In Mole Calculations, many students lose marks in questions related to Volumetric Analysis, especially on those that requires them to understand the concepts of CONCENTRATIONS of solutions. Volumetric Analysis are always a highlight in our annual Mole Calculations Mastery Workshop.

Concentrations of a solution refers to the amount of solute dissolved in 1 dm3 of the solution.

Do note that

Solution = Solute (minor component) + Solvent (major component)

Usually in basic chemistry such as O Levels, the solvent used is Water. As such, the solution formed is usually aqueous solutions whereby certain solid or liquid are dissolved in water.

Concentrations can be expressed in:

• Grams of solute per dm3 (unit will be g/dm3)
• Moles of solute per dm3 (unit will be mol/dm3)

The most important formulae student need to know in volumetric analysis are:

Conc. (mol/dm3)  = Amt. of Solute (mol) / Vol. of Solution (dm3)

&

Conc. (g/dm3)  = Amt. of Solute (g) / Vol. of Solution (dm3)

Let’s check out a question to see how we can use the formulae above.

Question:

60g of NaOH is dissolved in water and make up to the total volume of 500cm3. Calculate the concentration of solution in i) g/dm3 and ii) mol/dm3.

Answer:

i)

Mass of NaOH = 60g

Volume of solution in dm3 = 500/1000 = 0.5dm3

Concentration of solution in g/dm3 = 60 g /0.5 dm3 = 120 g/dm3

ii)

Mr, Relative molecular mass of NaOH = 23 + 16 + 1 = 40

Moles of NaOH = Mass / Mr = 60/40 = 1.5 mol

Concentration of solution in mol/dm3 = 1.5 mol / 0.5 dm3 = 3 mol/dm3

Isn’t it easy if you know the strategy of doing it? I would love to hear from you. Leave me a comment.

For those itching to try out a question on your own. Check out the Quick Check below with a little bit of twist.

Quick Check 1:

Given concentration of a NaOH solution is 1.5 mol/dm3. How many moles of NaOH are contained in 2.0 dm3 of solution?

PS: Try it out and leave your suggested answer (working even better) below!

As you can see from the on-going survey on the right hand side, Mole Calculations (commonly known as Mole Concepts) has been voted by students as the #1 Killer Topic in O Level Chemistry (and other equivalent basic Chemistry Syllabus).

In fact, a large number of students skip the calculation-based questions when they see it on Chemistry examination, fearing that they spend alot of time on that question, and yet could not answer it.

Over the years, this topic is often voted the “Hot Favorite” on students’ challenging topics. Some:

• have problems understanding what the question is asking for
• do not know where to start
• totally blank when they read the question
• too confused with the formulae to use
• gave up on Mole Calculations totally

Many a times, question will require you to write a Balance Chemical Equation in order to solve the question. Other times, they will also require you to write the Ionic Equation and state the Spectator Ion.
As such you will need the following basic foundations that i have shared some time ago:

4 Steps-Process to write a Balanced Chemical Equation

7 Steps-Strategy to write the Ionic Equation & Spectator Ion

The best way to learn is NOT TO READ, but to DO IT!

Let’s try out one exam-based question(similar to O Level Chemistry Exams):

Question 1:

Copper (II) oxide reacts with dilute sulfuric acid to produce a salt, copper (II) sulfate and water. This process is known as Neutralisation.
i)  Write a balanced chemical equation with state symbols
ii) Write the ionic equation and state the spectator ion of this reaction

PS: Try it out and leave your answers below.

Do you find Mole Calculations difficult?

Mole Calculations, or commonly known as “The MOLE” or “Mole Concepts” is regarded by many of the GCE ‘O’ Level Chemistry students (as well as IGCSE and IB / IP Chemistry students) to be one of the most challenging and difficult topic.

Let’s not mention that many students are not able to score in this topic!

