Calculate the mass of sodium acetate that must be added to 200.0 ml of a 0.20 M solution of acetic acid to prepare a buffer with a pH value of 4.75? The addition of sodium acetate does not change the overall volume of the solution. (Ka= 1.8 x 10^-5)
I am looking for the steps showing how to solve this problem. Thank you! :)
I am looking for the steps showing how to solve this problem. Thank you! :)
1 Answer
3.32 g
Explanation:
Ethanoic acid is a weak acid and dissociates:
For which
Please remember that these are equilibrium concentrations .
This is the initial moles of ethanoic acid. This will be slightly less than the equilibrium moles. Because the value of
This is an important assumption which is often not stated. As a guide this assumption is valid for a
Since
From this
Rearranging the expression for
Note I am using moles instead of concentrations. I can do this because the total volume is common to both acid and co base so just cancels.
This means that the assumption that the question asks you make regarding no volume change is actually irrelevant.
Rearranging: