AP Chemistry
Determination of the Molecular Weight of an Acid
Background Information and Pre-Lab Questions
Discussion
In this experiment you will be performing an acid-base titration where the titrant is a base, sodium hydroxide (NaOH),
and the analyte is an acid, citric acid (H3C6H5O7). When doing titrations the titrant is chosen such that the titrant and
analyte react completely and with a known stoichiometry. In this case three moles of sodium hydroxide are required to
neutralize one mole of citric acid, given that citric acid contains three acidic hydrogens.
H3C6H5O7 (aq) + 3 NaOH (aq)
Na3C6H5O7 (aq) + 3 H2O (l)
The equivalence point is reached when sufficient sodium hydroxide is added to react completely with the citric acid. At
this point the number of moles of sodium hydroxide is three times the number of moles of the citric acid, therefore:
nNaOH = 3 x ncitric acid
Knowing that molarity multiplied by volume (L) equals moles, we can rewrite the previous equation as:
MNaOH x VNaOH = 3 x ncitric acid
The concentration of the sodium hydroxide is known and the volume required to react completely with citric acid is
determined in the titration. With this information we can calculate the moles of citric acid. Since we know the mass of
the sample of citric acid, we can then calculate its molecular weight by using the following equation.
ncitric acid = mass of citric acid sample/ molecular weight of citric acid
The end point is the experimental estimate of the equivalence point. An indicator is added to the analyte before titration
begins in order to determine when the end point is reached. Once the analyte is neutralized, the next drop of titrant
reacts with the indicator and changes the color of the solution. Therefore the equivalence point and the end point do
not coincide. In this experiment we use phenolphthalein. This indicator is colorless up to a pH 8.5, at which point it
changes to pink.
In order to have a successful titration, the concentration of the titrant must be known with accuracy. The concentration
of a titrant can be determined through a process known as standardization. Standardization occurs by titrating an
accurately known amount of an extremely pure analyte or primary standard. The reason we standardize sodium
hydroxide is because when solid it reacts with water and carbon dioxide in the air when solutions are being prepared,
thus rendering inaccurate concentrations. In this laboratory, the sodium hydroxide is standardized (Part I) using acetic
acid as the primary standard. At the equivalence point the number of moles of sodium hydroxide equals the number of
moles of the acetic acid.
HC2H3O2 (aq) + NaOH (aq)
NaC2H3O2 (aq) + H2O (l)
nNaOH = nacetic acid
therefore
nNaOH = Macetic acid x V nacetic acid = nacetic acid
The concentration and volume of the acid are known, the number of moles can be calculated. The volume of the sodium
hydroxide required to react completely with the acid is determined by the titration. With this data the concentration of
the sodium hydroxide can be calculated:
MNaOH = nNaOH / VNaOH
Name: ________________________________
AP Chemistry
Determination of the Molecular Weight of an Acid
Pre-Lab questions
1. Calculate the molarity of a sodium hydroxide solution if 24.3 mL were required to titrate 25.00 mL of a 0.873 M
acetic acid solution.
2. Calculate the molecular weight of citric acid if 25.0 mL of a 0.750 M sodium hydroxide solution were required to
titrate 1.20 g of citric acid.