EXPT #4: Alternative PRE-LAB: ANALTITR Excel Spreadsheet
Objective
To obtain ideal titration plots for the following titrations:
Titration A. 100.mL of the strong acid HCl (0.040M) with NaOH (0.20M)
Titration B. 100.mL of the weak acid acetic acid (0.040M) with NaOH (0.20M)
Procedure for using the spreadsheet for Titration A
1. Open the spreadsheet and select the sheet entitled Monoprotic
2. Enter the volume and concentration data in the pink cells H1, H2 and H4:
Volume of acid: 100mL: Enter 100 [don’t enter the units!]
Concentration of acid: 0.040M Enter 0.040
Concentration of NaOH: 0.20M Enter 0.20
Do not change any other cells on this sheet!
3. Set a value for Ka using the green slider: move the scroll bar or tap the arrows for fine adjustments.
For HCl, a strong acid, set Ka = 1000 (i.e. a very large value)
4. Click near the outer edge of the titration graph, so that the entire graph is selected, and print the graph.
Analysis of Titration A
1. Read off the volume of NaOH at the equivalence point (the center of the near-vertical region of the pH curve). You can do this by resting the mouse cursor at this point, OR by moving the blue slider until the pink spot is at this point, and reading the volume in the blue table below the graph.
2. Separately, calculate the volume of 0.20M NaOH needed to react completely with 100mL of 0.040M HCl.
3. Check that the measured and calculated NaOH volumes agree.
Procedure for using the spreadsheet for Titration B
Follow the same procedure as in the HCl titration, except that the Ka value for acetic acid should be set to 1.74 x 10-5 (i.e. 1.74E-5), using the green slider. Once again, print the graph.
In addition, move the blue slider, which controls the volume of NaOH added to the 100mL of acid. The volume and associated pH are indicated by the pink spot on the graph and shown in the blue table below the graph.
Also watch the bar chart and notice how the concentration of HA (i.e. acetic acid) decreases and that of A- (i.e. the acetate ion) increases as the titration progresses. Note in particular the relative amounts of HAc and A- when about 10mL and 20mL of NaOH have been added.
Analysis of Titration B
1. Calculate the volume of 0.20M NaOH needed to react completely with 100.mL of 0.040M acetic acid. Check that the calculated value agrees with the volume of NaOH needed to reach the equivalence point on the graph.
2. Does the volume of NaOH needed to reach the equivalence point depend on whether the acid is strong or weak?
3. Compare the pH curves for HCl and acetic acid before the equivalence point. Does the acetic acid curve satisfy the definition of a buffer, i.e. a region with an intermediate value of pH, with a slow variation of pH as the NaOH is added?
4. At the equivalence point, how much molecular acetic acid remains in solution? What happens to the acetic acid and acetate ion concentrations after the equivalence point?
Further tests (optional)
You are welcome to vary Ka of the acid and (within reason) the volume of acid and the concentrations of the acid and base, i.e. the entries in cells H1, H2 and H4.
The other sheet shows titration curves for reaction of a diprotic acid with NaOH. On this sheet you need to enter the two Ka values of the acid manually, in cells E1 and E2.
C:\Y0708D\Sum08\Chem0120-Ex4-prelab-Excel.doc