Aqueous Reactions and Solution Stoichiometry

Solution Stoichiometry


Solution Stoichiometry

For balanced chemical equations involving solutions we calculate the number of moles by knowing the concentration (moles/liter, or Molarity) and volume (in liters).

How many moles of water form when 25.0 mls of 0.100 M HNO3 (nitric acid) solution is completely neutralized by NaOH (a base)?

1.  Let's begin by writing the balanced equation for the reaction:

2.  The stoichiometric relationship between HNO3 and H2O is HNO3H2O, therefore, for one mole of HNO3 that is completely consumed (i.e. neutralized) in the reaction, one mole of H2O is produced.

3.  How many moles of HNO3 are we starting with?

HNO3

4.  Therefore, we should have 0.0025 moles of H2O produced

Titrations

How can we know the concentration of some solution of interest? One answer to this problem lies in the method of titration. In titration we will make use of a second solution known as a standard solution which has the following characteristics:

  1. The second solution contains a chemical which reacts in a defined way, with known stoichiometry, with the solute of the first solution
  2. The concentration of the solute in this second solution is known.

Classic titrations include so-called acid-base titrations. In these experiments a solution of an acid with an unknown concentration is titrated with a solution of known concentration of base (or vice versa). For example, we may have a solution of hydrochloric acid (HCl) of unknown concentration and a standard solution of NaOH. To a fixed amount of the HCl solution is added incremental amounts of the NaOH solution until the acid is completely neutralized - i.e. a stoichiometrically equivalent quantity of HCl and NaOH have been combined. This is known as the equivalence point in the titration. By knowing the concentration of the standard solution, and the amount added to achieve stoichiometric equivalency, we can determine the amount of moles of HCl in the original sample volume.

How do we know when we have reached the equivalence point in such a titration experiment? In this type of acid-base titration, so called indicator-dyes are used. For example phenolphthalein is colorless in acidic solutions and turns red in basic solutions. Thus, in the above experiment we will add a small amount of this indicator-dye and add base until we barely begin to see a color change to red.

Mike's Handy Science Tip

Phenolphthalein is not only useful in acid-base titrations, but its also a darn good laxative used to be the active ingredient in ExLax. Just thought you'd like to know...

25 mls of a solution of HCl with an unknown concentration is titrated with a standard solution of 0.5 M NaOH. The phenolphthalein indicator dye begins to turn color after the addition of 2.8 mls of standard solution. What is the concentration of the HCl?

Balanced equation for the reaction:

HCl + NaOH -> NaCL + H2O

of NaOH was added

Since the stoichiometry of the NaOH and HCl is 1:1, the sample of HCl must have contained 0.0014 moles of HCl. The concentration of the HCl solution is therefore:

, or 0.056 M


1996 Michael Blaber