Chemistry Notes: TitrationsChemistry 2014-2015

A titration is a lab procedure which uses a solution of known concentration to determine the concentration of an unknown solution. This is accomplished by putting one of the solutions in a flask, and filling a buret with the other solution. The stopcock on the buret allows you to slowly add one solution to the other until the reaction reaches an endpoint, the conclusion of the reaction.

Acid-Base Titrations

The endpoint is often observed by a change in color. This is usually determined using an indicator during acid-base titration.

For example, if you fill the flask with HCl of unknown concentration and a few drops of phenolphthalein, and fill the buret with 1.0 M NaOH, you can let just enough NaOH react with the HCl to give a pink color (phenolphthalein is pink in base).

The equation for this reaction is HCl + NaOH NaCl + H2O. Let’s say it takes 25.0 mL of the 1.0 M NaOH to neutralize 20.0 mL of the unknown HCl.  0.025 L x 1.0 M = 0.025 moles NaOH

You can use the data from a titration to determine the concentration of the unknown

solution.

If you look at the reaction equation, there is a 1:1 ratio of HCl:NaOH (the coefficient for both reactants is 1). If it took 0.025 moles NaOH to neutralize the HCl, there must have been 0.025 moles HCl in the flask. Remember that the flask contained 20.0 mL of HCl, so 0.025 moles / 0.020 L = 1.25 M HCl In other words, the concentration of the HCl is 1.25 M.

HCl + NaOH NaCl + H2O

A simpler way to carry out this kind of calculation is to use the equation M1V1 = M2V2, where M = molarity and V = volume. For this problem, we would have  1.0 M NaOH x 25.0 mL NaOH = Molarity of HCl x 20.0 mL HCl

 or (1.o)(25.0) = 20.0x

If we divide both sides by 20.0 mL HCl, we get molarity of HCl = 1.25 M.

It can be easier!

Note that this only works if we use a monoprotic acid—an acid that has one proton, H+--and a monobasic base—a base that has one hydroxide ion, OH-. We’ll only give you problems like that so you can use M1V1 = M2V2 to keep things from getting too complicated.

M1V1 = M2V2

You have 50.0 mL of an unknown HCl solution. It takes 80.0 mL of 0.50 M NaOH to neutralize this acid. What is the concentration of the hydrochloric acid solution?

Titration Practice