Stoichiometry is the study of the numbers of atoms that combine to form a chemical compound. It is defined as the relationship that exists between the mole ratio of the reactants and the products in a given chemical reaction.
A stoichiometric compound is one in which the ratio of the elements in the compound are small whole numbers. For example in benzene (C6H6) it is 1 to 1 while that of propane (C3H8) it is 3 carbon atoms and 8 hydrogen atoms.
Non-stoichiometric compounds have atoms in ratios that are not whole numbers. This usually happens in crystalline materials, where ions can be missing from the lattice or extra ions can be present, or there can be impurities, where atoms of one element have been swapped for those of another.
Experiments designed to study stoichiometry of chemical reactions may include precipitation, various displacement reactions, synthesis as well as reduction of metallic oxides.
Stoichiometry can also be determined from a balanced equation since a mole, as we know, represents the molar mass of the substances in grams. Consequently, if the reacting mass of any one of the substances in a reaction is given, one may calculate the reacting masses or the number of moles of the rest from a balanced chemical reaction. This is demonstrated in the following worked examples:
WORKED EXAMPLES
Ethane burns completely in oxygen according to the equation:
2C2H6+ 7O2--->4CO2+ 6H2O How many moles of carbon (IV) oxide will be produced when 9.0g of ethane are completely burnt in oxygen? [O=16; C=12; H=1]
{First determine the compounds, elements or ions that are really involved in finding out the solution to the question. Here, it is CO2and ethane, which we are given information on}
{Then, calculate the molar mass of ethane that will form 4 moles of CO2as shown in the balanced equation}
2(30)g of ethane forms 4 moles of CO2
i.e. 60g of ethane will form 4 moles of CO2
Therefore for 9g of ethane, we will have 9/60 x 4 moles of CO2which is 0.6 mole
Find the volume of oxygen produced by 1 mole of potassium trioxochlorate (V) at s.t.p in the following reaction:
2KClO3------->2KCl + 3O2
{In the equation, we are asked for the volume of oxygen and information is given on KClO3, so these are the species that we are concerned with}
{Note that when volume is required, we relate it to the molar volume which is 22.4dm3for every mole of a gas at s.t.p.}
2 moles of KClO3gives 3 moles of O2
Therefore, 1 mole of KClO3will produce 3/2 moles of O2
This is 1.5 moles and is multiplied by the molar volume, which is 22.4dm3to give 33.6dm3
Now try the following problems:
exercises
12.5g of ZnCO3, were heated very strongly to a constant mass and the residue treated with excess HCl.Calculate the mass of ZnCl2that would be obtained [Zn=65;C=12;O=16;H=1;Cl=35.5]
Calculate the mass of pure NaCl that will yield enough hydrogen chloride gas to neutralize 25cm3of 0.5M K2CO3solution [NaCl=58.5,HCl=36.5,K2CO3=138]
Calculate the number of moles of calcium chloride,CaCl2, that can be obtained from 25g of limestone,CaCO3in the presence of excess HCl [Ca=40;C=12;O=16;H=1;Cl=35.5]
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