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Explain how the method of continuous variations might be used to determine the stoichiometries of the following types of reactions:

a. an endothermic reaction between two substances in solution

b. a reaction between two gaseous reactants to produce gaseous products where n=/= 0.

c. a reaction between two strong electrolytes in solution to forma soluble, nonionized product

Explanation / Answer

Chemical reactions are represented by equations that identify reagents and side products at the other, separated by an arrow indicating the transformation.

Equations must be adjusted so that there are the same number of atoms of a given element on both sides of the equation, since the atoms are preserved.

To determine the stoichiometry of the reaction can be used the method of continuous variations which measure a common property of a series of preparations with different amounts of reagents to study process systems.

For example:

For colored products remain in solution, measuring the absorbance of the final product is a widely used method (continuous variations of Job).

For precipitation reactions, that property would be the mass of precipitate, which would be proportional to the volume occupied by it, under equal conditions of sedimentation. Therefore, if the precipitation takes place in graduated tubes of the same diameter, the amount of precipitate will be approximately proportional to the height occupied in the tube.

Thus, using the weight (volume or height) of the precipitate formed from the mixture of two solutions containing the reagents may determine the stoichiometric amounts in which the two are combined to form a final product.

Data obtained from the precipitates (weight, volume or height) is plotted on a graph versus composition used in each system (initial concentration of reagents). The evolution of the curve will indicate the e stoichiometric of the reaction, when the graph reaches its peak.

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