Question

DATE: NAME: NAME: SECTION/GROUP: NAME: NAME LAB 11: PRE-LAB HOW DOES THE BUFFER CAPACITY DEPEND ON THE COMPOSITION OF A BUFFER MIXTURE?, 1. Use 10-mL Mohr pipettes to prepare the solutions described in the Data Table below and measure their initial pH using a pH electrode. Make sure the electrode bulb is fully immersed before measuring. Add 1.0 mL of 0.10 M NaOH to each solution and measure the pH again. Calculate the pH change caused by the addition of NaOH. Table 11.2. pH Data for Acetate Buffers Solution mL of 0.100 M HC2H302 (weak acid) mL of 0.100 M NaC2H302 (salt of conjugate base) Initial pH pH after addition of 0.100 M NaOH. pH Change 10.0 2.0 10.0 5.0 10.0 10. 0 4 3.74 4.76 11.02 3.79 4. 30 10.51 .194 .58 0.39 4.45 5.33 10.88 4.91 11.57 6.661 5.0 10.0 2.0 10.0 2. Which solution(s) above is(are) better for handling the addition of NaOH? Explain why. 3. Of the solutions above, which one exhibits the maximum buffer capacity? What is its composition? 4. The buffer capacity of which solution above is exhausted upon addition of y0 ml of 0.100 M NaOH? Justify your claim. 2.0mL Investigation 11 l Buffers Exploration 179

Answer 1

2.)The given reaction is the case for acidic buffer

i.e. the buffer solution formed by the addition of weak acid and its conjugate base

When NaOH is added the OH ion of NaOH and the H+ ion of the acid forms H2O.The aacidic reaction occurs in forward direction thus resisting the change in pH.

The 3. solution is better for handling the addition of NaOH, since the pH change is small.Which serves the actual meaning of a buffer solution.

3.) Buffer calacity=no. of moles of OH or H3O added/ pH change

So,

The 3 rd solution will give the maximum buffer capacity of 11.74 mM/pH

The composition of the buffer is acetic acid(weak acid) and sodium hydroxide (strong base).

4.) The buffer capacity of the 5th solution will get exhausted since the amount of weak acid is much lesser than the strong base.