Determination of the Molarity of a Strong Acid, Experiment 3
Titration of a Weak Base with a Strong Acid Leads to Neutrality at the Endpoint
The purpose of the lab “Determination of the Molarity of a Strong Acid” was to
determine the molarity of a dilute solution of Hydrochloric acid (HCl).
A solution of 5 M
HCl is diluted with less than 250 mL of water. Then, in order to determine the
concentration of this new, diluted solution of HCl, the weak base
tris(hydroxymethyl)aminomethane (TRIS) was titrated with a strong acid, HCl.
lab, the indicator bromocresol green (BCG) was used to determine the equivalence
point, also known as the endpoint, (where moles of the acid equals the moles of the
base) by turning from blue (indicating basicity of the solution) to green (indicating
neutrality of solution). Based on the volume of dilute HCl solution used to titrate the
TRIS, one is able determine what the concentration or molarity of the dilute solution is.
The results demonstrated that the concentration of the dilute HCl solution was
calculated to be 0.1120 M.
(Douskey 29, 35)
As mentioned previously, the objective of this lab was to determine the molarity of a
dilute solution of HCl. To do this, the TRIS, a weak base, was titrated with HCl, a strong
A titration is used to determine quantitative amounts of a base or an acid, depending on
whichever is the reactant of unknown concentration. In this lab, a specific amount of
grams of TRIS was weighed and recorded, and then this amount was used to determine
the moles of TRIS in the base solution. The moles of the base equaled the moles of the
acid at the endpoint of the titration. The endpoint of a titration is when the moles of the
acid and base are equal, signaling that the titration has come to an end.
In an acid-
base titration, an indicator is used to determine the endpoint by switching from one color
to another color, indicating that the solution has changed in pH level (whether acidic or
basic) to achieve neutrality.
In this lab, bromocresol green was the indicator employed;
it was applied to the basic solution of TRIS, where it appeared to be blue; and once the
moles of HCl of the dilute solution equaled that of TRIS, the bromocresol indicator
turned the solution green. BCG is blue when the solution is basic and yellow when the
solution is acidic, so when it is green that means that half the moles of the solutions are
moles of base and the other half are moles of acid. So, there is an equal amount of
moles of acid and moles of base, which causes the solution to appear green.
indicator as such is useful in a titration because it indicates the equivalence point, which
then means that if the moles of acid are known, then the concentration of the acid can
TRIS is a base, meaning that it accepts hydrogen ions (H+) or gives off hydroxide ions