07. Determining Ka of a Weak Acid

What information/material do you need from a previous experiment?
a. The value of the molal boiling point elevation constant.
b. The ability to graph a linear relationship.
c. The NaOH solution you or your partner made.
d. The value of the rate constant from the kinetics experiment

c. The NaOH solution you or your partner made.

Derive Equation number 4 on page 53 of your lab manual

pH=pKa + log [A-]/[HA]

pH=pKa + log(1)

pH= pKa + 0

pH= pKa

Where does the data need to be collected most carefully?
a. at the beginning of the titration
b. near the equivalence point
c. when the titration is nearly complete

b. near the equivalence point

What will be the independent axis and the dependent axis on your
titration curve?

independent: x-axis (volume mL) → dependent: y-axis (pH)

Lab Purpose:

To determine the Ka of an unknown weak acid through the use of a titration with a pH meter and interpretation of a titration curve

A weak acid ionizes less than ____ and thus forms a dynamic equilibrium.
▫ Ka

→ 100%

= [A–] [ H3O+]/[HA]

A titration can be performed between a strong base and this weak acid. The half-equivalence point can be used to determine the Ka of the acid

pKa =pH

How is the half-equivalence point determined?

The half equivalence point is determined by finding that plotted point that is half of the volume or molarity at the equivalence point

How is the pH at the half-equivalence point determined?

The pH of the half equivalence point can be determined by finding the equivalence point and halfing the volume and finding that value on the graph, then reading the pH. Or one could half the pH at the equivalence point

What can be stated about the concentrations of the acid-base conjugate pairs at the half-equivalence point?

The concentrations of the weak acid and conjugate base will equal each other.