Introduction to atomic masses and molecular masses
Given that the atom of an element is small, determining the actual mass of a single atom is difficult. The IUPAC came up with a scale basing on C-12 Isotope where the Carbon was used as a base for calculating the atomic mass of all other elements. C-12 was recommended since it is a stable and very familiar element. Carbon-12 was assigned12.00 a.m.u by the IUPAC. In most cases, calculating the individual atomic masses is quite impossible hence, the scientists take the relative atomic masses of a group of atoms by weighing them as a whole. So the mass of the entire group divided by the total number of atoms participated will give the relative atomic mass.
Table Of Contents
1. Introduction to atomic masses and molecular masses
2. Definition of AMU
3. Relative Atomic mass
4. AMUs of the most common elements
5. Molecular mass
6. Law of Conservation of Mass
7. Gay Lussac’s law
Definition of AMU
“The atomic mass unit is defined as the ratio of the average mass of one atom of the element to one-twelfth of the mass of one atom of C-12”. The atomic weight of an atom is a dimensionless number when it is divided by unified atomic weight or Daltons. This is called the relative isotopic mass. The atomic masses of elements vary from 1.008 amu for hydrogen up to 250 amu for elements that have a very high atomic number. Mass of molecules can be determined by adding the average atomic mass of each atom in the molecule.
Relative Atomic mass
The RAM of an element is the sum of the product of the percentage abundance and the corresponding atomic masses of the isotopes. For example: calculate the RAM of the chlorine atom whose relative abundances and corresponding masses are given below.
AMUs of the most common elements
Atomic Mass units( AMU)
Atomic Mass units( AMU)
Molecular mass is the total atomic masses of the elements that are contained in a molecule. It is calculated by finding the product of the atomic masses by the corresponding number of atoms and adding them together. Example molecular mass of H2SO4 can be obtained as shown below
(1x2) + 32 + (16x4)
2 +32 + 64
Law of Conservation of Mass
The law states that the mass in an isolated system is neither created nor destroyed by chemical reactions or physical transformations. According to this law, the mass of the reactants is always equal to the mass of the products in any given chemical reaction. Consider the reaction between Sulphur and Oxygen.
S(s) + O2 (g) -----------------> SO2 (g)
Gay Lussac’s law
The law was discovered by Gay Lussac’s in the year 1805. It summarizes the relationship between the reacting volumes of gases. The law states that when gases react, they do son in volumes that bear a simple ratio to one another and to the volumes of products if gaseous, provided temperature and pressure are kept constant.
2CO (g) + O2 (g) -------------> CO2 (g)
20cm3 10cm3 10 cm
2vol 1vol 1 vol
Thus the ratio becomes 2:1:1
In an experiment, 20cm3 of Sulfur (IV) Oxide are found to react completely with 10cm3 of Oxygen gas to produce 30cm3 of Sulfur (IV) Oxide. Determine the equation for the reaction.
2SO2 (g) + O2 (g)--------------> SO3 (g)
20cm3 10cm3 20cm3
2vol 1 vol 2vol
And the ratio is 2:1:2
Results from experiments have shown that equal volumes of all gases under the same conditions of temperature and pressure contain an equal number of molecules. This relationship leads to Avogadro’s Law which states that "Equal volumes of gases will contain an equal number of molecules." The Avogadro's constant is denoted by "L" is always 6.02× 1023