In fact, a large number of chemistry students simply “hate” Mole Calculations, because they:

• Don’t know which formulae to use
• Don’t even know how to start when they read the question
• Can’t relate how mathematical manipulations are connected to chemistry theories
• Can’t handle the fact the Mole Calculations are everywhere & anywhere in Chemistry Exams – they appear in questions that are linked to various other chemistry topics

As such, many students have in fact given up on Mole Calculations! But question is: Can they afford to give them up? Let me know your views on it? Do you think students can afford ignore Mole Calculations?

Even worst, we have received emails from chemistry teachers and chemistry tutors (tuition centre teachers) on the challenges in teaching their students. Many students seemed to “understand” the examples shared by their chemistry teachers during lecture but once the question is rephrased a little bit, students will be stunned and do not know how to carry on. Are you one of them? I would love to hear from you.

Recently, we have seen more students asking us to discuss more about Mole Calculations in this chemistry blogsite. One of my chemistry student, who is committed to do well in the October’s GCE ‘O’ Levels Chemistry Exams, had sent me a series of questions related to Mole Calculations. And one of them is a challenging application question that i have addressed last year (2009). Let’s check out the question:

Question:
12dm3 of polluted air is passed through limewater so that all the carbon dioxide is precipitated as calcium carbonate. The mass of calcium carbonate formed is 0.05g. What is the percentage by volume, of carbon dioxide in the air sample?

Now, take out your calculator, pen & paper – and try it out! Leave your answer (and even better, leave down your working) in the Comment section below.

I will be providing the solution soon. But before that, i would strongly suggest that you attempt the question first to see if you have the correct answer. =)

Suggested Solution:
Updated on 2nd March 2010:

For full screen video – click HERE

PS: If you are based in Singapore, there is a chance now that you can master Mole Calculations quickly & effectively. Click HERE for more information.

Last week, we have discussed on the Common Properties of a Group (in general).

Today, let’s take a closer look at the properties of elements from 3 very important groups that examiners like to test you:

Properties of Impt Groups in Periodic Table:

Group I (Alkali Metals)

• Soft & Silvery Metals
• Low Densities & Low MPs
• Has 1 valence electron
• Forms ions with 1+ charge by giving away 1 valence electron
• Reacts vigorously with cold water to form metal hydroxide alkaline solution & hydrogen gas
• Reacts vigorously with chlorine gas to form ionic metal chlorides

Group VII (Halogens) (more…)

In the previous blogpost, we have discussed about the General Features of the Periodic Table.

Besides that, students taking GCE ‘O’ Levels / IGCSE / IB / IP examinations would also need to master the Common Properties of a Group.

Common Properties of a Group

Elements in the same group have similar properties:

• Same number of valence electrons
• Usually form ions with the same charge
• Form compounds with similar formulae
• Similar physical properties
• Similar chemical properties

In addition to the above, the following are observed as we go down a particular group:

• Proton number increases
• Number of electron shellls increases
• Relative atomic mass increases

Now, go ahead and try out some exam-based question and leave your suggested answers below.

Quick Check 1:

Given that Strontium is (more…)

More Chemistry Tips & Exam Strategies

(Photo by ma-kasu)

In Chemistry, Periodic Table has long been regarded by students, educators and chemists as the “Bible” of Chemistry.

We need to have a good grasp of the General Features & the important Patterns (Group Trends) in order to appreciate many other topics and to score in exams.

General Features of Periodic Table:

1. Elements are arranged in the order of increasing proton / atomic number
2. A horizontal row of elements is called a Period
3. A vertical column of elements is called a Group
4. Elements are further divided into (more…)

(Photo Courtesy Loupe Clean)

In the previous post, i have discussed on the overview checklist that students need to know about this topic of Particulate Nature of Matter (commonly known as Kinetic Particle Theory).

Besides the “connection between relative molecular mass of a gas & its rate of diffusion” which is listed as very important, students should also know how to describe & differentiate the 3 States of Matter: Solids, Liquids & Gases, with respect to:

1. Arrangement of the particles
2. Forces between the particles
3. Movement of the particles
4. Energy of the particles
5. Density

Let’s take a look at it now.

With the above comparisons, we shall now discuss which States of Matter can be compressed upon the application of (more…)