Identification of a Weak Acid by Titrimetry

Pre-lab Assignment
This lab has prelab quiz (two tries) which you can find on the myCourses page. Read through all
the background information first, and then answer the prelab questions.
Background Information:
Two distinguishing characteristics of an acid are its molar mass and its acid strength
(identified by its Ka), both of which will be used to identify an unknown acid in this lab. The acid
strength is measured by the extent to which the acid transfers H+
to a base, which is usually
water. The following equation and equilibrium expression for the proton transfer from an acid to
water are shown below:
HA(aq) + H2O(l) ⇌ H3O
(aq) + A−
(aq) (Eq. 1)
The equilibrium constant expression for the reaction in Equation 1 is shown in Equation 2:
[H O ][A ] 3
+ −
= (Eq. 2)
The greater the extent to which the acid transfers H+
to water, the greater the acid strength
of HA, and the larger the value of the acid dissociation constant, Ka. For example, HCl is a
strong acid in water and transfers virtually all of its H+
to water. The Ka of HCl is very large,
approximately 108
. By contrast, acetic acid (HC2H3O2) and mandelic acid (HC8H7O3) are weak
acids that transfer only a fraction of their H+
to water. The Ka of HC2H3O2 is only 1.8×10−5
, and
that of HC8H7O3 is 3.89×10−4



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