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The Mole (Part 1)

Chap 9: The Mole

(Part 1)

 

Understanding the concept of mole is extremely important in chemistry, as it will be there in almost everything related to this subject. From paper point of view, it will be important in MCQs and theory paper, as well as practical or A.T.P paper. To make it simpler, I have decided to teach this chapter in two parts. This is the first part of the lecture.

Now, don’t get confused with the amount of things you must learn and understand; trust me, it gets easier once you get the hang of it, and if you practice it enough. For this, we will be doing some questions together, too.

 

A mole is a certain amount of substance. It is a general term to describe an amount of atoms, ions or molecules, and it enables chemists to count these particles by weighing. A mole is defined as the amount of substance which contains the Avogadro Number of particles. The Avogadro Number (or Avogadro Constant) is defined as the number of atoms in 12g of the carbon-12 isotope, and its value is equal to 6.02 x 10²³.

 

 

Relative Atomic Mass:

 

The mass of one mole of atoms is its ‘relative atomic mass’ in grams. In other words, the relative atomic mass of any atom is the number of times the mass of one atom of an element is greater than 1/12 of the mass of one carbon-12 atom. Its symbol in chemistry is Ar.

 

Relative Molecular Mass:

 

The mass of one mole of molecules is its ‘relative molecular mass’ in grams.

Or you could say that the relative molecular mass in grams of any compound contains the same number of molecules, equal to the Avogadro Number. Its symbol is Mr.

The relative molecular mass of ionic compounds is more accurately known as the ‘relative formula mass’, as ionic compounds do not have molecules and thus using the word molecular would be wrong. The symbol for this is also Mr. In all places that Mr is mentioned, it may mean either.

To calculate the Mr of a compound, you must first know its molecular formula. First note down the masses of all the elements present in the compound (you can take this information from the periodic table), and then multiply the mass of each element with the number of atoms in one molecule (the number in the subscript). Add the resulting numbers. This is your Mr.

 

Molar Gas Volume:

 

One mole of any gas at room temperature pressure occupies a volume of 24 dm³, or 24000 cm³ (1 dm³ = 1000cm³). This is sometimes known as the ‘molar gas volume’ as it contains the Avogadro Number of particles. In more general terms, Avogadro’s Law states that equal volumes of all gases at the same temperature and pressure contain the same number of particles.

 

Calculations with Moles:

 

Memorize the following formulae:

 

 

Number of Moles =    mass   in   grams  .

Relative atomic mass

 

Or:

 

Number of Moles =    mass   in   grams   .

Relative molecular mass

 

These two formulas (which are basically the same; one has Ar and the other Mr) will help you as long as you are studying chemistry.

 

Let’s do some questions on moles together. Remember, in the paper you will be provided the periodic table from where you can see the atomic numbers and masses of respective elements, but it is always good to memorize the repeated ones.

 

  1. How many moles are there in 54g of Water?

 

  1. (keeping the above formulae in mind)

Mr of H2O = (2 x 1) + 16 = 18

Number of Moles = 54/18 = 3 moles.

 

Another type of question could be:

 

  1. How many grams are there in 7 moles of Carbon dioxide?

(In this question, we reverse the formula to obtain what we need.)

 

  1. Mr of CO2 = 12 + (2 x 16) = 44

Mass in grams = 7 x 44 = 308 g

 

 

Now, let’s deal with volumes of gases. Memorize the following formulae:

 

  1. Volume of gas =  mass of gas    x  molar gas volume

Mr of gas

 

  1. Vol of gas = number of moles x molar gas volume

 

These two formulae, in this form or another, are going to be used repeatedly by you. Let’s do some questions for practice.

 

  1. What volume (at r.t.p) would 0.2 moles of oxygen gas occupy?
  2. Vol of O2 = 0.2 x 24 = 48 dm³.

 

  1. What volume would 32 g of methane gas occupy (at r.t.p)?
  2. Mr of CH4 = 12 + (4 x 1) = 16

Vol of CH4 = 32/16 x 24 = 48 dm³.

 

  1. What mass in grams would 3 dm3 of Hydrogen gas occupy?
  2. Mr of H2 = 2 x 1 = 2

Mass of H2 = 3/24 x 2 = 0.25 g

 

These were basic questions. Let’s now go to some trickier ones: ones that involve equations as well as calculations.

 

  1. How many ions are there in 20 g of Magnesium Oxide?
  2. Mr of MgO = 24 + 16 = 40

Number of moles of MgO = 20/40 = 0.5 mol

 

MgO  Mg2+ + O2-

 

From the equation we can see that 1 mol of MgO contains 1 mol of  ions and 1 mol of  ions. 0.5 mol of MgO will contain 0.5 mol of  ions and 0.5 mol of  ions.

Hence, 0.5 mol of MgO contains 1 mol of ions.

Number of ions = 1 x 6 x 10²³ = 6 x 10²³ ions.