Experiment 6: STANDARDIZATION OF A BASE; MASS PERCENT OF
AN ACID
Introduction
The reaction of an acid and a base to form a
salt and water is known as neutralization. In
this experiment; potassium acid phthalate,
abbreviated KHP, is used as the acid. It has
a molecular weight of 204.22 and has only
one acidic hydrogen atom per molecule. As
a result, one mole of KHP reacts with one
mole of NaOH.
HKC8H4O4 + NaOH
NaKC8H4O4 + H2O
Titration is the process of measuring the
volume of one reagent required to react with
a measured volume or weight of another
reagent. We will use this technique in two
ways. In Part A of the procedure, we will
determine the molarity of a NaOH solution
by titrating it with a weighed sample of KHP
dissolved in water. In Part B, we will
determine the mass percent of pure KHP
present in an unknown mixture by titrating it
with a NaOH solution of known molarity. In
a titration, the goal is to stop the addition of
reagent when stoichiometrically equal
amounts of the two reactants have been
combined. This is called the equivalence
point of the titration and is usually indicated
by a change in the color of the solution
being titrated. The color change is achieved
by adding an indicator to one of the
reagents. Indicators are substances which
change color at a specific pH. The point at
which an indicator changes color is called its
endpoint. In this experiment, the indicator is
phenolphthalein, which is colorless in acidic
solution, but pink in alkaline solution.
Part A: Standardization of a NaOH Solution
The molarity of a NaOH solution is found
by titrating it with a weighed sample of
KHP. From the weight of KHP used, we
calculate the moles of KHP. Exactly the
same number of moles of base is needed to
neutralize this number of moles of KHP
since based on the balanced equation, one
mole of NaOH reacts with one mole of
KHP. We then calculate the molarity of the
base solution from the titration volume and
number of moles of NaOH in that volume.
These relationships can be described
mathematically as follows:
1. The number of moles of KHP is found by
dividing the mass of KHP by the molecular
weight of KHP.
moles KHP = _____mass KHP______
MW KHP (204.22 g/mol)
2. The number of moles of NaOH is found
by multiplying the moles of KHP by the
mole ratio of NaOH to KHP given by the
above, balanced chemical reaction.
moles NaOH = moles KHP x 1 mole NaOH
1 mole KHP
3. The molarity of the NaOH solution is
found by dividing the moles of NaOH by the
liters of NaOH solution required to reach the
endpoint of the titration.
molarity NaOH sol = moles NaOH
liters used
