"Published in Newark, California, USA"
What mass in grams of NaC2H3O2 must be dissolved with 500 mL of 0.1 M acetic acid to make a buffer solution of pH = 5? Ka = 1.8 x 10-5 for acetic acid.
Solution:
Buffer solution is a solution which is a mixture of weak acid and its salt or a mixture of weak base and its salt. The purpose of making a buffer solution is to prevent the rapid change of pH if acid or base is added to the solution. In this problem, the buffer solution is a mixture of acetic acid and sodium acetate.
Let's consider the ionization of acetic acid as follows
The ionization constant for acetic acid is
If pH = 5 for the buffer solution, then the concentration of [H+] is
From the ionization of acetic acid, it follows that
Let's consider the ionization of sodium acetate as follows
If x is the concentration of sodium acetate, then it follows that
Again, let's consider the ionization constant of acetic acid as follows
If sodium acetate is added to acetic acid solution, then the concentration of acetate ion will increase as follows
Since 1 x 10-5 is a very small number, then we can neglect it at the above equation as follows
Let's assume that the change of the volume of acetic acid is negligible if sodium acetate is added, then the volume of solution is also equal to 500 mL.
Therefore, the weight of sodium acetate to be added with 500 mL of acetic acid is
