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774 Chapter 14 | Acid-Base Equilibria
Step 1. Determine the moles of NaOH. One milliliter (0.0010 L) of 0.10 M NaOH contains:
Step 2. Determine the moles of CH2CO2H. Before reaction, 0.100 L of the buffer solution
contains:
Step 3. Solve for the amount of NaCH3CO2 produced. The 1.0 10−4 mol of NaOH neutralizes 1.0 10−4 mol of CH3CO2H, leaving:
and producing 1.0 10−4 mol of NaCH3CO2. This makes a total of:
Step 4. Find the molarity of the products. After reaction, CH3CO2H and NaCH3CO2 are contained in 101 mL of the intermediate solution, so:
Now we calculate the pH after the intermediate solution, which is 0.098 M in CH3CO2H and 0.100 M in NaCH3CO2, comes to equilibrium. The calculation is very similar to that in part (a) of this example:
This series of calculations gives a pH = 4.75. Thus the addition of the base barely changes the pH of the solution (Figure 14.17).
(c) For comparison, calculate the pH after 1.0 mL of 0.10 M NaOH is added to 100 mL of a solution of an unbuffered solution with a pH of 4.74 (a 1.8 10−5-M solution of HCl). The volume of the final solution is 101 mL.
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