Question

Experiment 4: Identification of an unknown acid by titration Page 2 of 15 Background In this experiment, you will use both qualitative and quantitative properties to determine an unknown acid's identity and concentration. To do this analysis, you will perform a titration of your unknown acid sample-specifically a potentiometric titration where you use a pH meter and record pH values during the titration, combined with a visual titration using a color indi- cator to show the progress of the reaction. By comparing your data to a titration of a different unknown acid, you will also be able to compare the changes in the titration curve due to the equilibrium acid ionization constant (K), and compare the effectiveness of the two titration methods. What is a titration? Titration is a method used to determine the concentration of one solution by reacting it with a second solution that we know the concentration of very precisely. A typical set-up for a titration is shown in Figure 1. Usually, the buret is filled with titrant (the solution of known concentration - yellow in the diagram), while some of the analyte (the solution we are trying to analyze) is placed in a flask. The titrant is slowly added, until exactly enough titrant has been added to react with all of the analyte in the flask - this is the equivalence point. FIGURE 1: TITRATION SETUP Usually, we cannot see the equivalence point directly. To help make the endpoint visible, we can use a color indicator - a compound that will be one color before the equivalence point, and a different color after. One color indicator you may be familiar with is phenolphthalein, which is clear in acidic solution, changing to bright pink in basic solutions. Since the color change of the indicator may not happen exactly at the equivalence point, the point where the indicator changes color and we usually stop titrating) is called the endpoint. The volume of titrant needed to reach this point is the titre. We read the burette from the bottom of the meniscus, to a precision of 2 decimal places, e.g. 1.85 ml. More information on taking pre- cise buret readings can be found on Page A13 (Appendix F).
It's a weak acid strong base titration

Answer 1

1.) Decrease. This is so as the concentration of weak acid increases, it is more dissociated as per equilibrium Eq.

HA + H2O= H3O+ + A- (a)

Hence, Ka (pH at half equivalence) which is equal to [H3O+] [A-]/[HA] would increase which would decrease pKa value given by -log Ka.

2.) Increase. This is so as more Ka value means acid is stronger. More of H+ ions are being formed. Hence, to neutralize a stronger acid, more volume of NaOH will be required.

3.) Increase. Higher the concentration of weak acid, more of H+ ions are being generated as per Eq. (a). Hence, larger volume of NaOH will be required to neutralize the acid completely.

4.) Decrease. pH at half equivalence is pKa which is -log pKa. If value of Ka increases, pKa will decrease.

5.) Stay the same. pH at half equivalence is pKa which is dependent on the type of acid, its concentration and hence, concentration of H+ ions produced. It is the property of an acid. It is independent of the concentration of NaOH used.

6.) Decrease. Increasing NaOH concentration would require lesser volume of NaOH to neutralize the acid completely as for the same concentration of H+ ions, higher concentration of NaOH gives more OH- ions to neutralize at lower amount.