be able to apply concepts on real exam questions to earn additional of 20+ marks in ‘O’ Levels
• Expose themselves to the variety of possible Organic Chemistry questions which can be asked
• Avoid common mistakes in Organic Chemistry

This workshop is for students:

• Who haven’t been doing well in Chemistry all along and want to improve themselves in the remaining 15 weeks. It’s possible since Organic Chemistry is an independent topic from the rest of Chemistry.
• Who wants an overall revision just before their prelims & ‘O’ Levels, focusing on the essential concepts in Organic Chemistry.

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Organic Chemistry Mind-Map:

(Click Image for larger view)