Solution Stoichiometry

Molarity allows us to do mole/mole stoichiometric calculations when the reaction occurs in solution. Consider the chemical reaction:

Suppose we want to know what mass of CaCO3 is required to react with 25 mL of 0.75 M HCl. We can solve this problem by using the same mole/mol stoichiometric concepts already discussed.



In a titration a known volume of a reactant (titrant), with known concentration, is slowly added to a vessel with another reactant until the reaction is complete. The point at which the reaction is complete is known as the "end-point". This situation is depicted in the figure on the left. Typically the end-point is determined visually through either a color change or formation of a precipitate. Consider the acid/base reaction:

NaOH(aq) + HCl(aq) ----> NaCl(aq) + H2O(l)

Suppose we have flask of HCl but do not know what its concentration is. We are told that we need to know how many moles of HCl there is in this solution. We do a titration experiment: In our laboratory we find a bottle labeled 0.035 M NaOH. We fill a buret with this NaOH solution and put 1 drop of phenolphthalein* indicator in the flask with the HCl. We slowly add the NaOH to the HCl until we notice that the solution in the flask turn a slight pink color. The amount of NaOH added was 27.5 mL. From this information and the balanced reaction above we can determine the moles of HCl:

*phenolphthalein is an organic compound that is colorless in acidic solution and red in a basic solution.

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C101 Class Notes
Prof. N. De Leon