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.

(more…)

(Geodesic Domes @ Eden Project in Cornwall. Photo Credit)

In Chemical Bonding chapter of Basic Chemistry Syllabus (O Levels, etc), you have learned about the term Allotropes as well as Macromolecules.

To be more exact, you learned that Allotropes = Compounds with the same element with different structure.

The two examples you learned are Diamond and Graphite, which are Allotropes of Carbon.

Now, i would like to introduce to you another allotrope of carbon that have found so many applications into our daily modern lives. Think of Apple, iPod, iPad, Macbook, iPad, Blackberry, Nokie, smaller and thinner electronics consumable products.

This is FULLERENE. C6o – highly symmetrical spherical football-shaped molecule – which was obtained by firing a powerful laser at a sample of graphite at a temperature of 10, 000 degree Celsius. It was named as Buckminsterfullerene in honour of the architect R. Buckminster Fuller, who used to be the principle of the geodosic dome in many of his buildings.

Molecular Model of C60, Fullerene (Photo credit St Stev)

Look at the molecular structure and you will realised that the alternating 5- and 6-membered rings in C60 gives a bonding pattern similar to the struts in a geodosic dome.

In fact, from the point C60 was found and honoured, other fullerenes such as C70, C76, C78, C90, etc has been synthesized and i expect many more to be synthesize to open up more exciting new areas of Chemistry and its application in the modern world.

As stated earlier on, fullerenes found its many application in the semi-conductors industry.

Now, time for you to put on your thinking caps:

Question:

Is fullerene a good or non-conductor of electricity? Explain with reference to its structure and bonding properties.

(Clue: Refer to structure and bondings in Graphite (Good Conductor) and Diamond (Non-Conductor))

PS: I would love to hear from you. Think about it and leave your comments below. It is actually very easy. =)

(Photo credit Ingy The Wingy)

Besides sharing Chemistry Tips and Exam Strategies, the other aim of this chemistry blogsite is to encourage students to learn the Chemistry Theories and Concepts in a more experiential and interesting way – HOW ? By relating chemistry theories to everyday life processes and experience.

Thermit Process came to my mind several days back when i was passing by the railway track in the west area of Singapore, and today i am going to share with you more on how Thermit Process – which is a highly exothermic reaction is used to weld together steel railway lines. It involves the reduction of iron (III) oxide to iron by aluminium.

Now, let’s check out a chemistry question that requires your concepts in several chemistry topics, namely:

• Energy Changes (Endothermic/Exothermic Reactions)
• Redox (Oxidation/Reduction)
• Mole Calculations

Question:

In the highly exothermic Thermit Process to weld railway lines, aluminium is used to reduce iron (III) oxide to iron based on the balanced chemical equation below:

2Al   +   Fe2O3   ->   Al2O3   +   2Fe

(a) Indicate the state symbols for all the reactants and products involved

(b) Calculate what mass of aluminium is needed to react completely with (more…)