Stoichiometry is the art of measuring chemical elements in quantitative relationship in chemical reactions. It is ordinary chemical arithmetic. Experiments can be designed to study the stoichiometry of reactions. The types of reaction that are commonly studies include
- Precipitation reactions;
- Displacement of hydrogen from acids;
- Displacement of metallic ions;
- Synthesis of metallic oxides; and
- Reduction of metallic oxides.
Stoichiometry of a Precipitation Reaction
When solutions of barium chloride and potassium tetraoxochromate(VI) are mixed, a precipitate of barium tetraoxochromate(VI) forms instantly. By using molar solutions of the reactions, it is possible to find the ration in which they combine.
Stoichiometry of Hydrogen Displacement from an Acid
Magnesium readily displaces gaseous hydrogen from dilute hydrochloric acid. To study the stoichiometry of this reaction, we react a known mass of magnesium with an excess of the acid, and measure the volume of hydrogen liberated.
Set up the apparatus as shown in equalize the water levels in the aspirator bottle and the cylinder by raising or lowering the cylinder. Record the volume of water in the cylinder. Tilt the conical flask and let the acid react with the magnesium. The hydrogen gas given off will displace its own volume of water from the bottle into the cylinder. Allow the apparatus to cool and equalize the water levels in the cylinder and bottle. Record the volume of water in the cylinder. The difference in the two volumes gives the volume of the hydrogen gas evolved. Note the atmospheric pressure and room temperature.
Mass of magnesium = 0.36g
Volume of hydrogen displaced = 378cm3
Room temperature = 25C
Atmospheric pressure = 755mm Hg
Molar mass of mg = 24g
From the readings given, we can calculate the mole ratio of the magnesium and hydrogen in this reaction in the following way;
Let V2 be the volume of hydrogen obtained experimentally at s.t.p.
T1 = (25+273) K – 298K T2 = s.t. = 273K
P1 = 755mm Hg P2 = s.p. = 760mm Hg
V1 = 378cm3
Molar volume of any gas = 22.40dm3 i.e 2240cm3 = 1 mole H2