Stoichiometric arguments in equilibria calculations

It is important to distinguish between the actual concentrations present in a solution, and the total stoichiometric concentrations (see Section 2.11). 

When a solution of, for example, benzoic acid is prepared by dissolving a given number of mol in a specific volume of solution, the concentration is given as number of mol/volume of solution. When a concentration is worked out in this way, i.e. in terms of what was weighed out, it is called the stoichiometric concentration. But what really is of interest is what is actually present in solution, and a concentration based on this is called the actual concentration. If, on dissolution, the solute is involved in any reaction or equilibrium, then the stoichiometric concentration will not correspond to what is actually present. 

When a weak acid such as ethanoic acid is dissolved in water equilibrium is established, the species present in solution are HaO Caq), the ethanoate ion, CH3COO (aq), and the remaining undissociated ethanoic acid, CH3COOH(aq). 

These distinctions between actual and total concentrations are also important when formulating the expression for the fraction ionised  

A decision may have to be made as to whether it is legitimate to approximate actual concentrations with total concentrations. Sometimes this may be justihed and sometimes it is not, and each case must be decided individually. In the present case, ethanoic acid is a weak acid, and unless the total concentration is very low it will be valid to approximate  

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