Question 4: Why is it more useful to have two units for amount of substance, kilogram and mole?

Answer

Kilogram measures, in fact, how heavy something is. Mole measures how many atoms or molecules are in a substance.

In scientific work, sometimes we are concerned with the mass of substances and we use the kilogram for it. For example, if we want to calculate the density of a substance, we divide mass (kilogram) by volume. It will be incorrect to divide number of atoms on volume because same number of atoms of different elements has different masses and hence, the density (mass per unit volume) calculation will be incorrect.

In other cases, we may be concerned with the number of particles like atoms in a substance. For example, in Chemistry, we want to know how many of the atoms or molecules are required to react with a certain amount of other reactant. In such cases, it will be inaccurate to say how many kilograms are required. Because, same number of kilograms of different elements would have different number of atoms and the calculations on the reaction will become wrong.