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Stoichiometry is the calculation of mass / volume relationships of a chemical reaction. So whenever we hear the word Stoichiometry, we should be thinking about problems related to chemical reaction. Stoichiometry is all about calculations. One must know how to write and balance a chemical equation before one can solve Stoichiometry problems. A chemical equation is a statement written in form of formulae and symbols where the reactant entities are on the left hand side and the products are on the right hand side. When we are talking specifically about chemical reaction (not radioactivity), we say that the sum of masses of the reactant is equal to the sum of masses of the product.
Now, what is Mass? We all know that mass is any thing that has weight and occupies space. Human beings have mass, the table in your room has mass, stones have mass and even atoms have mass. Or don’t you believe? This will take us to the atomic mass and molecular mass. The atomic mass of an element can be found on the periodic table. For example; from the periodic table, the atomic mass of Oxygen (O2) is 16, atomic mass ofChlorine is 35.5 and so on. The atomic mass is equal to the number of protons and neutrons in the atom.Molecular Mass, on the other hand , is the mass of themolecules of an element or compound. Some elements are monoatomic where as some are diatomic or triatomic. Note that mono means one, dia means two and tri means three. The molecular mass of a monoatomic element is the same as the atomic mass of the element. Examples of monoatomic elements are, Helium (He), Argon (Ar) etc. Examples of diatomic elements are Oxygen (O2), Chlorine (Cl2) etc example of triatomic element is Ozone (03). The molecular mass of diatomic or triatomic element is the mass of one atom multiplied by the number of atoms of the element. For example the molecular mass of oxygen is 16 (atomic mass of Oxygen) multiply by the number of atoms ( 2 atoms of Oxygen). This will give us 32. The molecular mass of a compound is the sum of the masses of the elements that make up the compound. For example, the molecular mass of Sodium Chloride (NaCl) is equal to sum of the mass of Sodium and mass of Chlorine which is (58.5).