SimpleChemConcepts Tips

O Level Chemistry: Molecular & Empirical Formulae

sean — Tue, 31 Aug 2010 04:30:34 +0000

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.Related Articles:

O Level Chemistry: Air (Atmosphere)

sean — Tue, 24 Aug 2010 01:47:56 +0000

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:

Ask the students to read textbook on their own
Give them 1 page summary mindmap or worksheet and ask them to just memorise it
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.Related Articles:

O Level Chemistry: Mole Calculation Question & Common Mistakes Made

sean — Sun, 22 Aug 2010 02:30:33 +0000

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!

O Level Chemistry: Challenging Mole Calculation / Electrolysis Questions

sean — Sat, 14 Aug 2010 00:29:34 +0000

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 HERERelated Articles:

O Level Chemistry: Challenging Mole Calculation / Electrolysis Questions

sean — Thu, 12 Aug 2010 05:28:27 +0000

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.

Thought Porcess:

1. Half equations are required to see how many mole of electrons is required for discharge equation

2. Using Molar Ratio comparison, find out the actual mole of electrons required for “no. of mole of ions” given in each choice

Suggested Answer:

A) Al³⁺(aq) + 3e^- –> Al(s)

1 mol of Al³⁺(aq) ions require 3 mol of electrons to discharge

Thus, 4 mol of Al³⁺(aq) ions require 3 x 4 = 12 mol of electrons to discharge

B) 4OH^-(aq) –> 2H₂O(l) + O₂(g) + 4e^-

4 mol of OH^-(aq) ions require 4 mol of electrons to discharge

Thus, 5 mol of OH^-(aq) ions require 5 mol of electrons to discharge

C) 2O^2-(aq) –> O₂(g) + 4e^-

2 mol of O^2-(aq) ions require 4 mol of electrons to discharge

Thus, 4 mol of O^2-(aq) ions require 8 mol of electrons to discharge

D) Na⁺(aq) + e^- –> Na(s)

1 mol of Na⁺(aq) ions require 1 mol of electrons to discharge

Thus, 10 mol of Na⁺(aq) will require 2 mol electrons to discharge

With the above mole of electrons comparison, (A) has the largest number of moles of electrons required for discharged.

Common Error: Many students will choose (D) because they simply look at the “10 mol” and assume it has the largest no. of electrons directly.

So far so good for you? Let me know your thoughts on this question and the suggested solution. I would love to hear from you.

If you have a different way of solving this question, feel free to write down your thoughts on the “Leave A Reply” section below.

PS: If you are finding it difficult to solve any mole calculations – related questions, there is an upcoming Mole Calculations Mastery Workshop 2010 during the 1-week September School Holidays. Check it out! Seats are extremely limited.Related Articles: