The Mole (Part 2)

Chap 9: The Mole

(Part 2)


In the previous lecture, we learnt what the mole was, and did some related numerical questions related to it. Let’s go through a quick revision of the major points.

  • A mole is a quantity of substance in grams which contains the Avogadro Number of Particles.
  • The mass of one mole of atoms is its relative atomic mass in grams. Same goes for molecules.
  • One mole of any gas at r.t.p occupies a volume of 24 dm³.


We also learnt some formulae. Now, let’s learn some more.


Percentage Composition of Compounds:


This is to determine what percentage of each element is present in the compound. It also involves calculations. It can be found if one is given the compound’s formula, as well as the Ar of the elements in the compound.

In general, the percentage by mass of an element in a compound can be found by using the formula:


Percentage by mass of an element in a compound


= Ar of element × no. of atoms in formula × 100%

Mr of compound


Percentage Yield:


In any reaction, the amount of product produced experimentally is always less than that which should have been produced theoretically. The percentage yield is a measure of efficiency of the reaction.


Percentage Yield = experimental value x 100%

theoretical value


Concentration or Molarity:


(This is mainly going to be used in your practical or A.T.P paper).


1 mol/dm3 means 1 mole of solute is dissolved in 1 dm3 of solution. Therefore, we can say that concentration = no. of moles x 100

Vol. in cm3



Empirical Formula:


The simplest formula of a compound that shows the type of elements present in it in the simplest ratio is known as the empirical formula. The steps for determining an Empirical Formula are as follows.

  1. Start with the number of grams of each element, given in the problem. If percentages are given, assume that the total mass is 100 grams so that

The mass of each element = the percent given.

  1. Convert the mass of each element to moles using the molar mass from the periodic table.
  2. Divide each mole value by the smallest number of moles calculated.
  3. Round to the nearest whole number. This is the mole ratio of the elements and is represented by subscripts in the empirical formula. If the number is too far to round, then multiply each solution by the same factor to get the lowest whole number multiple.
  • ■ g.  If one solution is 1.5, then multiply each solution in the problem by 2 to get 3.
  • ■ g.  If one solution is 1.25, then multiply each solution in the problem by 4 to get 5.

The molecular formula is the formula that shows the exact number of atoms of each element in a molecule.

Once the empirical formula is found, the molecular formula for a compound can be determined if the molar mass of the compound is known. Simply calculate the mass of the empirical formula and divide the molar mass of the compound by the mass of the empirical formula to find the ratio between the molecular formula and the empirical formula. Multiply all the atoms (subscripts) by this ratio to find the molecular formula.