Tag Archive: Chemistry MCQ

O-Level Chemistry: The pH Scale

The pH Scale

The pH Scale is a set of numbers from 0 to 14 which is used to indicate whether an aqueous solution is acidic, neutral or alkaline.

All aqueous solutions contain hydrogen ions, H+ and hydroxide ion, OH-. The concentration of these ions differs in solutions of different pH values. Therefore, the pH of a solution is related to the concentration of hydrogen ions or hydroxide ions present in a solution.

It is important to appreciate the formal definition of “pH” in order to master Chemistry. It is formally defined as the “measure of the concentration of the hydrogen ions in a solution”.

Based on the definition above, let’s look at three different sections of the pH Scale:

A) Solution with pH equal to 7

It is NEUTRAL!

This if the pH of pure water i.e. distilled water.

The concentration of hydrogen ions and hydroxide ions is the same.

B) Solution with pH less than 7

It is ACIDIC!

Acids have concentration of hydrogen ions more than the concentration of hydroxide ions.

The lower the pH value, the greater the concentration of hydrogen ions and the lower the concentration of hydroxide ions, which means the more acidic the solution.

C) Solution with pH greater than 7

It is ALKALINE!

Acids have concentration of hydroxide ions more than the concentration of hydrogen ions.

The higher the pH value, the greater the concentration of hydroxide ions and the lower the concentration of hydrogen ions, which means the more alkaline the solution.

Let’s take a look at a Chemistry MCQ question.

Question 1:

An unknown aqueous solution has a pH value of about 2.

Which of the following is correct about the ions present in the solution?

A) Concentration of H+ ions = High  & Concentration of OH- ions = None

B) Concentration of H+ ions = High &  Concentration of OH- ions = Low

C) Concentration of H+ ions = Low &  Concentration of OH- ions = High

D) Concentration of H+ ions = Low & Concentration of OH- ions = None

Work on it and write your answer in the comment section below. See you soon!

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2014 GCE O-Level Chemistry 5073 – Paper 2 Post Mortem

[For Singapore Students taking 2014 GCE O-Level Pure Chemistry Exam; Subject Code: 5073]

Chemistry Paper 2 Post

How do i put it? Besides getting our students to study very hard on ALL topics, i must say that we were pretty lucky this year. In the last 2 weeks of revision, we went through a couple of concepts which came out almost exact in Paper 2 (Written) on Tuesday 28th Oct 2014 and are worth lots of marks, namely:

  • Titration Curves which appear in 4 pages of Paper 2
  • Multiple-Step reactions involving Dibasic Acid e.g. H3po4 + NaOH

In terms of topics, let’s take a look at what have been heavily tested in Paper 2 (Written Paper) according to our own students as well as comments that we gather from facebook/twitter/blogs:

  1. Experimental Chemistry (not tested in Paper 2)
  2. Methods of Purification & Separation (not tested in Paper 2)
  3. Identification of Ions and Gases a.k.a. Qualitative Analysis (not tested in Paper 2)
  4. Kinetic Particle Theory (not tested in Paper 2)
  5. Atomic Structure (not tested in Paper 2)
  6. Chemical Bonding and Structure of Matter (about 3 – 4 marks in Paper 2)
  7. Formulae, Stoichiometry and the Mole Concept (about 4 marks in Paper 2)
  8. Electrolysis (one whole question in Paper 2)
  9. Energy from Chemicals (a few marks in Paper 2)
  10. Speed of Reaction (about 2 marks in Paper 2)
  11. Redox (about 2 marks in Paper 2)
  12. Acids and Bases (about 2 marks in Paper 2)
  13. Preparation of Salts (not tested in Paper 2)
  14. Ammonia (about 2 marks in Paper 2)
  15. The Periodic Table and Group Trends (one whole Data Bank Question in Paper 2)
  16. Metals and Reactivity Series (about 2 marks in Paper 2)
  17. Air and the Atmosphere (not tested in Paper 2)
  18. Organic Chemistry: Fuels, Alkanes, Alkenes, Alcohols and Carboxylic Acids (lots of marks in many questions in Paper 2)
  19. Organic Chemistry: Macromolecules (few marks being tested in Paper 2)

Based on what have been tested in Paper 2 (Written), we are looking forward to the following concepts to be tested in Paper 1 (MCQ) on 11th Nov 2014:

  • Experimental Chemistry
  • Methods of Purification & Separation
  • Identification of Ions and Gases a.k.a. Qualitative Analysis (lots of marks?)
  • Kinetic Particle Theory
  • Atomic Structure
  • Preparation of Salts (lots of marks?)
  • Speed of Reaction
  • The Periodic Table and Group Trends (lots of marks?)
  • Metals and Reactivity Series (lots of marks?)
  • Air and the Atmosphere (lots of marks?)

Now, let us be even more strategic.

Paper 1 consists of 40 multiple-choice questions (total of 40 marks) and the weightage is a WHOPPING 30%, in case you are not aware of it. Doing well in Paper 1 will definitely bump up your overall Chemistry grade. In view that Paper 1 is only for 1 hour, you must not waste time. In fact you have an average of 1.5 minutes for each question.

If you are a student taking your GCE O-Level Pure Chemistry (syllabus code: 5073) and would like a final dash to score 40/40 marks, i welcome you to join us in our annual Score 100% MCQ Workshop on 8 Nov 2014.

All the best for the final Chemistry paper. See you at the finishing line.

PS: Feel free to share this blog post with your friends who will benefit from it! Keep sharing & learning! :)

O-Level Chemistry: Question on Chemical Bondings

In the last blog post, we went through a common exam-based question on Chemical Bondings and revised  briefly on:

  • Types of Bondings
  • Types of Structures & its properties

Today, we shall look at a few more exam-based Chemistry MCQ questions on this topic:

Question 1:

Which of the following is most likely to be the melting point of an ionic solid?

A. -182oC

B. 0oC

C. 114oC

D. 943oC

Suggested Solutions:

Ionic compounds tend to have high melting point and boiling point. They tend to exist as solids at room temperature and pressure.

Strong electrostatic forces of attraction holds the positive ions and the negative ions together in the giant ionic/crystal lattice structure.

As such, a large amount of energy is required to overcome these strong forces.

As such, answer in (Option D).

 

Question 2:

Which one of the following substances;

(i) is an element?

(ii) also forms crystals composed of small molecules?

A. Copper

B. Graphite

C. Carbon dioxide

D. Iodine

Suggested Solutions:

Option A: Copper is an element BUT it exists as positive metal ions surrounded by ‘sea of delocalised valence electrons’.

Option B: Graphite is an element BUT it exists as giant molecular structure of carbon atoms held together by strong covalent bonds.

Option C: Carbon dioxide exists as simple molecular structure (i.e. small molecules) BUT it is a compound.

Option D: Iodine is an element and it exists as a diatomic molecule (i.e. simple molecular structure). At room temperature, iodine is a solid so that diatomic molecules are packed very closely together in a regular pattern, to form “crystals”. DO NOT be tricked to think that iodine is an ionic compound due to the word “crystals” which is usually associated with ionic compound. It could also mean that simple molecular structures are packed closely together in a regular pattern when they are in solid state. You can refer to Kinetic Particle Theory to learn more on 3 states of matter.

As such, answer is (Option D).

PS: Hmm, just realised this is quite a tricky question. Going to test my students in our O-Level Pure Chemistry Tuition Classes on the same question and see how they fare for it :)

I am sure you have learned something useful today.

Feel free to share it with your friends who need help in Basic Chemistry.

Cheers! :)