Pre-Lab Discussion

In the chemistry laboratory, it is sometimes necessary to experimentally determine the concentration of an acid solution or a base solution. A procedure for making this kind of determination is called an acid-base titration. In this procedure, a solution of known concentration, called the standard solution, is used to neutralize a precisely measured volume of the solution of unknown concentration to which one or two drops of an appropriate acid-base indicator have been added. If the solution of unknown concentration is acidic, a standard base solution is added to the acid solution until it is neutralized. If the solution of unknown concentration is basic, a standard acid solution is added to the base solution until it is neutralized. When carrying out an acid-base titration, you must be able to recognize when to stop adding the standard solution, that is, when neutralization is reached. This is the purpose of the acid-base indicator mentioned above. A sudden change in color of the indicator signals that neutralization has occurred. At this point, the number of hydronium ions from the acid is equal to the number of hydroxide ions from the base. The point at which this occurs is called the end point of the titration. When the end point is reached, the volume of the standard solution used is carefully determined. Then, the measured volumes of the two solutions and the known concentration of the standard solution can be used to calculate the concentration of the other solution. The following steps tell how to calculate the unknown concentration:

1. Write the balanced equation for the reaction. From the coefficients, determine how many moles of acid reacts with 1 mole of base (or vice versa). Use the coefficients to form a mole ratio. 2. If the mole ratio is 1:1, the following relationship can be used to calculate the unknown concentration:

Ma x Va = Mb x Vb

Where Ma =