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.

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.

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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).

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(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…)

Recently, i received several emails and comments from readers (students) to share more challenging Mole Calculations (or Mole Concepts) questions in this chemistry blog.  So here we go………..

Question:

What would be the concentration of the hydrochloric acid produced if all the hydrogen chloride gas from the reaction between 50g of pure sulfuric acid and an excess of sodium chloride was collected in water, and the solution made up to a volume of 400 cm3 of water?

Based on equation: NaCl(s)   +   H2SO4(aq)    –>   NaHSO4(s)   +   HCl(g)

Now, try it out and leave your suggested answer (and possibly your solution/steps) in the comment section below.

The best way to learn is NOT by reading or looking, instead it is by Taking Actions.

I look forward to seeing you answers. Cheers! Enjoy! =)